imattHiBMHUwH^fiBaiffiii^ i.'U'ii'iJ ■■•^■—■■•^ ;■■-, ■';,.}■: ■ L -y±r.r~::xi'.:z :,^. 1312 CORNELL UNIVERSITY LIBRARY ENGINEERING LIBRARY Cornell University Library QC 876. J31 1912 Weather and weather instruments for the 3 1924 004 523 142 DATE l DUE $p — I )mLjU£ J5K^Tf , sr Qr — 5 \ J^tw — *^-/ — I I J i I I GAYLORD PRINTED IN U.S.A. Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924004523142 Weather AND Weather Instruments For the Amateur BY P. R. JAMESON, F.R.Met.Soc. Second and Revised Edition PUBLISHED BY Taylor Instrument Companies ROCHESTER, N. Y., U. S. A. w in "Everywhere, skin deep below our boasted science, ive are brought up short by mystery impalpable, and by ada- mantine gates of transcendental forces and incompre- hensible laws, of which the Lord, who is both God and Man alone holds the key, and alone can break the seal." — Chas. Kingsley. 1 n* Copyright 1912 by laylcr Instrument Companies Rochester, N.Y..U.S. A. Kp, «« Fair Weather After You" -Shakespeare. HE atmosphere surrounding the earth Tmay be regarded as an "ocean" of air extending upward from the earth's surface. Obeying the law of gases it exerts, in all directions, a pressure varying according to the density of the air. It is impossible to tell accurately to what height air extends. Formerly some authorities claimed eight miles, while others said forty to fifty miles. The existence of an atmosphere at more than a hundred miles above the surface of the earth is revealed to us by the phenomenon of twilight and the lu- minosity of meteors and fireballs. Should you measure off the "ocean of air" in layers of equal thickness, the top layer would nat- urally be lightest because it is not weighted down and compressed by any layers above. Each succeeding layer would increase in weight until the earth is reached. This layer is heaviest as it must support the p [m «e.. s ;iie M a s .«*u,- w t entire volume of air above. m».i«£%**m. ...A... ris — -«H — as •■■* L 1 -M~) fl i -•" « ■•— «----- %. — 36 - - v, An altitude of 8s»27o feet was registered at Uccle Observatory, Belgium, by 'balon-sondes' the pressure at this point being only 0.67 inch. 4 WEATHER THE AIR AT GREAT HEIGHTS. It is almost out of the question for man to ascend higher than five or six miles because of lack of air to breathe. At six miles it is too thin to supply a human being with the requisite oxygen for breathing. At great heights the atmosphere becomes more and more attenuated, and thins out by insensible gradations into a perfect vacuum. There is no defi- nite boundary immediately below which there is an atmosphere, and immediately above which there is none. The pressure at an altitude of a few miles is very small, decreasing with increase in altitude, as the higher the ascent, the less air remains above. PRESSURE OF THE ATMOSPHERE. The air at sea level (weighted down by the air above it) exerts a pressure of about 14.7 pounds per square inch of surface. The pressure on a grown per- son (average 16 square feet) would be about 35,000 pounds. Were it not for the ease with which the air (under this pressure) penetrates the body, very slight changes in pressure would prove disastrous. THE WEIGHT OF AIR IN POUNDS. Like terrestrial solids and fluids, the atmosphere is held in place by the attraction of the earth. As the area of the earth's surface is one hundred and ninety- seven million square miles, or seven hundred and Generally speaking, the fall of one inch in the barometer indicates a rise of about 900 feet in the elevation: 017 feet above sea level the barometer falls 1 in. i860 " " " " " " " 2 in. 2830 " ' " 3 in. 3830 " ' ' 4 in. 4861 " " " " " " " s in. MEASURING HEIGHTS ninety quadrillion inches, the total weight of the atmosphere is eleven and two-thirds quintillion pounds. Of the enormity of these values, some idea may be ob- tained by instituting a few inter- esting comparisons. One million trains each composed of one mil- lion powerful locomotives would represent but the hundredth part of the weight of the atmosphere. A leaden ball equal in weight to the atmosphere would have a diameter of 60 miles. This law (decrease of pres- sure) being known, its principle is used in measuring the height of hills and mountains by means of barometric observations at the two points. THE ANEROID. The word "Aneroid" is a Greek compound, expressing* "without fluid, "thus distinguish- ing this barometer from one which measures the pressure of the air by means of a mercury column. The Aneroid is so arranged that the pressure of the air ac- tuates the upper surface of a vacuum chamber, which is per- The average height of the barometer in England at sea level is 29.94 inches. 'The average height of the barometer in the United States at sea level is 29.92 inches. 6 WEATHER fectly balanced between this pressure and a main spring. The vertical action thus given to the vacuum chamber is multiplied and transmitted to an index hand moving over the dial , which has been graduated into divisions (inches and fractions of an inch) to agree with the scale of the mercurial barometer. EARLY WEATHER RECORDS. The earliest records of weather are found in mythi- cal stories, some of which still survive. In England and Sweden "Noah's Ark" is still seen in the sky, while in Germany the "Sea Ship" still turns its head to the wind before the rain. In Scotland the "Wind Dog" and the "Boar's Head" are still the dread of the fisherman, while such names as "Goat's Hair" and "Mares' Tail" recall some of the shaggy monsters of antiquity. It is said that some of the prognostics of the Greek "Diosemeia" (270 B. C.) are in current use at the present time, having been incorporated by Virgil in his Georgics and then translated into English. OLD WEATHER PROVERBS. The enormous extent to which such a foretelling has been carried on, is shown by the vast array of weather proverbs and adages handed down from the past, while the faultiness of their generalizations has been proved by the utter failure of most attempts at their verification. Among the most common of these wise sayings are those which assert a controlling in- fluence of certain days over the weather for consider- able periods to follow. A cubic foot of dry air at 32 F. at sea level weighs 0.080728 lbs. PROVERBS 7 The most potent of these special days seems to have been sacred to some particular saint, and perhaps the most powerful of all in this respect was the far- famed St. Swithin, whose wonderful prowess as a rain- maker is shown in the verse: "St. Swithin's day, if thou dost rain, For forty days it will remain. St. Swithin's day, if thou be fair, For forty days 'twill rain nae mair." A class of proverbs has to do with some supposed relation between one meteorological condition and an- other soon to follow, or of certain conditions existing at one time of day being indicative of immediate change. As an example of the first: Or: 'A storm of hail Brings frost in its tail. "If the rain comes before the wind, Lower your topsails and take them in ; If the wind comes before the rain, Lower your top sails and hoist them again." And: "The rainbow in the morning Is the shepherd's warning, The rainbow at night Is the shepherd's delight." There are grounds for suspecting that the exist- ence of many of the most "catchy" of all the proverbs is due to the tendency which existed a century or two ago, especially in England, where the crop of sayings seemed to be most prolific, of putting words together in such a way as to form rhyme, even at the expense of truth. A case in point, though not from weather "Evening red and morning gray Are sure signs of a pleasant day." WEATHER lore, is the epitaph upon a seven- teenth century tombstone in an English country churchyard: "Here lies the body of Thomas Wood- hen, The kindest of husbands and best of men. ' ' Directly beneath is the ex- planation : "His name was Woodcock but it wouldn't come in rhyme.' ' THE INFLUENCE OF WEATHER ON PEOPLE. The records of the police courts of New York City, studied in connection with those of the Weather Bureau, show conclu- sively that not only on, the hot day but during certain meteoro- logical conditions, (unknown per- haps by name to the author of "Romeo and Juliet"), was the "mad blood stirring."* Records of deportment in the public schools, of suicide, of death, of general health, and of the behavior of the insane simi- larly studied, show unmistakable evidence of a weather influence, and in spite of the fact that it seemed to Samuel Johnson a very sorry thing that ' 'a being endowed with reason should resign his * "I pray thee, good Mercutio, let's retire : The day is hot; the Capulets abroad, And if we meet, we shall not 'scape a brawl, For now, these hot days, is the mad blood stirring." FIRST USE OF BAROMETERS q t powers to the influence of the air, and live in depend- ence upon weather and wind," even the most phleg- matic of us must acknowledge the potency of the east wind and the leaden sky. It was not until 1643, twenty-three years after the landing of the Pilgrims on Plymouth Rock, that Torricelli discovered the principle of the barometer. Torricelli's great teacher, Galileo, died without know- ing why Nature, under certain conditions, abhors a vacuum; but he had discovered the principle of the thermometer. The data from the readings of these two instruments form the foundation of all meteoro- logical science. THE FIRST USE OF THE BAROMETER. As soon as men began to observe the barometer attentively,- they began gradually to recognize that the rising and falling of the barometer had an evident connection with the weather. It was the celebrated burgomaster, Otto von Guericke, of Magdeburg, who first used the barometer as a "weather glass." He ap- plied, even then, to his water barometer the "weather scale," which is at present in such general use, on which the highest reading occurring at any place, is designated as "fine weather," the lowest reading as "rain and wind," etc. The barometer as a weather glass has taken its course throughout the world, and is to-day used almost universally. ROTARY MOTION OF STORMS. About one hundred years after the invention of the barometer (1747), Benjamin Franklin divined that certain storms had a rotary motion and that they pro- gressed in a northeasterly direction. Although his Lowest U. S. barometer reading was taken at Galveston, Texas, during the year of flood when the barometer reached 28.48 inches or nearly $ lb. per square inch below normal. TO WEATHER ideas in this respect were more important than his act of draw- ing the lightning from the clouds and identifying it with the elec- tricity of the laboratory, his con- temporaries thought little of his philosophy of storms. It remain- ed for Redfield, Espy, Maury, Loomis and Abbe, one hundred years later, to gather the data and completely establish the truth of that which the great Franklin had dimly yet wonderfully out- lined. STUDY OF CONDITIONS AT GREAT ELEVATIONS. We have at present no method by which we can forecast the weather with absolute certainty even for one day in advance, to say nothing of longer periods. The Weather Bureau has es- tablished an Observatory at Mount Weather to study con- ditions of temperature, pres- sure, humidity and wind velocity and direction at great elevations to increase our knowledge of the laws governing the atmosphere, which should eventually enable our successors (if not ourselves) to add to the accuracy of weather forecasts and to make them for a longer period in advance. A temperature of in degrees below zero was taken at St. Louis, Mo., at an altitude of 48,700 feet. Hall Barometer PREDICTIONS ii As one of the primary objects in view in establish- ing Mount Weather Observatory was to make a study of the relations existing between the various forms of solar radiation and terrestrial weather conditions, much attention had to be given to the instrumental equipment ar.d to securing men to study the varia- tion in the amount of heat energy given off by the sun from day to day and variation in the amount of heat absorbed by the atmosphere. So important to the study of the sun is a continu- ous record of the magnetic variations that one of the first steps in the establishment of the Observatory was the installation of a magnetic plant consisting of the best modern instruments for the direct observation and for continuous registration of the variation in the magnetism of the earth. Researches are also carried on to determine the existence and measure the extent of probable direct relation between meteorological disturbances and mag- netic variations. AT PRESENT WEATHER PREDICTIONS ARE MADE ; (a) From local observations and refer to the lo- cality where made. (b) From' weather charts (covering an extended region) and refer to any region on the chart. (c) From weather charts in connection with local observation and refer to the region where the local observations are made. As storms usually occur where the air pressure is low, the aneroid not only determines the height of mountains but also forecasts the weather. The sun setting after a fine day behind a heavy bank of clouds, with a falling barometer, is generally indicative of rain or snow, according to the season, either in the night or next morning. 12 WEATHER WEATHER AND THE EFFECT OF THE SUN. We speak of weather as mean- ing the atmospheric condition as shown by the meteorological ele- ment of a particular time, for a day, a season, or even a year. Climate is the aggregate of weath- er conditions. The sun regulates our weather ; it gives rise to winter and sum- mer; by evaporation it raises the aqueous vapor into the air, and this vapor by cooling, produces clouds, rain, snowstorms and hail ; it is the primary cause of the diff- erences in atmospheric pressure, and. in this way produces the winds. This heating influence of the sun, as also its modifications by cloudiness, by the wind, by the change from day to night or from I winter to summer, and by the properties of the earth's sur- face, which, consisting as it does of water and land either covered with vegetation or barren, has varying capacities for absorbing -the sun's heat. This influence of the heat of the sun has been established with the most abso- lute certainty by the most exact observations. Hall .Barometer The sun is the great source of light and heat, which is transmitted to the earth. It is 853,000 miles in diameter and spherical in shape. TAKING. READINGS i'3 THE METEOROLOGICAL ELEMENTS. The meteorological elements are the temperature, the" barometric pressure, the humidity, precipitation, evaporation, the wind, the clouds and the electrical conditions of the air. Aerial Meteors are winds, hurricanes, whirl winds, etc . Aqueous Meteors are fogs, clouds, rain, dew, snow, etc. Luminous Meteors are lightning, the rainbow and the Aurora Borealis. Metal Case Compensated Aneroid Barometer HOW TO FORECAST. To make a good forecast, it is essential that the observer take into consideration the direction and force of winds, appearance of the sky, humidity of the air and a comparison of the barometer reading with the indicated pressure for several days preceding. An important fact, too often overlooked, is that the Aneroid foretells, rather than indicates weather that i4 WEATHER is present. The Aneroid generally indicates changes in weather 12 to 24 hours in advance. After "setting" the barometer if the hands at the next observation coincide, the barometer is "station- Wood Frame Aneroid Barometer ary." If the blue hand has moved to the right, the barometer is "rising." If it has moved to the left, the barometer is "falling." The extent of the rise or fall being the distance (in fractions of an inch) be- tween the two hands upon the dial. The possibility is always for a continuation of exist- ing weather unless some phenomenon presents itself which foretells a change. A very low barometer is usually attendant "upon stormy weather, with wind and rain at intervals, but the latter not necessarily in any great quan- tity. If the weather, notwithstanding a very low barometer, is fine and calm, it is not to be depended upon; a change may come on very suddenly. EFFECT OF WIND 15 EFFECT OF WIND. The wind which causes the barometer to rise and fall, has more to do with the weather than the preval- ence or deficiency of sunshine. The shifting of the wind is the most trustworthy of weather forecasts. A rise in the barometer shows that heavier air is drifting to a place just before occupied by light air. As heavy air is air that has been condensed by cold, a rise in the barometer indicates a cold wind. A fall in the barometer shows that light air is drift- ing to a place just before occupied by heavy air, or, in other words, a warm wind is blowing. A falling barometer usually indicates a high or a low atmospheric pressure existing near at hand. The fall is then due to the gradual drifting up of the lighter air and the drifting away of the heavier air in the giddy whirl of some aerial conflict. A fall- ing barometer shows that lighter pressures are ap- proaching the station of the observer. WEATHER WORDS ON ANEROID USELESS. Whoever has provided himself with an instrument of this kind believes himself to be the possessor of a self-registering weather prophet, and is generally highly indignant if it rains when his barometer stands at "fine" or astonished if it is fine weather when the barometer says "rain." The reading 29.5 (29^ inches) was at one time assumed to be the midway line separating "Fair" from "Rain" and was accordingly marked "Change." 30 inches was marked "Fair;" 31 inches, "Very Dry;" 28.5 inches, "Rain;" and 28 inches, "Stormy." A fixed standard was thus assumed for a condition of Nature that is literally as unstable as the wind. It was A sudden rise in the barometer is nearly as threatening as a sudden fall because it shows that the level is unsteady. 1 6 WEATHER supposed that the instruments were to be used only in places about at the same level as the surface of the sea. This is the ideal barometer, as the scale reads only from 28 to 31 inches, and has no weather words on it. One thousand feet of altitude represents, roughly, an inch of pressure on the barometer. So that if two barometers were placed, one at sea level and the other at an altitude of i ,000 feet, the one at sea level might read "Fair," while the other, under practically similar meteorological conditions, would read "Rain." Even at the sea level, if a barometer which has been standing at, say, 30.9 inches for some days, suddenly fell to 29.9 in 24 hours, it would give a positive indica- tion of change, intimating the approach of strong wind ALTITUDE. The scientific word for "height." The altitude of a cone or pyramid is the height of its vertex above the plane on which it stands. The altitude of a star is its height above the horizon. The altitude of a mountain or hill is its greatest height above sea level. EFFECT OF WINDS 17 and probably rain, yet according to the dial, it would read "Fair." In a similar manner, if a barometer that, had been standing at 28 inches for some days, rose in about 24 hours to 29 inches it would indicate the ap- proach of a cold, dry wind although the dial would read "Rain." It follows that these words on the dial have no significance but are simply relative. SINGLE OBSERVATION USELESS. A single observation of the barometer, without reference to the conditions prevailing at definite inter- vals preceding is liable to be misleading. The im- portant thing to know is — Has the rise or fall been a gradual one or has it been rapid? If the barometer is stationary, how long has this condition existed? Weather prognostications from barometer observa- tions are based on a knowledge of all these conditions, and never from a single observation. RAPID CHANGES INDICATE. A rapid fall or a rapid rise intimates that a strong wind is about to blow, and that this wind will bring with it a change in the weather. What the precise nat- ure of the change is to be must, in the main, depend upon the direction from which the wind blows. If an observer stands with the wind blowing on his back, the locality of low barometric pressure will be at his left and that of high barometric pressure at his right. With low pressure in the west and high pres- sure in the east, the wind will be from the south; but with low pressure in the east and high pressure in the west, the wind will be from the north. When the glass falls low, Prepare for a blow; When it rises high, Let all your kites fly. 1 8 WEATHER The barometer rises for northerly wind (including from northwest, by the north, to eastward) for dry, or less wet weather, for less wind, or for more than one of these changes — except on a few occasions when rain, hail or snow comes from the northward with strong wind. The barometer falls for southerly wind (including from southeast, by the south, to the westward), for wet weather, for stronger wind or for more than one of these changes — except on a few occasions when moderate wind with rain (or snow) comes from the northward. The above applies to readings in the northern hemis- phere. The readings in the southern hemisphere are practically the reverse of these. For a change of wind towards northerly directions a thermometer falls. For change of wind towards southerly directions a thermometer rises. Moisture or dampness in the air (shown by a hygrometer) increases before rain, fog or dew. GENERAL BAROMETER INDICATIONS. A gradual but steady rise indicates settled fair weather. A gradual but steady fall indicates unsettled or wet weather. A very slow rise from a low point is usually associated with high winds and dry weather. A rapid rise indicates clear weather with high winds. A very slow fall from a high point is usually connected with wet and unpleasant weather with- out much wind. A sudden fall indicates a sudden shower or high winds, or both. When the barometer falls considerably without any particular change of weather, you may be certain that a violent storm is raging at a distance. WINTER i 9 A stationary barometer indicates a continuance of existing conditions, but a slight tap on the barometer face will likely move the hand a trifle, indicating whether the tendency is to rise or fall. In the warm months the winds are light and rather variable, and changes in direction have not the same importance as in the colder months. The rain of sum- mer generally occurs in connection with thunder- storms; it will be found that these are most frequent from a certain direction and with the wind in a partic- ular quarter. Beyond the fact that more thunderstorms come from a westerly quarter than from any other direc- tion, little can be said that will be of value in forecast- ing their approach by the direction of the surface winds only. The coming of a thunderstorm can gen- erally be foretold a few hours in advance by the form and movement of the clouds. STORMY WEATHER IN WINTER. The signs of falling weather in the colder months are the formation of a high sheet cloud covering the whole sky, an increase in the temperature and moisture of the air, and the change of the wind to some east- erly quarter. The precise direction that the wind takes, whether northeast, east or southeast, varies for different localities and the direction from which the storm is approaching. In New England, the Middle States and the Ohio Valley, northeasterly winds precede storms that ap- proach from the southwest, and southeasterly winds precede storms that approach by way of the Lake Rapid changes in the barometer indicate early and marked changes in the weather. 20 WEATHER Region. On the Pacific coast southeasterly and south- erly winds precede rain storms. HI In Wyoming and other Northwestern States the heavy snowstorms of winter and spring generally come from the north or northwest with a strong wind from the same direction. The direction of the wind depends very much on the position of traveling storms that pass across the country. In every locality, there is one direction of cloud motion that; betokens bad weather, and another, gen- erally the opposite direction, which portends. fine weather, etc. Weather rules relative to red morning and evening sky have been deduced. LOCAL SIGNS. The rules that bad weather is expected when in any given locality the summit of a certain mountain is covered with a cap; that a small, "watery" halo around the moon indicates rain; that the weather will continue bad if, when the clouds break up, a second light covering of clouds is seen above them; that it will be fine weather if, after rainy weather, according to the locality, a certain wind sets in; that a slow breaking up of the clouds gives promise of fine weather, etc., all of these rules have been formulated from long-continued and accurate observation, and are ex- ceedingly well adapted for local weather forecasts from one day to the next. FORECASTING FROM COLOR OF CLOUDS. Experienced observers also know from the color and nature of the clouds whether the, prevailing weather will continue or will change and, by these delicate distinctions, generally acquire the reputation of being especially good weather prophets. Should the barometer continue low when the sky becomes clear, expect more rain within 24 hours. The Spicier as a Barometer HE spider is a good example of a living barometer. Every twenty-four hours he makes some alteration in his web to suit the weather. When a high wind or heavy rain threatens, he may be seen taking in sail, shortening the rope filaments that sustain the web structure. If the storm is to be unusually severe or of long duration, the ropes are strengthened as well as shortened. On the contrary, when you see the spider running out the slender filaments, it is certain that calm, fine weather has set in, whose duration may be measured by their elongation. When the spider sits quiet and dull in the middle of its web, rain is not far off. If it be active, however, and continues so during a shower, then it will be of brief duration, and sunshine will fol- low. When you see the spiders coming out of the walls more freely than usual, you may be sure that rain is near. THE FROG AS A BAROMETER. A small green frog is found in Germany, which always comes out of the water when cold or wet weather is approaching. These frogs are caught and kept in glass jars furnished with a tiny ladder and half filled with water. The frog weather prophet sits high and dry on the top of his ladder for several hours before a storm, and climbs down to the bottom "Everything is lovely and the goose honks high." 22 WEATHER when the weather is to be fair and clear. Other re- markable weather prophets are leeches. About 1867 a new treatment of weather problems (known as the synoptic method weather charts) was introduced. Lines were drawn through all places where the barometer read 30; others through all places reading 2 , etc . These were called ' 'isobars ' ' be- cause they marked out lines of equal pressure. Dots drawn through places where the temperature was equal at the moment were called "isotherms" or lines of equal temperature. Arrows marked velocity and direction of wind. Letters and symbols denoted appearance of sky, amount of clouds and occurrence rain or snow on Synoptic Chart. WHEN THE CHARTS WERE EXAMINED IT WAS FOUND: 1. That in general the configuration of the isobars assumed one of seven well defined forms. 2. That, independent of the shape of the isobars, the wind always took a definite direction relative to the trend of those lines and the position of the nearest area of low pressure. 3. That the velocity of the wind was always nearly proportionate to the closeness of the isobars. 4. That the weather — that is, the kind of cloud, rain, fog, etc., at any point was related to the shape (not the closeness of the isobars), some shapes enclosing areas of fine, others of bad weather. 5. That the regions thus mapped out were constantly shifting their position so that changes of weather were caused by the drifting past of these areas of good or bad weather, just as on a small scale rain falls as a squall drives by. The motion of these areas was found to follow certain laws, so that foretelling weather changes in advance be- came possible. Birds fly high when the barometer is high — probably because the air is heavier and denser, therefore has more sustaining capacity. FIRST U. S. WEATHER BUREAU 23 6. That sometimes in the temperate zone and habit- ually in the tropics, rain fell without any appreciable change in the isobars, though the wind conformed to the general law of these lines. So far the science rests on observation that such and such wind or weather comes with such a shape of isobars. The same shape of isobars appear all over the world, but their motion and the details of weather are modified by numerous local, diurnal and annual vari- ations which must be studied out. Isobars represent the effect on our barometers of the movements of the air above us so that by means of isobars we trace the circulation and eddies of the atmosphere. THE FIRST U. S. WEATHER BUREAU. Although American scientists were the pioneers in discovering the rotary and progressive character of storms and in demonstrating the practicability of weather services, the United States was the fourth country to give legal autonomy to a weather service. Congress authorized the first appropriation of $20,000* to inaugurate a tentative weather service in 1870. Gen. Albert J. Myer, to whom was assigned the Chief ship of the new Meteorological Service, doubtless had no conception of the future wonderful extension of the system that he was then authorized to begin. STORM WARNING ON THE COAST. Whether on the Atlantic, on the Pacific, or on the Lakes, there is either a full meteorological observatory or else a storm-warning display-man who attends to If the barometer and thermometer both rise together, it is a very sure sign of coming fine weather. 24 WEATHER the lighting of the danger lights on the storm-warning towers at night, to the display of danger flags by day, and to the distribution of storm-warning messages among vessel masters. This system is so perfect that the Chief of the Weather Bureau, or the forecaster on duty at the Cen- tral Office, can dictate a "storm warning" and feel cer- tain that inside of one hour a copy of the warning will be in the hands of every vessel master in every port of material size in the United States, provided that it is his desire that a complete distribution of the warning be made. ADVANCE REPORTS OP STORMS REDUCE LOSS 75%. The marine ^warnings of the service have been so well made that in over ten years no protracted storm has reached any point in the United States without the danger warnings being displayed well in advance. As a result of these warnings the loss of life and prop- erty has been reduced to a minimum, being doubtless not more than 25 per cent, of what it would have been without this extensive system. When a marked cold wave develops in the northern plateau of the Rocky Mountains and, by its broad area and great barometric pressure, threatens to sweep southward and eastward with its icy blasts, the meteorological stations of the Bureau are ordered to take observations every few hours in the region im- mediately in advance of the cold area and to telegraph the same to headquarters. By this means every phase of the development of the cold area is carefully watched, and when the danger is great each observatory in the threatened region It is estimated that 80 per cent, to 85 pe r cent, of weather predictions are successful. WEATHER BUREAU 25 becomes a distributing center, from which warnings are sent to those who have produce or perishable articles of manufacture that need protection against low temperatures. Travelling Set The United States Government spends $1,500,000 a year on its Weather Bureau, which is more money than the combined governments of Europe spend. It is not uncommon for the Bureau to distribute 100,000 telegrams and messages inside of the space of one or two hours, so that nearly every city, village In England the observed range of the barometer is about 1.3 inches, or about 0.3 in. greater than in U. S. 26 WEATHER and hamlet receives the information in time to profit thereby. What this means to the farmer and shipper is well illustrated by the fact that it was gathered from those personally interested, statements relative to the sweep of one cold wave, which showed that over $3,400,000 worth of property that would have been destroyed by the low temperatures was saved. Even when severe storms are not imminent there is, in addition to the printing of the forecasts in the daily press, a daily distribution of 80,000 telegrams, maps and bulletins, that place the information in the hands of millions whose personal interests are materi- ally affected by the weather. FINE WORK OF WEATHER BUREAU. Not a single storm has swept across the United States or up or down its coastline within many years that has not been foretold hours, and possibly days, in advance by the Weather Bureau. The same applies to cold waves and floods. The time at the disposal of the forecast official of the Weather Bureau at the Central Office in Wash- ington for the purpose of forecasting probable weather changes, cold waves and severe storms is about thirty minutes in the morning and forty at night. It is impossible in this short time to do more than express the character of the anticipated changes for each state or district east of the Rocky Mountains in any but the most general terms. LOCAL FORECASTING. The local or state forecast official, on the other hand, is concerned with but a single district. He is at liberty to amplify the national forecasts or to put The principal maximum barometric pressure occurs before noon and the principal minimum after noon. WEATHER BUREAU 27 forth a statement of his own, in which the anticipated changes may be given in as much detail as the condi- tions seem to justify. Persons who use the forecasts constantly should cultivate the habit of carefully noting the weather changes in their respective localities, especially the sequences in which such changes occur, for it is only by acquiring a knowledge of local weather signs that they can use government forecasts to the best advan- tage. Hall Barometer If the barometer falls gradually for several days during fine weather, expect considerable rain. If it keeps rising while the wet continues, the weather, after a day or two, will probably be fair for some time. 2S WEATHER The barometer and wind indications of the United States are generr ally summarized in the following table of the U. S. Weather Bureau: Barometer Reduced to Sea Level. 30.10 to 30.20 and steady 30.10 to 30.20 and rising rapidly 30.10 to 30.20 and falling slowly 30.10 to 30.20 and falling rapidly 30.20 and above and sta- tionary 30.20 and above and fall- ing slowly 30.10 to 30.20 and falling slowly 30.10 to 30.20 and falling rapidly 30.10 to 30.20 and falling slowly 30.10 to 30.20 and falling rapidly 30. 10 and above and falling slowly 30.10 and above and falling rapidly 30 or below and falling slowly 30 or below and falling rapidly 30 or below and rising slowly. . 29.80 or below and falling rapidly 29.80 or below and falling rapidly 29.80 or below and rising rapidly Wind Direction. SW. to NW. SW. to NW. SW. to NW. SW. to NW. SW. to NW. SW. to NW.' i S. to SE. S. to SE. SE.toNE. i SE.toNE. i i E.toNE. ; E toNE. SE. to NE. SE.toNE. S. to SW. S. to E. E. to N. Going to W Character of Weather Indicated. Fair with slight temperature changes for 1 to 2 days. Fair followed within 2 days by warmer and rain. Warmer with rain in 24 to 36 hours. Warmer with rain in 18 to 24 hours. Continued fair with no de- cided temperature change. Slowly rising temperature and fair for two days. Rain within 24 hours. Wind increasing in force with rain within 12 to 24 hours. Rain in 12 to 18 hours. Increasing wind with rain within 12 hours. In summer, with light winds, rain may not fall for sev- eral days. In winter, rain within 24 hours. In summer, rain probably within 12 to 24 hours. In winter, rain or snow, with increasing wind will often set in, when the barometer begins to fall and the wind sets in from the NE. Rain will continue 1 or 2 days. Rain with high wind, fol- lowed within 24 hours by clearing and cooler. Clearing within a few hours and continued fair for several days. Severe storm of wind and rain or snow imminent, followed within 24 hours by clearing and colder. Severe northeast gales and heavy rain or snow, fol- lowed in winter by a cold wave. Clearing and colder. Explanation of Weather Map HE U. S. Weather Bureau makes tele- graphic reports of the weather each day at 8 a. m. and 8 p.m., seventy -fifth meridian time. The reports consist of observations of the barometer and thermometer, the velocity and direction of the wind, amount, kind and direction of movement of clouds, and amount of rain or snow. On the weather maps solid lines (isobars) are drawn through points that have the same atmospheric pres- sure, a line being drawn for each one- tenth of an inch in the height of the barometer. Dotted lines (isotherms) are drawn through points that have the same atmo- spheric temperature, a line being drawn for each ten degrees of temperature. Heavy dotted lines are some- times used to enclose areas where decided changes in temperature have occurred during the preceding twenty-four hours. The direction of the wind at each station is indicated by an arrow that flies with the wind. The state of the weather — clear, partly cloudy, cloudy, rain or snow, is indicated by symbols. Shaded areas are used to show areas within which precipita- tion in the form of rain or snow has occurred during the preceding twelve hours. The rapidity of the storm's approach and its intensity will be indicated by the rate and amount of the fall in the barometer. 3° WEATHER TABULAR DATA OF WEATHER MAPS. The tabular data give details of maximum and minimum temperature and twenty-four hour temper- ature changes, wind velocities, and amount of precipi- tation during the preceding twenty-four hours. The text printed on the maps presents forecasts for the state and the station, and summarizes general and special meteorological features that are shown by the lines, symbols and tabulated data. HOW HIGHS AND LOWS MOVE. The centers of areas of low barometric pressure, or general storms, are indicated on the map by the word "Low," and the centers of areas of high bar- ometric pressure by the word "High." The gen- eral movement of "Lows" and "Highs" in the United States is from west to east, and in their progression they are similar to a series of atmospheric waves, the crests of which are des- ignated by the "Highs" and the troughs by the "Lows." These alter- oljowtog •increase's,- lecrease nating "Highs" and oj "tttesswe.. "Lows" have an aver- age easterly movement of about 600 to 700 miles a day. The "Lows" usu- ally move in an easterly, or north of east, direction, and the "Highs" in an easterly, or south of east, direction. In the tropics a rapid barometric fall is dangerous because, in a general way, it shows the observer is nearly in path of cyclone. Any fall of more than ,02 is dangerous. WEATHER MAPS 31 In advance of a "Low" the winds are southerly or easterly, and are, therefore, usually warmer. When the "Low" passes east of a place the wind shifts to westerly or northwesterly with lower temperature. The eastward advance of "Lows" is almost invariably preceded and attended by precipitation in the form of rain or snow, and their passage is usually followed by clearing weather. The temperature on a given parallel west of a "Low" may be reasonably looked for on the same parallel to the east when the "Low" has passed, and when the night is clear and there is but little wind, frost is likely to occur along the north of an isotherm of 40 . A "Low" is generally followed by a "High," which in turn is followed by another "Low." WHAT ISOTHERMS INDICATE. When isotherms run nearly east and west no de- cided changes in temperature are likely to occur. When isotherms directly west of a place incline from northwest to southeast the temperature will rise; when from northwest to southwest, the temperature will fall. Southerly to easterly winds prevail west of a nearly north and south line passing through the middle of a "High" and also east of a like line passing through the middle of a "Low." Northerly to westerly winds occur west of a nearly north and south line passing through the middle of a "Low," and also east of a similar line passing through the middle of a "High." An absence of decided and energetic "Lows" and "Highs" indicates a continuance of existing weather When the air becomes colder with a low barometer and a southwest wind, squalls from the northeast will certainly follow, and in winter it is nearly always accompanied by snow. 32 WEATHER that will continue until later maps show a change, that usually appears in the west. At first glance, weather maps look very con- fusing. The storms of the United States follow, how- ever, year after year a series of tracks, not capricious, but related to each other by very well denned laws. MEAN TRACKS AND AVERAGE DAILY MOVEMENT OF STORMS IN THE UNITED STATES. The chart shows the general result of a study of tracks of storms in the United States. There are two sets of tracts running westerly and easterly, one set over the Northwestern boundary, the Lake Region, and the St. Lawrence Valley; the other set over the middle Rocky Mountain districts and the Gulf States. Each of these is double, with one for the "Highs" and one When the wind sets in from points between east and northeast and the barometer falls steadily, a storm is approaching from the south or southwest. Its center will pass near or to the south or east of the observer within twelve to twenty-four hours, with wind shifting to northeast by way of north. FORECASTING TERMS 33 for the "Lows." There are lines crossing from one main track to another showing how storms pass from one to the other. The transverse broken lines show the average daily movement. On the chart the heavy lines all be- long to the tracks of the "Highs" and the lighter lines to the "Lows." HOW " HIGHS " TRAVEL. A "High" appearing on the California coast may cross the mountains near Salt Lake, and then pass di- rectly over the belt of the Gulf States to the Florida coast; or it may then pass directly over the Florida coast; or it may move farther northward, cross the Rocky Mountains in the State of Washington, up the Columbia River Valley, then turn east, and finally reach the Gulf of St. Lawrence. The paths are determined by the laws of the general circulation of the atmosphere and the configuration of the North American Conti- nent. This movement of the "Highs" from the mid die Pacific coast to Florida or to the Gulf of St. Lawrence is confined to the summer half of the year — April to September, inclusive. In the winter months, on the other hand, the source of the "Highs" is different, though they reach the same terminals. TERMS USED IN FORECASTING. "Fair Weather" — that is, the absence of rain or snow, is indicated by several terms. The first of these is the words themselves. It may be used singly or pre- ceded by the word "generally." "Generally fair," as When the wind sets in from points south and southeast and the barom- eter falls steadily, it indicates a storm approaching from the west or northwest. Its center will pass near or north of the observer within twelve to twenty-four hours, with wind shifting to northwest, by way of southwest and west. 34 WEATHER used by the forecast, is less positive than "fair" alone. It signifies that the probability of fair weather over the whole district and for the entire period is not so great as when "fair" alone is used. PARTLY CLOUDY — RAIN — SNOW. "Partly cloudy," is used when the indications favor clouds but no precipitation. "Threatening" is used when the weather will be overcast and gloomy, with the appearance of rain or snow at any moment, yet a measurable amount of precipitation is not antici- pated. A forecast of "rain" or "snow" may be expressed in various ways. In the late fall, early spring and the winter season it is most commonly indicated by the single word "rain" or "snow," when it isexpected that the rain will continue for several hours. In other seasons of the year any one of the following terms, viz.: "local rain," "showers," and "thunderstorms," may be used. Forecasts of local rains, showers or thunderstorms indicate that the conditions are favorable for the oc- currence of precipitation in that district. CLEARING. "Clearing" is a word frequently used which car- ries a broader meaning than the word itself signifies, viz.: the occurrence of precipitation in the early part of the period; thus, "Clearing to-night" would indi- cate that rain or snow, whichever might be falling at the beginning of the period, would cease shortly thereafter and that the weather would be clear dur- ing the greater part of the time. No rule can be laid down for forecasting even a single country. The details vary indefinitely and each observer must use his judgment. a Construction of the Aneroid Barometer Movement "A" is a metal box or vacuum chamber consist- ing of two circular cor- r-i_ rugated discs of thin Ger- *' j j man silver firmly soldered |~~1 together at the edges (^3GGGz (Fig. 2) and fastened to ~ feT base plate, "B". When « a the air is exhausted the top and bottom discs close, as shown in Fig. 3. Spanning this chamber is the bridge "0" which is held from the plate by Kg. 3- the finely pointed screws, "C," "C," these screws also being used to finely regulate the tension upon the chamber, "A." The knife edge, "E," is inserted in the post of the vacuum chamber (which passes through a hole in the spring, "D") thus pulling the two cor- rugated discs of the vacuum chamber apart, leaving it in a poise with the atmosphere. It is upon the movement of the vacuum chamber that the working of the barometer depends. An in- crease in pressure allows the vacuum to overcome the power of the spring, the action then being down- wards; a decrease of air pressure producing the contrary result. An accurate Aneroid will show the "altitude" of a table. If lifted from the floor to the top of a table it will register equal to 2 or 3 feet, according to the height of the table. 36 WEATHER The lever "F," is fixed to the spring, "D," which (being in connection and working with the vacuum chamber as previously described) multiplies the move- ment considerably. The rod or main lever, "F," is connected to the lever, "B"; the lever, "B," is again connected to the lever, "H." To this a fine chain is attached which is wound upon the central pinion, "L," by the hair-spring, "I." The projecting arm, ANEROID MOVEMENT 37 "K," (with the two small pillars and cross piece) supports the arbor and hair-spring. To the pin, "L," (which passes through the center of the hair-spring) is attached the hand, "Ji," which indicates upon an accurately divided dial, the correct amount of bar- ometric change. The hand "J2," is the auxiliary C. & T. hand indicating sea level pressure. The hand, "J$," is not connected with the move- ment but is set (by turning the milled head extend- ing through the glass). As this hand remains sta- tionary, a glance shows the movement of hand "Ji." Weather Barometers For Use Above Sea Level The pressure of the air is neither uniform or station- ary, but it is different in different places and changes in many ways. Firstly, it decreases as we ascend. At the sea- level the whole weight of the air above our heads is pressing mightily upon us, at a thousand feet there is not so much air above us and consequently the pres- sure is less. "WEATHER BAROMETERS 39 The ideal barometer therefore is one that is easily adjusted so it will register at an elevation, the equiva- lent of the air pressure at the sea-level. This is faithfully carried out in the barometer illustrated. Inserted in the back of the instrument is a plate of about 2^4 in. diameter divided from o to 3500 feet which revolves. Engraved on the case and pointing at the divided plate is an arrow. If the user is living at an altitude of 1000 feet above sea level all it is neces- sary to do is to turn the plate until the 1000 mark is against the arrow. The instrument will then reg- ister sea-level readings, which are the readings pub- lished daily on the weather maps. This plate will take care of all adjustments from o to 3500 feet above sea-level. 40 WEATHER Occasionally (say once or twice a year) it is de- sirable to "check" the aneroid. The easiest way to do this is to consult the local Weather Bureau and obtain THE ACTUAL PRESSURE READING UN- CORRECTED FOR ALTITUDE. Turn the plate in the back of the instrument until the "O" is coin- cident with the arrow head, and if the barometer agrees with the standard reading no alteration is necessary. If, however, it should be a little high or low, it can be set to agree by placing a screw-driver in the screw seen through the hole in the back of the case and when turned the hand on the dial will follow in the same direction. No greater correction than THREE TENTHS of an inch should be made by this method. If the instrument reads more than three tenths off the standard reading it should be put in competent hands for readjustment for occasionally dirt clogs some of the delicate working parts as in a clock. The brass plate can then be revolved until the alti- tude of the town of observation is indicated against the arrow on the divided revolving plate. The instru- ment is then adjusted to a standard reading corrected for a difference in altitude between your city and the level of the sea. Too much faith must not be put in the weather words. They are simply relative and it does not follow that any condition should follow that the in- dicating hand points to. On the re-arranged dial as supplied with these barometers, the weather words are arranged at AVERAGE readings, i. e., at the point 29.72 inches the hand points to Rain — the average point at which rain occurs and the same applies to the "Change," "Fair" and "Very Dry" marks. It must be remembered that the weather is FORE- CASTED by an aneroid for probably twelve to twenty- four hours in advance and the readings do not indi- cate present conditions. The rapidity of a weather WEATHER BAROMETERS 41 change and its intensity will be indicated by the rate and amount of movement of the hand. The large figures on the dial represent inches of pressure. The small figures between represent tenths of inches. Each tenth of an inch is divided into fifths making each division value two one hundredths of an inch. Another satisfactory solution is the C. & T. Patent Altitude Adjustment, which consists of an auxiliary hand of copper adjustably attached to the pressure hand and moving with it. While the pressure hand shows the actual atmospheric pressure at the altitude at which the aneroid is used, the copper hand may be so adjusted as to always show the corresponding sea-level pressure. 42 WEATHER Aneroid Barometer with C. & T. Patent Altitude Adjustment The illustration shows the hands set for an alti- tude of about 300 feet. The table on page 46 gives the amount to move the copper hand for various heights above sea-level. METEOROLOGICAL STATIONS 43 LIST OF METEOROLOGICAL STATIONS UNITED STATES STATION « g P5 SlJ STATION » <8 a> X ° > 1718.0 18.2 586.3 3615.0 18.9 1033.0 8.3 100.4 3441 .0 98.0 6.4 862.2 1670.0 16.4 2492.0 —5.0 962.0 37.8 575.8 197.5 269.6 7.1 6.4 4660.0 5866.4 6.5 9.6 725.0 630.6 6054.0 579.5 15.5 546.9 594 3 Albany, N. Y Columbus, Ohio 5977.0 737 5 Amarillo, Tex 709 1372 Atlanta, Ga Corsicana, Tex 427 5 Atlantic City, N. J Davenport, Iowa Dayton, Wash 536.4 1604 4543 Baltimore, Md Denison, Tex 747 8 Barnegat, N. J 5183 Binghamton, N. Y Des Moines, Iowa Detroit, Mich 799.0 584 8 Block Island, R. I Dodge, Kans 2482 Boise, Idaho Boston, Mass Dubuque, Iowa Duluth, Minn 643.0 601 Breckenridge, Minn Eagle, Alaska 575 Brownsville, Tex Eagle Pass, Tex 692.9 Buffalo, N. Y Eastport, Me Elkins, W. Va El Paso, Tex Erie, Pa 5 1 Burlington, Vt Cairo, 111 1920.0 3692 Cape Henry, Va 572 9 Cape May, N. J Carson City, Nev Cedar City, Utah Cedar Keys, Fla Escanaba, Mich Eureka, Cal Evansville, Ind 593.0 25.7 382.6 6886 Charlestown, S. C Charlotte, N. C Fort Apache, Ariz Fort Bridger, Wyo 5000.0 6639.0 Chattanooga, Tenn Fort Canby, Wash Fort Custer, Mont Fort Davis, Tex Fort Gibson, Ind. T Fort Grant, Ariz 192.0 3040.0 4923.3 536.0 4833.0 44 WEATHER METEOROLOGICAL STATIONS — Continued STATION a'fj ojg S > j « $-° S Fort Griffin, Tex Fort Keogh, Mont Fort Smith, Ark 1270.0 2367.0 4310.0 415.0 6151.5 3050.0 1593.0 5498 . 600.3 290.0 5.6 581.3 4579.0 587.3 488.1 317.0 2483.0 3932.0 1285.0 4714.0 3721.0 708.0 9.0 7.5 1.0 2949.0 721.9 3607.0 481.9 806.6 678.5 951.0 5368.6 Lansing, Mich Las Animas, Colo Leavenworth, Kans Lewiaton, Idaho Lexington, Ky Lincoln, Nebr Little Rock, Ark Los Angeles, Cal Louisville, Ky Lynchburg, Va Mackinaw City, Mich Macon, Ga Manchester, N. H Marquette, Mich Memphis, Tenn Meriden, Miss 827.9 3884 . 737.5 737 8 Fort Stanton, N. Mex Fort Stockton, Tex Fort Sully, S. Dak Fort Washakie, Wvo Fort Worth, Tex Fresno, Cal Galveston, Tex Grand Haven, Mich Grand Junction, Colo Green Bay, Wis Hannibal, Mo Harrisburg, Pa 965.5 1147.0 286.5 255.6 456.5 523.3 582.0 334.0 180.8 627.9 271.3 341 Miles City, Mont 2355 Helena, Mont Milwaukee, Wis 586 2 Huron, S. Dak Montgomery, Ala 162 Idaho Falls, Idaho 5796 Independence, Cal Moorhead, Minn 909 Indianapolis, Ind Morgantown, W. Va 789 6 Mt. Tamalpais, Cal. . . 2353 3 Mt. Washington, N. H Nashville, Tenn . . 6300.0 434.8 3 Kalispell, Mont New Haven, Conn Kansas City, Mo New London, Conn 23 1 Newport, E,. I New York, N. Y Norfolk, Va 8 5 Knoxville, Tenn La Crosse, Wis 13.6 37.4 — 1 4 Northfield, Vt 739 North Platte, Nebr 2803 METEOROLOGICAL STATIONS 45 METEOROLOGICAL STATIONS — Continued STATION. Oklahoma, Okla Olympia, Wash Omaha, Nebr Oswego, N. Y Palestine, Tex Parkersburg, W. Va Pembina, N. Dak Pensacola, Fla Philadelphia, Pa Phoenix, Ariz Pierre, S. Dak Pike's Peak, Colo Pioche, Nev Pittsburg, Pa Pocatello, Idaho Poplar River, Mont Port Angeles, Wash Port Crescent, Wash Port Eads, La Port Huron, Mich Portland, Me Portland, Oreg Prescott, Ariz Pueblo, Colo Puerto Principe, Cuba, W. I Puntarasa, Fla Raleigh, N. C Rapid City, S. Dak Red Bluff, Cal Richmond, Va Rio Grande City, Tex Rochester, N. Y Roseberg, Oreg Sacramento, Cal St. Louis, Mo St. Michael's, Alaska St. Paul, Minn St. Vincent, Minn Salt Lake City .Utah San Antonio, Tex d a ill ;j3 0J £X1 » 1195 17 1040 252 494 616 798 11 8 1084 1441 14107 6100 697 4466 1955 11 8 4 581 47 8 5320 4656 324 2 317 3175 306 164 200 509 482 2 412 23 693 798 4268 683 STATION. San Diego, Cal Sandusky, Ohio Sandy Hook, N. J San Francisco, Cal San Luis Obispo, Cal. . . . Santa Fe, N. Mex Sault Ste. Marie, Mich. . Savannah, Ga Seattle, Wash Shreveport, La Sioux City, Iowa Sitka, Alaska Southport, N. C Spokane, Wash Springfield, 111 Springfield, Mass Springfield, Mo Tacoma, Wash Tampa, Fla Tatoosh Island, Wash. . . Thatchers Island, Mass.. Titusville, Fla Toledo, Ohio Tucson, Ariz Umatilla, Oreg Unalaska, Alaska Valentine, Nebr Vicksburg, Miss Virginia City, Mont Visalia, Cal Walla Walla, Wash Washington, D. C Wichita, Kans Williston, N. Dak Wilmington, N. C Winnemucca, Nev Woods Hole, Mass Yankton, S. Dak Yuma, Ariz V a Aa . s > s 5.8 572.9 9.2 8.0 240.0 6954 . 607.3 41.7 22.3 187.2 1107.0 62.9 14.0 1010.0 600.2 70.0 1348.0 31.0 —1.1 162.0 53.0 1.0 572.8 2389.0 297.0 8.0 2581.0 223.2 5824 . 325.1 923.0 91.2 1300.0 1854.0 31.9 4335.0 4.0 1197.3 140.5 4 6 WEATHER Table No. 2 For Aneroids with "C. & T." Adjustment Table No. 2 gives the fraction of an inch, in which the copper hand should be moved to the right upon the dial for each succes- sive thousand feet of elevation at which the Barometer will be used above sea level. Above Sea Level. Move Copper Hand to the Right. 500 feet 1,000 " 1,500 " 2,000 " 2,500 " 3,000 " 3,500 " 4,000 " 0.5 scale inches 1.1 1.6 " 2.2 " 2.7 " 3.2 " 3.7 " 4.2 " Cyclones CYCLONES LOW. AT times the barometric pressure over a part of a country is much below the average, sometimes as low as 29 inches or even less. In such cases the pressure increases in wid- ening circles for a distance of several hun- M dred miles from the place of lowest pres- ^^ sure. A system of isobars of this kind is called a "Cyclone." It is usually accom- panied by rain and high winds in the coun- try over which it lies. The "Lows" are sometimes called storms. The center of the smallest isobar is called the storm center. When the shape of the isobar representing an area of low pressure are not rounding nearly circular, the area is called simply a "Low" or a "Depression." CAUSES OF CYCLONES OR STORMS. Cyclones are due primarily to the unequal heating and moisture, or cooling and drying, of the air over large regions of the earth's surface, disturbing the level of the surfaces of equal density. This results in a convectional ascensional movement of the lighter air near the ground and the coming down of heavier air from above to restore the equilibrium. The light air moves spirally inward and upward, and at a greater height flows outward to the sides. This flow is similar to that of water from a basin through a hole in the Rain falling at the rate of 0.02 inch per hour is considered light; at 0.05 inch heavy. 48 WEATHER bottom. The motion from opposite sides gives rise to the rotation. When the upward convection extends to a height at which the temperature is lowered by dynamic cool- ing below the temperature of the dew-point of the air, there is a condensation and cloud formation. When this occurs, the initial gyratory impulse of the air be- comes of secondary consequence. The principal part in maintaining and extending the ascending motion is taken by the latent heat set free from the vapor. The cloud canopy in the daytime also increases the tendency of the air to ascend by transferring the point of application of the sun's heat from the ground to the top surface of the clouds at a height in the air. DRY CYCLONE. Convectional ascending motion in the air is going on at all times during the day, but for the most part is not sufficient to carry the air high enough to pro- duce any great amount of condensation, sometimes on account of the feebleness of the ascensional force, and again because of the dryness of the air requiring as- cent to a very great height to reduce it to the dew- point. This condition sometimes produces a dry cy- clone of feeble action, with cloud formation only, and no rain. The decrease of pressure in a cyclone pro- duced by rainfall alone in very slight. The centrifugal force developed by the gyration and the deflecting in- fluence of the earth's rotation on the currents are the main causes of the production of low pressure at the centre of a cyclone. ADJUSTMENT NECESSARY. An aneroid barometer may be out of adjustment, so far as not agreeing with the reading of a mercurial On only a few occasions in any year will .10 be exceeded, though .20 has been recorded. EFFECT OF CLOUDS 49 barometer, and still give accurate measurements of the amount of change in atmospheric pressure. It is more satisfactory to the observer, however, if his in- strument be compared with a Standard Mercurial Barometer. If they do not agree, the aneroid may be adjusted by turning the small adjusting screw until the indi- cating hand on the dial coincides with the height of the mercury column. It should however never be moved more than 0.3 inches. The finest quality barometers require a slight adjustment at the end of say six months and then about once in nine months. After a time they become so nearly permanently accurate that they require no re-setting. The ordinary grade of instruments natur- ally require more frequent adjustment. COMPENSATION OF ANEROIDS. All fine quality aneroids are compensated to coun- teract the expansion and contraction of the metals, which alters the leverage of the mechanism, mak- ing the indications very inaccurate. In compensating a barometer, it is necessary to make the lever "F" (See cut page 36) of a composite bar of two metals (steel and brass), the quantity of each being altered until it is correctly "compensated" for any change in temperature. This avoids the ne- cessity of making allowances for temperature, which is necessary in reading a mercurial barometer. Fast rise after low, For tells stronger blow; Long foretold, long last, Short notice, soon past. So WEATHER Approximate compensation for temperature can be made by leaving a small quantity of air in the vacuum chamber. When heated this increases its pressure upward and tends to offset the weakening effect upon the springs. This compensation alone is not sufficient. It is necessary to compensate the lever "F" as described. SYMPIESOMETER. _ra_ ANEROIDS EOR MARINE USE. The Aneroid Barometer is the best instrument that can be divised for marine use, not only on account of its extreme sensitiveness, but also because it is not affected by the motion of the vessel. It is now recog- nized as a necessity for the mariner and is made in many compact forms for use in yachts. An important testimonial for their excellence for mariners was given in the generous action of the Life Boat Insti- tution of Great Britain, when, in order to promote its use and prevent the loss of life amongst this fine class of fisher- men, they offered to provide the master of any fishing smack with an aneroid at half price. o *• li A barometer in which the atmos- pheric pressure is exerted directly on a short column of oil or similar liquid, causing compression of a portion of air 8j nf w*a«i«T or gas enclosed in the tube above the liquid; highly sensitive, but very liable to derange ment and great inaccuracies. Rainbow in morning, Shepherds take warning; Rainbow at night, Shepherds' delight. Watch and Pocket Aneroids OR the tourist, engineer and surveyor, the F Aneroid Barometer is not only very inter- esting but also indispensable, as it measures, if properly used with great accuracy, the height of hills, mountains and gradients. There are many forms in use, but the reg- ular watch or pocket style is the most popular. They are made with both fixed and revolving altitude scales. They will accur- ately register alti- tudes up to 20,000 feet. Those which register to 3,000 feet have the finest div- isions, the value of each being but 10 feet. By sub-divid- ing, a careful observ- er can take even closer readings. As the value of the altitude scale decreases, as the pressure lessens, the "0" of the altitude scale should always be exactly opposite 31 inches on the barometer dial before taking an alti- tude reading. Watch Aneroid with Altitude Scale A red morn, that ever yet betoken Wreck to the seamen, tempest to the field, Sorrow to shepherd, woe unto the birds, Gust and foul flaws to herdmon and herds. 52 WEATHER Pocket Aneroid with Altitude Scale For example, suppose the aneroid indicated a pressure of 27 inches. If we ascend a hill and the hand (due to decreasing pressure) moves to 22 inches, the correct method of determining the difference in altitude, would be as follows: Approximately the value of 27 inches (with the "O" feet at 31 inches) is "Mackerel sky, Twelve hours dry." ZISO" EFFECT IN INCREASE IN ALTITUDE S3 3,750 feet, while the value of 22 inches, under the same conditions, is 9,350 feet. 9,35°' 3>7So' Subtraction shows the differ- ence in altitude to be 5, 600' Now suppose the aneroid indi- cates a pressure of 27 inches, but in- stead of having the o feet at 3 1 inches (as we should) we move the milled ring so that the O feet is standing op- posite 27 inches. If we then ascend the mountain until the hand reaches 22 inches, the altitude registered will be only 4,800 feet, or 800 feet in error. The explanation is simple. As the air at sea-level is far heavier than at an altitude of a few thousand feet, it exerts a greater pressure. The graduations on the altitude scale of a watch or pocket aneroid grad- ually diminish in size. The first inch of pressure (from 31 inches to 30 inches) represents an ascent of about 900 feet, while an inch of pressure, say from 18 inches to 17 inches, represents about 1,570 feet. tl.OOO — JQSOO 1 0,000 SiSOO 9,000 8.S00 — 8,000 — 7&0 7000 G$00 __ 6,000 tpoo SfiOO ASOO 4000 """ 3500 — 3000 ZSoo — 2000 ~ 1,500 1,000 soo ALTITUDE SCALE I 8400 a*>" 2400" J45o' 2S00" 2550" 2600" 26.50" 2700" 2750' 2400- tS.SO" 2900- £950' 3000* Slflo" INCHES PRESSURE When rainfall exceeds a inches a day or 10 inches a month, it is excessive. 54 WEATHER 30" to 31" represents 900 ft. 17" to 18" represents 1, 580 ft. The following table of altitudes (by Professor Airey, Astronomer Royal of England) has been adopted as a standard: No dew after a warm day foretells rain. AIREY'S ALTITUDE TABLE 55 Aneroid or Height Aneroid or Height Aneroid or Height Aneroid or Height Aneroid or Height Corrected In Corrected In Corrected In Corrected In Corrected In Barometer Feet Barometer Feet Barometer Feet Barometer Feet Barometer Feet in. ft. in. ft. in. ft. in. ft. in. ft. 31.00 28.28 2500 25.80 5000 23.54 7500 21.47 10000 30.94 50 28.23 2550 25.75 5050 23.50 7550 21.44 10050 30.88 100 28.18 2600 25.71 5100 23.45 7600 21.40 10100 30.83 150 28.12 2650 25.66 5150 23.41 7650 21.36 10150 30.77 200 28.07 2700 25.61 5200 23.37 7700 21.32 10200 30.71 250 28.02 2750 25.56 5250 23.32 7750 21.28 10250 30.66 300 27.97 2800 25.52 5300 23.28 7800 21.24 10300 30.60 350 27.92 2850 25.47 5350 23.24 7850 21.20 10350 30.54 400 27.87 2900 25.42 5400 23.20 7900 21.16 10400 30.49 450 27.82 2950 25.38 5450 23.15 7950 21.12 10450 30.43 500 27.76 3000 25.33 5500 23.11 8000 21.08 10500 30.38 550 27.71 3050 25.28 5550 23.07 8050 21.05 10550 30.32 600 27.66 3100 25.24 5600 23.03 8100 21.01 10600 30.26 650 27.61 3150 25.19 5650 22.98 8150 20.97 10650 30.21 700 27.56 3200 25.15 5700 22.94 8200 20.93 10700 30.15 750 27.51 3250 25.10 5750 22.90 8250 20.89 10750 30.10 800 27.46 3300 25.05 5800 22.86 8300 20.85 10800 30.04 850 27.41 3350 25.01 5850 22.82 8350 20.82 10850 29.99 900 27.36 3400 24.96 5900 22.77 8400 20.78 10900 29.93 950 27.31 3450 24.92 5950 22.73 8450 20.74 10950 29.88 1000 27.26 3500 24.87 6000 22.69 8500 20.70 11000 29.82 1050 27.21 3550 24.82 6050 22.65 8550 20.66 11050 29.77 1100 27.16 3600 24.78 6100 22.61 8600 20.63 11100 29.71 1150 27.11 3650 24.73 6150 22.57 8650 20.59 11150 29.66 1200 27.06 3700 24.69 6200 22.52 8700 20.55 11200 29.61 1250 27.01 3750 24.64 6250 22.48 8750 20.51 11250 29.55 1300 26.96 3800 24.60 6300 22.44 8800 20.47 11300 29.50 1350 26.91 3850 24.55 6350 22.40 8850 20.44 11350 29.44 1400 26.86 3900 24.51 6400 22.36 8900 20.40 11400 29.39 1450 26.81 3950 24.46 6450 22.32 8950 20.36 11450 29.34 1500 26.76 4000 24.42 6500 22.28 9000 20.32 11500 29.28 1550 26.72 4050 24.37 6550 22.24 9050 20.29 11550 29.23 1600 26.67 4100 24.33 6600 22.20 9100 20.25 11600 29.17 1650 26.62 4150 24.28 6650 22.16 9150 20.21 11650 29.12 1700 26.57 4200 24.24 6700 22.11 9200 20.18 11700 29.07 1750 26.52 4250 24.20 6750 22.07 9250 20.14 11750 29.01 1800 26.47 4300 24.15 6800 22.03 9300 20.10 11800 28.96 1850 26.42 4350 24.11 6850 21.99 9350 20.07 11850 28.91 1900 26.37 4400 24.06 6900 21.95 9400 20.03 11900 28.86 1950 26.33 4450 24.02 6950 21.91 9450 19.99 11950 28.80 2000 26.28 4500 23.97 7000 21.87 9500 19.95 12000 28.75 2050 26.23 4550 23.93 7050 21.83 9550 19.241 13000 28.70 2100 26.18 4600 23.89 7100 21.79 9600 18.548 14000 28.64 2150 26.13 4650 23.84 7150 21.75 9650 17.880 15000 28.59 2200 26.09 4700 23.80 7200 21.71 9700 17.235 16000 28.54 2250 26.04 4750 23.76 7250 21.67 9750 16.615 17000 28.49 2300 25.99 4800 23.71 7300 21.63 9800 16.016 18000 28.43 2350 25.94 4850 23.67 7350 21.59 9850 15.439 19000 28.38 2400 25.89 4900 23.62 7400 21.55 9900 14.883 20000 28.33 2450 25.85 4950 23.58 7450 21.51 9950 56 WEATHER SURVEYING ANEROIDS. For very accurate altitude measurements, larger aneroids (3" or 5" in diameter) are generally used, as a small movement of the indicating hand can be Surveying Aneroid Barometer more readily detected. Greater accuracy in the move- ment can be also attained than is possible in the small Aneroid, which is of necessity crowded. SCALE DIVISIONS. On watch and pocket aneroids, the divisions of the pressure scale are equal, while the divisions of the altitude scale gradually diminish. The surveying aneroid scale reverses this arrangement, the divisions — - — | _ Meteors on entering the gaseous envelope of the earth are set afire by friction. THE VERNIER 57 of the altitude being equal, while the pressure scale divisions diminish. By having an equally divided altitude scale, it is practical to sub-divide (by means of- the vernier applied to the altitude scale), which would not be possible were the scale unequal in value. In the larger surveying aneroids, it is possible to take readings showing differences of single feet. THE VERNIER. The vernier is a device by means of which each graduation can be sub-divided into decimal quantities. It was invented by one Peter Vernier of Brussels in the year 1631. It consists of a small scale moved by a rackwork adjustment, attached to the milled knob at the top of barometer case. If an aneroid scale is divided into ten-feet divi- sions, the vernier will be divided into ten divisions to exactly cover twenty-one divisions on the altitude scale. It is therefore possible for only one line on the vernier to coincide with any line on the altitude scale. If the second line on the scale coincides with a line on the altitude scale, it indicates that the odd number of feet to be added to the reading of the aneroid (as shown by the altitude scale) is two feet. If the third coincides, three feet should be added, and so on. If an aneroid is divided into 20-feet divisions, the vernier sub-divides them into two-feet divisions; if We are 253,000 miles from the moon. 58 WEATHER fifty feet, it sub-divides into five feet. A small magni- fying glass revolves around the case facilitating rapid accurate readings. For example, if an aneroid (ten-feet divisions) reads a few feet over 1,770, adjust the "O" of the ver- nier scale directly under the hand. Only one gradua- tion of' the vernier scale can coincide with a gradua- tion on the altitude scale. The seventh vernier gradua- tion coinciding, the odd number of feet to add is seven (See illustration page 57.) It is very essential that absolute accuracy be ob- tained on surveys, and the mode of procedure is as follows: Where a survey (which may take a consid- Barometric gradient means the degree or steepness of the slope of sobaric surfaces. DIFFERENCE OF LEVEL 59 erable time) is taking place, two aneroids are em- ployed. One is placed at the lower station (with an observer to record at stated intervals any change which takes place in the atmospheric pressure), the other being carried by the person making the ascent. When the survey is completed, the indicated changes at the lower station are added to or deducted from the observed readings of the aneroid used in the ascent and corrections made accordingly. (See P- 53-) Aneroids are compensated (see p. 49) but, as the atmosphere is affected by change in temperature the following rule for correction should be applied to the table of altitudes (which assumes a mean at- mospheric temperature of 50° F.). RULE FOR CORRECTION FOR TEMPERATURE. Add together the temperature of the upper and lower stations. If this sum (in degrees) is greater than ioo° F., increase the height by i-ioooth part for every degree in excess of ioo°. If the sum be lower than ioo°, diminish the height by i-ioooth part. For example: the reading of the barometer at the lower station is 30.146 — 500 feet altitude. Lower Station 30.146 500 feet Upper 21.019 10,500 feet Reading by the scale 10,000 feet Temperature at lower station 6o° F. Temperature at upper station 30 F. Total 90 Evening red, And morning gray ; Two sure signs Of one fine day. 60 WEATHER The total being less than 100°, the deduction would be 10 feet, therefore io°xio feet=ioo, deducted from reading of 10,000 feet equals correct height 9,900 feet. Surveying aneroids should always be read in a horizontal position, as there is quite an appreciable amount of difference between the reading of an aner- oid when held horizontally and when held vertically. Surveying aneroids are made in ranges from 3,000 feet to 25,000 feet. HYPSOMETERS. From the connection between the boiling point of water and the atmospheric pressure, the height of mountains can be measured by the thermo-barometer. Suppose, for example, it is found that water boils on the summit of a mountain at 90 C. and at its base 98° C. Since a liquor boils when its vapor pressure is equal to the atmospheric pressure, it is only necessary (in order to ascertain the atmospheric pressure at the top and the bottom of a mountain) to refer to a table giving corresponding temperatures and vapor pressures. By the aid of this table, the thermometer gives the same information as the bar- ometer. An ascent of 1080 feet produces a diminu- tion of 1° C. in the boiling point. CONSTRUCTION OE HYPSOMETERS. Instruments (hypsometers) used for this purpose, consist of a small metallic vessel for boiling water, fitted with a very delicate thermometer graduated from 8o° C. to ioo° C. only. As each degree thus In very high altitudes, they say it is impossible to boil eggs (hard) unless the cover of the kettle is weighted down so that the pressure of steam will allow higher temperature than is possible in an open vessel. HYPSOMETERS 61 occupies a considerable space on the scale (the i-ioths and even the i-iooths of a degree being estimable) it is possible to determine the height of a place to within about ten feet. AN EXPERIMENT BOILING. An interesting experiment on the effect of pres- sure on the boiling point is the following: Boil some water in a flask; while boiling is going on, cork the flask and remove the source of heat; when the glass vessel has somewhat cooled down, squeeze a sponge saturated with cold water over the flask, and boiling will be seen to recommence. This is owing to the fact that the sudden application of the cold water out- side condenses the vapor above the hot water within, and thus considerably reduces the pressure above it, so that bubbles of vapor can be again formed in the liquid, and boiling is renewed. On Mount Blanc water boils at 183.2 degrees F. On Mount Quito water boils at 194 degrees F. Barographs (Or Stormographs) AROGRAPHS (or stormographs) are an- Beroids arranged to record upon a chart the atmospheric changes, the amount of rise and fall and the time such changes occur. The mechanism consists of a "pile" or series of vacuum 'chambers, seven or eight in number, each secured to the one above and below, making a move- ment of the whole seven or eight times as sensitive as a single chamber. The movement of these chambers is still further greatly magnified and transmitted to the aluminum recording arm carrying the pen, by a series of connecting levers. This pen records the changes in pressure on a chart which encircles the drum containing barograph the clock move- ment. /' A week's record can be obtained on the chart, as the clock revolves once in that time. As the top of the chart is divided into seven spaces, (the seven days of the week), and sub-divided to spaces representing two "D' yxrw u»»a.-*o Tfcvae (nloujer Miciiuntib »ajiHfc-f«t* or. chart A Barogram is the record made by a Barograph. RECORDING BAROMETERS 63 hours each, it is possible to tell at what time of any day, atmospheric conditions undergo a change. While the ranges of charts vary, the one universally used shows pressure from 28" to 31", the value of each division on the chart being .05 inches. ADJUSTMENT OP BAROGRAPHS. Barographs should be adjusted (to read with a standard barometer) by turning the small milled head screw, directly over the bridge spanning the vacuums. The pen will rise or fall, dependent on the direction the screw is turned. The compensation for temperature is accomplished by leaving a sufficient quantity of air (ascertained by experiment when instrument is made) in the vacuum chambers so that the tendency of the barometer to register too low (on account of the weakening of the springs, the expansion of the levers and other parts) due to a rise in temperature, is counteracted by the increased pressure of air in the vacuum cells. An evening- grey. And a morning red; Will send the shepherd Wet to bed, 64 WEATHER Tuesday Convex. This Concave The instrument should, however, be kept in as uniform a temperature as possible. With a rising recording baro- meter the trace of the pen is convex for a decreasing rate and concave for an increasing one. The reverse is true of a falling barometer. If fall is steady the line will be straight diagonally. cut illustrates one advantage of the baro- graph. Two observations of an aneroid were made (at 10 p. m. and 8 a. m., re- spectively) both showing a reading of 30.10 in., which would indicate a "station- ary" barometer with a con- tinuance of present weather. A glance at the barograph record shows a rapid fall and rise between 10 p. m. and 8 a. m., which indicates a short but severe storm due at about n a. m. Speaking of a certain "delicate" barogram, Hon. Ralph Abercromby, F. R. Met. Soc, London, says: "A case of this sort shows, more than any other, the superior value of a continuous trace over an in- termittent barograph, for though the latter permits the tabulation of hourly values, they entirely lose all chance of following these minute alterations of pressure which are often accompanied by great changes of weather." In winter heavy rain is indicated by a decrease of pressure and an in- crease in temperature. RECORDING BAROMETERS 65 In Weather prognostication a single observation of the Barometer is of little or no value, and while frequent observations will, if recorded, convey the desired information provided changes in atmospheric Simplified Barograph pressure are gradual, yet when sudden changes occur between observations such records will be missing and probably lead to a misinterpretation of "weather signs." The Barograph is a most reliable form of Bar- ometer in indicating the present-time atmospheric pressure, but its special value lies in the continuous hourly record which it creates, of every fluctuation in pressure for seven consecutive days, showing not only the extent of the various changes, but also the time of their occurrence. Franklin ascribed the dry fog met with in London to the large quantities of coal tar and paraffin vapor sent into the atmosphere, which condense on the particles of fog, preventing their evaporation. 66 WEATHER USE OF BAROGRAPHS AT SEA. Barographs are invaluable for mariners, as they are not affected by the roll and motion of a vessel at sea. Here it is important to know not only the amount of rise or fall but also whether rapid or slow, as winds and seas depend upon these conditions. In all well appointed vessels it is now recognized as a necessity. Barograph with Dial An interesting attachment is made for recording barometers, in the shape of an auxiliary dial. Its hand is actuated by the same movement as the barograph, and therefore registers the same as the pen upon the chart. Instead of complicating the baro- graph the advantage to the lay user is obvious, as the present barometer readings are more readily deter- mined by reference to the dial. No dew after a hot day foretells rain. Thermograph or Recording Thermometer Experiments have been made with many different types of recording, thermometers, some depending upon a metallic spirit tube for their movement , others on a bi-metallic bar. As the latter style is more accurate, durable and constant in its action, it has been adopted as a standard by the best makers. DEFECTS OF SPIRIT TUBE THERMOGRAPHS. In the "spirit tube" thermograph the constant ex- posure to the air corrodes the metal, causing it to be- come more or less porous and leaky, making the in- strument highly inaccurate. The mechanism also necessitates a series of levers, magnifying the move- ment of the tube. The "pins" which fasten these levers often become rusted causing the instrument to register even more inaccurately. The thermograph now most generally used has a spiral coil of two different metals (brazed together) with the pen arm fixed directly to the coil. The ex- pansion and contraction of the spiral coil causes the pen arm to move up and down, recording the tempera- ture on the chart. It is three times as sensitive as a very sensitive mercurial thermometer. The temperature of the sun is 14,072 degrees F. 68 WEATHER Metal Case Thermograph USE OF THERMOGRAPHS. Accurate thermographs are an absolute necessity in ship's stores, refrigerators, ice plants, railroads and fruit vans, as in such places the question is not so much "What is the temperature?" as "What has been the temperature?" Where a uniform temperature of say 40 is neces- sary, a thermometer at the time of inspection may show 40° but it does not tell if the temperature has been above or below 40° in the past two, four, six or eight hours. The thermograph keeps a time record of all fluctuations in temperature, any altera- tion on the chart being easily detected. Thermogram is the record made by a Thermograph. THERMOGRAPH 6q COMBINED THERMOGRAPH AND BAROGRAPH. A recent improvement has been introduced into recording instruments by combining the barograph and thermograph in the same instrument. The records are given on the same chart but in different colored inks to prevent confusion. Barogram plus Thermogram plus Anemogram equals Metogram. 7o WEATHER GBSERAVTIONS AND ^REFtECTI0»8 COUCXHMlua'TIlI *> THERMOMETERS, WE cannot enough eoraiTteri(i:>nii.adriiiM ' that -e*celient invention'* of fb/rmwc ters, whereby' we sib enabled to make feme judgement of the variou* "degKes of heat ii> bodies. It iaiSrt our bufinefi 3t>'»refe«fto-M>a.-T)t.g1j. < Ti-tT;pet , o.'tur«. Our earth in its revolution around the sun intercepts less than one-half of one-billionth of the heat sent off by the sun. Q2 WEATHER area of high or low temperature, preceding page.) During the day, the ground receives from the sun more heat than it radi- ates into space. Thereverse is the ease during the night. It is necessary in mete- orological observations, to know the highest tempera- ture of the day and the lowest temperature of the night. Ordinary thermom- eters could only give these indications by a continuous observation, which would be impractical. The Thermograph (see p. 67) is of course the ideal instrument, as it gives all fluctuations and the time of their occurrence, but a maximum and minimum thermometer will give the extremes. The mercury pushes ahead of it an index (see cut). When the mer- cury recedes, the index remains at the highest point. In the left hand tube this is the lowest, while in the right hand tube it is the highest degree of heat reached. (See : drawing on ■ lyiM 1 ill IS UUIIIII [IHI'IIH 1 SI in iiiiiiiiii 'SI p 1 £ =W =L ■■■■■ mill *[3fl -b ': :| |h The Eastern Hemisphere is 2 F. warmer than the Western, due to the greater amount of land 8o° E long, and ioo° E. long, from Greenwich. HOW WATER FREEZES 93 HOW WATER FREEZES. Water contracts when its temperature sinks to about 25° F., but from this point (although the cooling continues) it expands to the freezing point so that 2 5 represents the point of greatest contraction. In winter, the water at the surface of a lake be- comes cooled and sinks to the bottom and a continual series of currents go on until the whole has a tem- perature of about 25° F. The cooling on the surface still continues, but water expands about 10% at the moment of solidify- ing and, in consequence, floats on the surface of the water. Were it not for this, a lake would freeze solid. The ice which forms protects the water below, the lower portions of which remain at a temperature of about 25° F. EFFECT OF CLOUDY SKY. In some winters it has been found that the rivers have not frozen, the sky having been cloudy, although the thermometer had been for several days below 25° F., while rivers freeze at higher temperatures when the sky is clear. DEPTH OF THE SEA. The depth of the open sea is very variable; the lead generally reaches the bottom at about 300 to 450 yards, in the ocean it is usually 1,300 yards, and in- stances are known where the bottom has not been reached at 4,500 yards. It has been computed that the total mass of the water does not exceed that of a liquid layer surrounding the earth with a depth of about 1,100 yards. An isothermal line is a line every point of which has the same temperature. Mercurial Barometers F~T~\ HE Mercurial Barometer of to-day I is essentially the same as origin- ally invented by Torricelli in 1643. Fit consists of a straight glass tube, 32 or 33 inches long, filled with mercury. The tube is in- verted with the open end in a cup of mercury, the column falls un- til counterbalanced by the weight of the surrounding atmosphere pressing upon the surface of the mercury in the cistern. In other words, were it possible to weigh a column of air (of the same diameter as the bore of the barometer tube) extending from the surface of the mercury in the cistern to the top of the atmosphere, the weight would be the same as that of the mercury contained in the tube above the surface of the mercury in the cistern. Hence, any change in atmospheric pres- sure produces an alteration in the height of the mercurial column in the tube. It only re- mains, therefore, to devise some method of measuring the height of the column to deter- mine the varying conditions of atmospheric pressure. It is almost impossible to safely ship Mer- curial Barometers (no matter how carefully packed) on account of the weight of the mer- cury. Humidity HEN you say "humidity" people shrug W their shoulders and look for something more interesting, not realizing that without moisture in the air there would be no life — that the lack of humidity causes discomfort, ill health, catarrhs, colds, and other diseases of the mucous membrane — that by having proper hu- midity in the houses in winter they could save 12^ per cent, of the total cost of heating. Many people have the idea that colds are taken (in winter) by the sud- den change in temperature in stepping out of doors, but as a matter of fact the change in humidity is much more liable to cause disease. You can better realize this if you will but consider that in buildings heated with steam and hot water, with an average temperature of 72 , the rela- tive humidity averages 28 per cent. UNHEALTHY INDOOR ATMOSPHERE. In the most arid regions of the world, only, is a humidity as low as 30 per cent, found. Imagine what parching and blindness this causes; what thirst and what dryness of the tissues in those lonely wastes — and this is just the "climate" we live in all winter. When 50% humidity is spoken of it means that half as much moisture is present, as would be necessary for the saturation of the vapor under the ex- isting conditions of temperature and pressure, 9 6 WEATHER Stepping from this atmosphere to an outside hu- midity of about 70 per cent., is it any wonder that such a sharp and violent change is productive of harm, particu- larly to the delicate mucous membrane of the upper air passages? The pneumonia period is the season of arti- ficial heat in living rooms. This artificial heat (espe- cially if a hot air heater is used) is dry enough to work nervous irritation to the per- son compelled to breathe it. THE PROPER HUMIDITY. Dr. Henry Mitchell Smith, M. D., in his book on "In- door Humidity," says: "It was most interesting and in- structive to find that on the perfect days in May arid early June, with all the win- dows open admitting freely the outdoor air, a thermom- eter stood at 65 to 68 de- grees and the hygrometer registered about 60 per cent, relative humidity." If a room at 68° is not warm enough for any healthy person it is because the humidity is too low, and Hygrometer The Zuni Indians in New Mexico said: "When the locks of the Navajos grow damp in the scalp house, surely it will rain." HUMIDITY 97 water should be evaporated to bring the moisture up to the right degree. In other words, water instead of coal should be used to make rooms com- fortable when the temperature has reached 68°. Humidity causes the temperature, as shown by the ther- mometer, to vary as much as 45° from the temperature as felt by your body. If it were not for the moisture in the air it would be too cold to live in. The reason^ for this is that if the air is dry the heat goes through it without warming it. If the air is moist, it stops, the radiated heat and warms it, so that humidity becomes Nature's great bed blanket. If the air lacks moisture, it lacks its clothing quality, so that we are obliged to heat our living rooms warmer, in order to feel comfortable. The dry air allows too Ordinary Grade, Tube Not Insulated If every particle of moisture in the air were precipitated it would cover the entire globe. to a depth of less than four inches. o8 WEATHER much radiation from the body and too rapid evapora- tion makes us feel cold. WHY WIND COOLS. The cooling effect produced by a wind or draught does not necessarily arise from the wind being cooler, for it may, as shown by the thermometer, be actually warmer, but arises from the rapid evaporation it causes from the surface of the skin. Authorities agree that if we were to stop having our "climate" indoors (in winter), the dryest climate known, and kept it at a humidity of 65, we would be comfortable at 65 to 68 temperature, save money and avoid sickness. Certainly the subject deserves con- sideration. Water-vapor in some shape forms, as it were, a blanket for the earth and saves it from being burned up and frozen alternately. HUMIDITY TERMS. The three terms used in referring to the moisture in the atmosphere are: Absolute humidity, relative humidity and dew point. The amount of water-vapor in the air (when ex- pressed in the number of grains per cubic foot of air) is called the absolute humidity; when expressed in the form of a percentage, it is called relative humidity. The relative humidity depends chiefly on the tem- perature of the air. If we make moist air colder, we shall increase its relative humidity without increas- ing its absolute humidity. If it is cooled sufficiently, its relative humidity will become 100 per cent., which is saturation. Sounds travel far when the humidity is high. THE HYGROMETER 99 The dew point is that temperature of the air at which its invisible moisture begins to condense into visible water drops. THE HYGROMETER. The Hygrometer is an instrument devised to determine the percent- age of moisture in the air. It consists of two thermometers, the bulb of one exposed to the air while the bulb of the other is constantly wet, being covered with silk cord or wick immersed in water. As evapora- tion causes a loss of heat, the wet bulb ther- mometer will read lower than the dry, providing there is any degree of dryness in the air. The more rapid the evaporation, the greater the cooling; hence the greater the difference between the readings of the two thermometers. If the air is fully satur- ated, both thermometers will read alike, as there can then be no evapora- tion. Better Grade, with Insulated Tube The region of least relative humidity is Southwest Arizona, where it averages but 40 per cent., as against 60 to 80 per cent, in other sections. The Hygrodeik The Hygrodeik is an improved form of hygrom- eter, being portable, easy to read and very accurate. The wet and dry bulb thermometers are mounted on the edges of a chart plotted from new and corrected tables prepared under the direction of THE HYGRODEIK 101 the Weather Bureau. At the top of the chart is a swinging index, to which is fitted a sliding pointer. HOW TO READ. All that is necessary to take a reading is to swing the index to the wet bulb side of the chart and slide the pointer either up or down the index arm until it points to the same degree of temperature on the chart that the wet bulb does on its tube. Swing the arm towards the dry bulb and note where the pointer intersects the line, curving downwards from the reading of the dry bulb thermometer. At this intersection the index hand will point to the relative humidity on the scale at the bottom of the chart. Example : Should the temperature of the wet bulb be 6o° and the dry bulb 70 , the hand will indicate a relative humidity of 55° when the pointer rests on the intersecting lines of 60° and 70°. After the first observation, it is very easy to find the "dew point." Observe the intersection as above and follow the curved line (passing through it, which runs from the "top downward to the right) to the point of contact with the dry bulb scale. The degree (53) at this point on that scale is the dew point required. The figure at the upper end of this line will give the Absolute Humidity cr the amount of water in grains (4.5 grains) per cubic foot of air. THE "SLING" PSYCHROMETER OR HYGROMETER. The Sling Psychrometer was designed for the pur- pose of obtaining quick and more accurate results than are possible with the stationary wet and dry bulb instruments. The original design has been improved upon by doing away with the link connection between About one-half of the entire quantity of moisture in the air is contained in the first seven thousand feet from the earth. 102 WEATHER the" thermometer back and the handle. The improved form les- sens the liability to breakage in swinging and enables the user to more quickly obtain the readings than is possible on the less rigid, link handle form. EVAPORATION. Water, when evaporated, becomes a vapor, which is transferred by the wind to regions where there is less vapor. The rapidity of evaporation de- pends on whether (i) it is free water surface, wet ground or vegetable growth, (2) the temperature, (3) the relative amount of water already in the air, (4) the motion of the air and (5) the atmospheric pressure. Increase of temperature accelerates the evaporation by increasing the pressure of the vapor. If air were freed from moisture, evaporation would reach its max- imum. If air is saturated, there would be no evaporation. Circulation increases evaporation, as, if the atmos- phere were not renewed, the air surrounding the liquid would soon become saturated when evaporation would cease. The amount of evaporation from plants (transpira- tion) is enormous, being five times as much as from water and twelve times as much as from ordinary land. The relative humidity within a forest exceeds that of the open by 2 to 4 per cent. Clouds "So foul a sky clears not without a storm." — Shakespeare. T is a trite saying that "clouds are the storm signals of the sky." Even the ama- teur, by watching the clouds scudding or drifting miles above, can often make a pretty sure guess of coming weather. FORMATION OF CLOUDS. When aqueous vapor (rising from a vessel of boiling water) diffuses in the colder air, it condenses, forming a sort of a cloud. Clouds form, first, through the direct cooling of the moist air by contact with colder bodies or through loss of the heat by radiation. Second, when ascending air currents are present and the moist air thus ex- pands (due to diminishing pressure) and cools. Third, the formation of clouds by mixture of air of different temperatures and humidities. If a current of water-saturated air meets a current of cold air also saturated, the air acquires the mean temperature of the two, but can retain only a portion of the vapor in invisible form, so that a cloud or mist is formed. HOW FOGS DIFFER FROM CLOUDS. Some clouds differ from fogs only in their eleva- tion from the earth. A fog, resting on the top of a As clouds contain more dust than the surrounding air, it is thought that the dust within them is drawn up from the earth's surface. io 4 WEATHER mountain, is called a cloud. A cloud, resting on the surface of the earth, is called a fog. When clouds form over a region in which the air is nearly saturated, the globules of water (forming the clouds) unite and descend through the moist air under- neath, falling as rain (if above 32°). Precipitation occurs when moist air is cooled be- low the dew point. This may take the form of rain, snow, hail, dew or frost. The amount of precipitation in the course of a year averages greatest at a distance of a few degrees from the equator, decreasing slightly towards the poles. The sun sets weeping in the lowly west, Witnessing storms to come, woe, and unrest. Since the colors and duration of twilight, espe- cially at evening, depends upon the amount of con- densed vapor which the atmosphere contains, these appearances should afford some indications of the weather which may be expected. The following are some of the rules which are re- lied upon by seamen : When after sunset the western sky is of a whitish yellow, and this tint extends a great height, it is prob- able that it will rain during the night or next day. Gaudy or unusual hues, with hard, definitely outlined clouds, foretell rain and probable wind. HOW THE SUN FORETELLS STORMS. If the sun before setting appears diffuse and of a brilliant white, it foretells storm. If it sets in a sky The effect of clouds is to prevent the minimum temperature from becom- ing as low as it would under a clear sky, because the radiation of heat from the earth is hindered. CLOUDS 105 slightly purple, the atmosphere near the zenith being of a bright blue, we may rely upon fine weather. Above the rest, the sun who never lies, Foretells the change of weather in the skies; For if he rise unwilling to his race, Clouds on his brow and spots upon his face, Or if through mists he shoots his sullen beams, Frugal of light in loose and straggling streams, Suspect a drizzling day and southern rain, Fatal to fruits, and flocks, and promised grain. Nearly every class of clouds attains a loftier alti- tude between the hours of 4 and 8 p.m. than at any other part of the day, whereas between noon and 4 o'clock they fall a trifle below the average. In velocity, conspicuous variations are attributable to a change of season. Stratus, for instance, floats along at a mean rate of thirteen miles an hour in warm weather, but accelerates its speed to twenty-four miles an hour in cold. The tops of Cumuli travel thirty-four miles an hour in summer and forty -seven miles an hour in winter. HIGH SPEED OF CLOUDS. The average for Cirrus in the former season is sixty-seven miles and in the latter seventy-eight. But in March, 1897, the maximum velocity observed was 187 miles, while in the previous December Cirrus was seen moving at a rate of over 200 miles an hour! Nice weather to get caught in with a flying machine! Prof. Bigelow says it looks as though the greatest speeds were realized at an elevation of seven or seven and a half miles, and from that level up to ten miles there was a slight falling off. But further observation is required to verify that inference. Ordinarily the height of clouds varies from 1300-1500 yards in winter to .3300-4300 yards in summer. io6 WEATHER CLOUDS. Soft looking or delicate clouds foretell fine weather, with moderate or high breezes. Hard-edged clouds, wind. A dark, gloomy, blue sky, windy but light. A bright blue sky indicates fine weather. A bright yellow sky at sunset presages wind; pale yellow, wet. By the prevalence and kind of red or yellow, or other tints, the coming weather may be foretold. Generally the softer look, the less wind (perhaps more rain) may be expected, and the harder, more "greasy," rolled, tufted, or ragged, the stronger the coming wind will prove. Small, inky -looking clouds foretell rain. Light scud clouds driving across heavy masses show wind and rain, but if alone, may indicate wind only. High upper clouds crossing the sun, moon or stars, in a direction different from that of the lower clouds, or the wind field below, foretell a change of wind toward that direction. THE SKY. Whether clear or cloudy, a rosy sky at sunset pre- sages fine weather. A sickly -looking, greenish hue, wind and rain. A dark (or Indian) red, rain. A red sky in the morning, considerable wind or rain. Clouds have been observed within 330 yards of the ground. CLOUDS FORETELL 107 A gray sky in the morning, fine weather. A high dawn, look out for wind. A "high dawn" is when the first indications of daylight are seen above a bank of clouds. A "low dawn" is when the day breaks on or near the horizon, the first streaks of light being very low down. PROGNOSTICATIONS. After fine weather, the first signs in the sky of a coming change are usually light streaks, curls, wisps, or mottled patches of white, distant clouds, which in- crease and are followed by an overcasting of murky vapor that grows into cloudiness. This appearance more or less watery, as wind or rain will prevail, is an infallible sign. Usually the higher and more distant such clouds seem to be the more gradual but general the coming change of weather will prove. Light, delicate, quiet tints of color, with soft, un- defined form of clouds, indicate and accompany fine weather, but unusual or gaudy hues, with hard, defi- nitely outlined clouds, foretell rain, and probably strong wind. Misty clouds, forming or hanging on heights, show wind and rain coming, if they remain, increase or de- scend, if they rise or disperse, the weather will improve or become fine. Dew is an indication of fine weather; so is fog; neither of these two formations occur under an over- cast sky, or when there is much wind; one sees fog Rainfall diminishes with the height of a station above sea level at a rate of 3 or 4 per cent, for each ioo feet increase of altitude. io3 WEATHER occasionally rolled away, as it were, by wind, but sel- dom or never formed while it is blowing. Remarkable clearness of atmosphere near the hor- izon, distant objects, such as hills, unusually visible or raised (by refraction) and what is called "a good hearing day" may be mentioned among signs of wet if not wind, to be expected. Much refraction is a sign of easterly wind, veering southward. Rainfall decreases both in quantity and frequency as the distance increases from the sea. Fogs and Their Cause SSI ^3g:*gJ* HE vapor in the atmosphere is quite T transparent, but when, from any cause, the air becomes cooled below the dew point, a portion of its vapor is precipi- tated in the form of drops of water (ex- tremely minute), which affects the transparency of the air, forming a fog if near the surface of the earth, or a cloud if in the upper regions of the at- mosphere. The chief cause of fogs is that the moist soil is at a higher temperature than the air. The vapor which ascends reaches its point of saturation, con- denses, becoming visible. Fogs are also caused when a current of hot and moist air passes over a river at a lower temperature. The diameter of the smallest visible particles of fog has been estimated at i-i8oth inch. When the diam- eter of the particles becomes equal to i-8oth inch, they fall with an appreciable velocity and are called rain- drops. On the Atlantic Ocean, from 30° to 35° north lati- tude, fogs are almost unknown. The Gulf Stream is caused by the warm waters of the tropics being con- tinually pushed by the trade winds into the Gulf of Mexico. Seeking an out- let, they p3ur eastward through the Florida Straits (forming a stream 32 miles wide) which ends at the Great Bahama Bank. Here it spreads to 50 miles, continuing as far as the capes of Chesapeake. It then spreads (like a fan) over the North Atlantic. "To suppose it could possibly affect the climate of the North Atlantic Coast is an obvious absurdity." no WEATHER Fogs never form when the air is very dry, and therefore are never known in deserts. On the northern side of the Gulf Stream they are of common occurrence, but most prevalent in summer, when the "Banks" are enveloped in fog nearly half the time. DRY FOG. The vapor which causes these fogs is furnished by the warm air of the Gulf Stream being condensed by the cold air of the banks, the contrast of temperature being most sudden. During the month of July the water on the banks frequently has a temperature of 45° P., while within a distance of less than 300 miles the Gulf Stream has a temperature of 78° F. Franklin ascribed the dry fog met with in London to the large quantities of coal tar and paraffin vapor sent into the atmosphere, which condense on the par- ticles of fog, preventing their evaporation. The particles of fog are sustained in the air in the same manner as a cloud of dust. A cloud of dust remains for a long time suspended in the air, although each particle may consist of matter 2,000 times as dense as the air in which it floats. When the air is perfectly tranquil these particles do indeed fall, but they descend so slowly that their motion is only per- ceptible after the lapse of a considerable interval of time. HAIL. When raindrops become frozen in their passage through the air they fall as hail. They may be frozen on their downward passage, but it is generally be- lieved that they are frozen by first being carried (by vertical air currents) upwards where the temperature The size of hail varies from i-ioth of an inch or less to more than four inches in diameter. DUST STORMS in of the air is below freezing, and that they have not sufficient time to melt before reaching the earth. The following is an extract from the "Memoirs of Benvenuto Cellini of a terrible hailstorm in Lyon in 1544. He writes of the storm "The hail at length rose to the size of lemons . . at about half a mile's distance all the trees were broken down, and all the cattle were deprived of life; we likewise found a great many shepherds killed; and we saw hailstones which a man would have found it a difficult matter to have grasped in both hands." SLEET. Sleet is solidified water consisting of small icy needles pressed together. Its formation is ascribed to the sudden congelation of the minute globules of the clouds in an agitated atmosphere. DUST STORMS AND RED RAIN. A dust-fall on a large scale occurred in May and August, 1883, when an enormous quantity of dust was hurled into the air during the Krakatoa eruption, being collected at various distances, the greatest be- ing more than 1,100 miles from the seat of the dis- turbance. The tremendous height to which the finer particles of dust were thrown, coupled with the move- ment of the air and this great distance from the earth's surface, were responsible for the magnificent colored sunsets which were observed nearly all over the world. On March 10th, 1901 (accompanying a depression traveling from Algeria to Pomerania), there occurred The maximum amount of rainfall a day is sometimes enormous. On one occasion in Japan 20.5 inches of rain fell in 24 hours, and in India 39.5 inches fell in 24 hours. This is as much as would fall in a favorably situated region in a cold temperature climate in a year. ii2 WEATHER a sirocco with red dust in the morning in Sicily, in the afternoon in .southern Italy; on March nth, there fell red and yellow dust generally with snow north- ward in Brandenburg and Pomerania. ON MARCH 20TH, PROF. RUCKER SAYS: "At 7:30 this morning the sky was copper colored, and it was evident that another fall of dust was taking place. The sirocco had been blowing for two days and it was raining slightly. The sky ceased to be cop- per colored about 8or 8:15 a.m." Under these circumstances he measured the dust that accumulated on various flat surfaces during the hour. The average, 0.00135, or about five and one- half tons per square mile, gives a fair idea of the density of the dust in the region of Taormina. As regards the total amount of dust that fell to the surface, rough estimates indicated that the weight of it would amount to about 1,800,000 tons, two- thirds of which were deposited to the south of the Alps. The dust was examined by Prof. Perhanz, both microscopically and chemically, and was found to be perfectly similar to the sands of the Desert of Sahara. The facts collected have led the investigators to form a very concrete survey of the whole phenomenon, tracing the origin of the dust to dust-storms that oc- curred on March 8th, 9th and 10th in the desert El Erg, situated in the southern part of Algeria, which carried the dust and transported it northward. This dust began to fall at Algiers and Tunis in the dry state on the night of the oth. The subsequent falls gradually took place northward, first Sicily, then Italy, the Alps, Austro-Hungary, Germany, Denmark and European Russia, practically in the order named, coming in for their share. In Sicily and Italy the dust DUST STORMS 113 was noticed to have fallen even without the aid of rain, but in the other countries it was only detected during and after showers. Not only did the dust-fall occur in these countries in the sequence mentioned, but the quantity that fell became gradually less the more north the places were situated, and the fineness of the dust, as shown by the analyses, increased at the same time. In January or February, 1890, the steamship "Queensmore," arriving at Baltimore from England, reported red rain and red dust off the coast of New- foundland. It would be very remarkable if this was Sahara dust. Formation of Snow, Dew and Frost INUTE ice crystals form when condensation takes place at a temperature below the freezing point. Snow flakes are produced by the union of these crystals. While the formation of snow flakes in the upper air necessitates freezing, they frequently reach the earth when the temperature of the lower air is considerably above the freez- ing point (3 2 F.). This is because they fall rapidly without melting to any extent. DEW. When the temperature of the earth's surface falls below the dew point of the air, the latter deposits' on the cooled surface part of its vapor in the form of small water drops, which we know as "dew drops." On account of the rapid cooling (by radiation) , especially on clear nights, the temperature of the ground and other solid substances becomes colder than that of the air above, and the "dew point" or even "frost point" are reached by the ground and the ad- jacent layer of air, while the temperature of the air (at a height of a few feet from the ground) is sev- eral degrees warmer. FROST. As before stated, the atmosphere of the earth al- ways contains more or less moisture in an invisible Frost suddenly following heavy rain seldom lasts. WHAT FROST IS 115 form. When at a considerable elevation above the earth, this moisture is condensed and clouds are formed; when the process of condensation is more active and the temperature of the air is above freez- ing, rain falls; and when the temperature of the air is below freezing, snow is produced. When the moisture of the air in immediate con- tact with the earth is condensed at temperatures above freezing, dew is formed; when at temperatures below freezing, frost is deposited. WHAT FROST IS. Frost is, therefore, the moisture of the air con- densed at freezing temperatures (32° F.) upon plants and other objects near the surface of the earth. In the process of frost formation, the temperature of the air a few feet above the earth is commonly sev- eral degrees above freezing. The surfaces upon which frost is deposited must, however, possess freezing temperatures. The manner in which frost is deposited on plants and other objects is very similar to that observed when the air moisture of a room is frozen and deposited upon window glass, the temperature of which has been reduced to freezing by the out-of-doors cold. WHEN TO EXPECT FROST. With other atmospheric conditions favorable, frost may be expected when temperature, as reported by the Weather Bureau, falls to a point 8° to io° above the freezing point. While the surfaces upon which frost is deposited must possess freezing temper- ature, the temperature of the air a few feet above the When the frost gets into the air (air becomes dull) it will rain. When the temperature is at 32° F., rain and hail often fall together. u6 WEATHER surfaces may be several degrees above freezing; and it is the temperature of the air, in some instances many feet above the ground, that is given by the Weather Bureau observations. CLEAR. Another atmospheric condition favorable for the occurrence of frost is a clear, cloudless and compara- tively calm night. The presence of cloud retards radiation or loss of heat from plants; the cloud acts as a screen in preventing the heat collected from the sun's rays during the day from escaping into the upper air. When clouds are not present, and a withdrawal of the sun's rays causes a rapid cooling of the air at moderate elevations, the warmer air which collects near the surface of the earth during the day rises, and the cooler upper air, owing to its greater density or weight, settles to the earth. CALM. Calm or comparatively still air is a condition which favors the formation of frost. On windy nights the air is disturbed and is not permitted to arrange itself in layers according to its density, with the densest and coldest air near the surface of the earth, but is kept mixed by the wind. RAINBOWS. Rainbows are produced by the refraction of the sun's rays by means of the rain drops in the air. The center of the bow is opposite the sun. Rainbows are most frequent in local showers in which the sun sud- denly breaks through the clouds at the edge. Rainbow in morning shows that shower is west of us and that we will probably get it. Rainbow in the evening shows that shower is east of us and is passing off. Lightning and Thunder HE atmosphere contains free electricity which is always positive in clear, but sometimes negative in cloudy weather. "The occurrence of negative electricity is a certain indication that within a distance of forty miles, it either rains, snows or hails." — Palmieri. The electricity of the air is carried by the vapor particles. If 1,000 par- ticles unite to make a droplet, the quan- tity of electricity it contains will be 1,000 times as great as in the small one; therefore, the potential will be 100 times as great. Instead of 1,000 vapor particles uniting to make a droplet, it usually takes billions. The spark (lightning) shooting from electrically charged clouds sometimes extends four to five miles in length. The duration of flash varies from 1 -300th of a second to a second. The reason lightning passes through the air in an irregular direction is probably due to the resistance of the air by the passage of a strong discharge, the spark taking the direction of least resistance. The ground (by the induction of the electricity from the cloud) becomes charged with contrary elec- tricity. When the tendency of the two electricities to In a vacuum, electricity passes in a straight line. n8 WEATHER combine exceeds the resistance of the air, the electric discharge strikes between a thunder cloud and the ground. Men and animals (like the ground) are sometimes charged with the opposite electricity to that of the thunder cloud. When the lightning is discharged, (even at a distance) the bodies revert rapidly from the electric to the natural state, causing a concussion (called a return shock) which has often proved fatal. Lightning usually strikes the tallest and best con- ducting objects. After the passage of lightning a peculiar odor is frequently produced, due to the forma- tion of ozone. HEIGHT OF THUNDER CLOUDS. Thunder clouds are sometimes limited to a height of 3-2Sths of a mile from the earth and sometimes they rise to a height of over a mile. Observers on the summits of hills less than a quar- ter of a mile in height report having seen thunder showers below them, while they were enjoying a cloudless sky. On the other hand, on the Cordilleras Mountains, a violent thunderstorm was experienced at the peak, which has an altitude of 15,970 feet. DIFFERENT FORMS OF LIGHTNING. Streak Lightning — A plain, broad, smooth streak or flash of lightning. Sinuous Lightning — A flash following some gen- eral direction, but the line is sinuous, bending from side to side. This is the most common type. If lightning is at a distance of 15 miles, thunder will not be heard. THUNDER 119 Ramified Lightning — Part of the flash appears to branch off from the main stem like the branches of a tree from the trunk; but whether these branches issue from the trunk or unite with it, is unknown. Meandering Lightning — Flash appears to wander without any definite course and forms irregular loops. Beaded Lightning — A series of bright beads of light appear along the white streaks of lightning. Dark Flashes — These have been photographed, but, as well as the others, are not really understood. Heat lightning is ascribed to distant lightning flashes, which are below the horizon, but illuminate the higher strata of clouds, so that their brightness is visible at great distances. They produce no sound, probably in consequence of the fact that they are so far off that the rolling of thunder cannot reach the ear of the observer. THUNDER. The occurrence of lightning and thunder is prac- tically simultaneous, but an interval of several seconds elapses before the thunder is heard. Sounds travel 1,120 feet a second, so that it is easy to calculate the distance of a storm by counting the seconds and multi- plying by 1,120. To reduce to miles divide by 5,280. (Allow one mile for every five seconds for approxi- mate estimate). WHY IT THUNDERS. The noise of the thunder arises in some such man- ner as the crack of a whip or the report of a cannon. The lightning compresses the surrounding air. This A summer thunderstorm which does not much depress the barometer will be' very local and of slight consequence. i2o WEATHER compressed air rushes in to fill the partial vacuum forming in turn a partial vacuum, making the wave motion, which produces the sound. Suspend an electric bell from inside the glass dome of a vacuum apparatus. You will hear the ringing of the bell distinctly, but as the air is gradually exhausted from the dome, the sound continues to diminish until it ceases when there is no air left to vibrate. If a gun were discharged in a perfect vacuum, no report would be heard. Thunder "rolls" because lightning is a series of discharges, each of which gives rise to a particular sound. Also because of the reflection from the ground, from clouds and from layers of air of different densities. FREQUENCY OF THUNDER STORMS. Thunderstorms occur oftenest in the summer months, more frequently in the afternoon. They are preceded by a decrease in air pressure and relative humidity and an increase in temperature. When storms burst, the pressure and humidity increase rap- idly and the thermometer falls. At the end of the storm the pressure and humidity is at the maximum while the temperature is at its minimum. WHEN THUNDER IS HEARD. The first thunder in a storm is heard before the thunder cloud reaches the zenith (the point directly overhead), the first rains commencing after it. The interval between the rain and thunder varies from a few minutes to about half an hour. About five min- utes after the rain begins there comes from the west When the sun in the morning is breaking through the clouds and scorch- ing, a thunderstorm follows in the afternoon. TORNADOES 121 or northwest a brisk wind which suddenly increases in violence, becoming a "squall." This wind dies down after the rain has begun. The heaviest rain in the storm varies; at times it occurs at the beginning and sometimes during the latter part of the storm. Some few minutes after the rainstorm has begun the lightning occurs, when the thunder is invariably- loudest. There are three kinds of thunderstorms: heat, winter and cyclonic. Heat storms are due to the unstable condition of the lower air, due to local heating. They need for their development a moist, quiet air, warmed by the sun's rays. CYCLONIC STORMS. Cyclonic storms are the result of well developed areas of low atmospheric pressure. They become in some extreme cases, tornadoes. TORNADOES. Tornadoes are caused by local differences in tem- perature, the air having become abnormally heated over a central area, causing a difference in pressure between the air of the inner region and that which surrounds it. From this, a flow of air rises spirally, increasing in velocity* as it approaches the center. This velocity varies from 7 to 100 miles an hour, 44 miles being con- sidered an average. They usually come from the southwest, moving in the direction of the northeast. Winter storms occur most frequently at night, especially in high latitudes, being more frequent near the coast. 122 WEATHER WHY TORNADOES ARE DESTRUCTIVE. The normal air pressure is about 14.7 pounds a square inch and if the pressure is reduced one-quarter of this amount in the center of a tornado it is lessened about 3.7 pounds per square inch, or 533 pounds a square foot. Therefore, if a tornado passes over a building (where the normal pressure on the inner and outer walls is an average of 2,117 pounds per square foot) the pressure on the outside of the walls is suddenly reduced to about 533 pounds per square foot the re- sult will be in the nature of an explosion, as the outer wall cannot support the pressure from the inside. Rainfall PITCHER of ice water on a hot sum- Amer day is not a bad sort of hygrom- eter. The pitcher is naturally cooler than the surrounding air, and conse- quently some of the water-vapor in the air is condensed and collects on the out- side of the pitcher. It will be remem- bered that water-vapor changes to the liquid state when the air is cooled be- low a certain point. The principle illus- trated by the pitcher of ice water is re- peated on a grand scale in nature every time rain or snow falls. The capacity of air to retain mois- ture, or the quantity of moisture which a given volume of air will hold, increases with in- creased temperature until saturation is obtained. It follows that with a reduction of temperature (from whatever cause) precipitation must take place from the inability of the air to sustain the amount of aque- ous vapor it has absorbed, the result being rain. Rain, briefly speaking, is caused by the chilling of the air which contains a certain amount of moisture. This chilling may take place either through the rise of the air into higher and colder levels, through its contact with a colder surface; or from its meeting a colder current of air. * Little globules form and fall by gravitation, form- ing into larger drops as one united with another. This Among the Chassia Hills, in India, the average rainfall is over 470 inches. i2 4 WEATHER increase of weight causes them to descend more rap- idly, overtaking other drops. The greater the height of clouds, the larger the rain drops will be when they reach the earth. Rain is often caused by the rushing of air from a low-land up over a mountain; some of the heaviest rainfalls take place on mountains near the sea. The air over the ocean gets thoroughly soaked with vapor, which while warm it can carry. Then it suddenly comes up against a mountain range and has to pour upwards, losing heat as it does so; becoming fast colder, it can no longer contain its surplus of hidden moisture. SEA WINDS BEING RAIN. When wind which blows over water first reaches land, rain will be precipitated. In Ceylon the rainy seasons on the two sides of the island occur in different months, which depend on the time each coast is exposed to the prevailing monsoon. ("Monsoon" is derived from Arabic word for "season.") Along the Atlantic coast of the United States, rain occurs most frequently with the wind from the north- east. Throughout most of the interior of the United States, the principal part of the rain comes with a westerly wind. In central Europe, about three-fourths of all the rain occurs with a westerly wind. In England, if a mountain under about 1,500 feet obstructs the prevailing westerly wind, the greatest amount of rain will fall on the east side of the range, the condensed vapof being blown over the top of the hill. If the range is higher, the rain clouds cannot blow over, and the rain falls on the west side. In some regions of India the total yearly rainfall is but 4 inches. CLOUDBURSTS 125 CLOUDBURSTS. Cloudbursts are sudden and excessive downpours of rain, which have been kept from falling by the as- cending air current until a large amount of water has been accumulated. TEMPERATURE AND HUMIDITY INDICATE RAIN. There is an increase in temperature and humidity of the air before rain. It does not follow, however, that every increase in humidity at the earth's surface indicates rain. In the coast districts, an increase in humidity may result from the wind shifting to blow temporarily from over the water, and a temporary increase is sometimes due to fog. Ignoring purely local and temporary conditions, it may be assumed that, as a rale, general rains are pre- ceded twelve to twenty-four hours by an increase in atmospheric moisture. The rain winds of the United States are from the oceans and the Gulf. RAIN GAUGES (PLUVIOMETERS). The amount of rainfall is measured on the basis of the depth of water which would accumulate on a level surface if all of it remained as it fell without loss by evaporation or otherwise. Snow, hail, etc., are measured both on the basis of the actual depth of the precipitation, and (more accurately) by melting the snow or hail, obtaining the equivalent depth in water. There are many gauges other than those illustrated, the most important being the "stick" gauge, which accurately measures (by means of a divided stick or rule) the depth of water in the "receiver." A gallon of rain weighs ten pounds, and if spread out in a layer one inch thick will cover an area of two If the sun sets in dark,' heavy clouds, expect rain next day. 126 WEATHER square feet. An inch of rainfall gives ioo tons of water to the acre, or 60,000 tons a square mile, yet in Khase Hills in Bengal, India, the rainfall exceeds 600 inches yearly — the greatest in the world. ELECTRICAL RAIN GAUGES. The "Electrical Tipping Bucket" rain gauge has a small bucket below the funnel, which "tips" after having received i-iooth of an inch of rain. The amount of rainfall is measured by the number of "tips," which is electrically recorded any reasonable distance away. Then there is the "Weighing Gauge," an instru- ment devised for weighing the amount of rain or snow fall. This is probably one of the most accurate styles, as no loss occurs from evaporation or melting if snow is measured. TO MEASURE SNOW. To measure the fall of snow, select a place where the snow has not drifted, invert the funnel of the gauge, pressing it through the snow to the ground, then give the funnel a sharp turn and it will lift up the snow in its circumference. It is necessary to reduce snow to a liquid condition for accurate measurement, the simplest method being to add a known volume of water, sufficient to reduce it to a state of "slush." The graduated measuring glass should be held so that the surface of the water is level to obtain a true reading. REGISTERING GAUGE 127 Fig. ! ZERO SETTING REGISTERING GAUGE Fig. i shows the type of rain gauge known as the "Zero Setting Registering Gauge." It is made on the tilting bucket principle, the rain falling through the opening in the top (8"x8"), passing through a small pipe and falling into the tilting bucket. ' When a given amount of rain has fallen (0.01") the ' weight of this rain causes the bucket to tilt over on the laden side, discharging the rain into a receiver. The tilting of the bucket operates a mechanical arrangement by which the hand is made to register the amount fallen. The large outer dial registers the fall in single i-iooths of an inch, one complete revolution showing a fall of one inch. The smaller hand in the second dial There is an increase in rainfall up to an altitude of about 4,000 feet. It decreases above this point. 128 WEATHER notes the number of revolutions of the large hand, registering up to twelve inches. The illustration shows the gauge reading at 13-iooths of an inch. Fig. z THE HOWARD RAIN GAUGE Fig. 2, known as the "Howard Rain Gauge," con- sists of a 5" metal funnel fitted into a glass bottle to receive the rain. When a reading is to be taken, the rain is poured from the bottle into a graduated glass jar which is divided in 0.01 inch graduations. During a rain on the sea there falls on the surface a coating of freshwater which does not immediately sink to the bottom, TYPES OF RAIN GAUGES 129 Fig. 3 THE BRITISITASSOCIATION GAUGE The "British Association Gauge" (Fig. 3) consists of a metal cylinder with a 5" funnel, which con- ducts the rain into a removable metal receiver, where it can be readily measured without disturbing the gauge. The heaviest annual rainfall of any place on the globe is on the Khasi Hills, in Bengal, where it is 600 inches, of which 500 inches falls in seven months. 130 WEATHER Fig. 4 THE GLAISHERS The " Glaishers'j" (Fig. 4) is probably the rain gauge most generally used. The chance of evapora- tion is reduced to a minimum by making, at the end of the funnel, a tube ending in a curve. In this curve is retained a certain amount of water already contained in the gauge. If this gauge is sunk in the ground to within 8 inches of the top, no evaporation will take place, even in the warmest seasons. EXPOSURE OE GAUGES. The exposure of rain gauges is a very important matter, as it is very necessary that they be placed where they will get proper exposure. Within a few When the sun draws water rain follows soon. Red skies in the morning precede fine to-morrows. EXPOSURE OF GAUGE 131 yards of each other, two rain gauges may show a dif- ference of 20% in the rainfall during a heavy rain- storm. Fig. s OFFICIAL BRITISH PATTERN RAIN-GAUGE Copper case with brass rimmed funnel 5 inches in diameter, inside bottle of white glass. Camden pattern jar. The wind is the most serious obstacle in collect- ing the true amount of precipitation. The stronger the wind, the greater the difference. In blowing against the gauge, the eddies of wind formed at the top and In the extreme northwest corner of the United States the most rain falls. 132 WEATHER about the mouth carry away rain (and especially snow) so that too little is caught. Snow is often blown out of a deep gauge after becoming lodged there. In a high location, eddies of wind (produced by walls or buildings) divert rain that would other- wise fall into the gauge. A gauge on a plot of ground with a fence three feet high around it (at a distance of three feet) will collect, roughly, 6% more rain than without the fence. These differences are entirely due to the wind currents. A gauge near the edge of a building collects less rainfall than one in the center of a roof. In the center of a fiat roof (at least 60 feet square) the rainfall col- lected does not materially differ from that collected on the ground. Rain gauges should not be exposed on roofs unless better exposure is unobtainable, when the center of a flat, unobstructed roof should be selected. A position in an open lot, unobstructed by large trees, buildings or fences is preferable. Low bushes, fences or walls in the vicinity of a gauge are, however, beneficial to break the force of the wind, but they must be at a distance of not less than the height of the object. Wind Pressure "Many can brook the weather that love not the wind." — Shakespeare. INDS are currents of air. The direc- tion of the wind is designated by the point of the horizon from which it blows. There are eight principal directions in which they blow, i. e., N., N. E., E., S. E., S., S. W., W., N. W. Mariners further divide the dis- tances between each of these eight directions into four others, mak- ing thirty-two in all, called "points" or " rhumbs. " A figure of thirty-two rhumbs on a circle in the form of a star is known as the ma- riners' card. In meteorological work, sixteen divisions are used. There would be a continual calm (air at rest) were it not for the unequal distribution of heat. There is a tendency towards a permanent interchange of air between the equatorial and polar regions due to the difference in temperature. Winds are produced as a result of a difference in temperature between adjacent countries. If the tem- The least rainfall in the United States is in the southwestern part of Arizona. i 3 4 WEATHER perature of a certain place increases, the air becomes heated and, as it expands, rises towards the higher regions. There it flows from hot to cold countries. In certain regions in the open ocean where the greatest heat and cold do not alter their relative posi- tions, the wind blows always in the same direction. At the same time the equilibrium is disturbed at the surface of the earth as the barometric pressure in the colder parts is greater than that in the warmer, which produces a current from the high to the low barometric pressure. Two distinct winds are therefore produced — the upper one from and the lower one towards the heated region. FROM GREATEST TO LEAST PRESSURE. Unequal atmospheric pressure tends to throw the air drift from the region of greatest pressure to re- gions of least pressure. The light air is driven up towards the clouds and above. It then flows over and back to fill up the space before occupied by the heavier air. SEA WIND. Along the seashore in midsummer, the wind blows from the sea to the land during the hottest part of the day. At night the direction of the wind is reversed. During the day the land retains the heat at its sur- face, while the sea diffuses it. The air on land conse- quently expands and becomes light. The heavier air over the cool sea, not being warmed and expanded to the same extent, presses with its greater weight in upon that resting over the land. After the setting of the sun, the land dissipates the heat much more rapidly than does the sea, so that The barometer falls lower for high winds than for heavy rain. KINDS OF WINDS 135 in a comparatively short time the land is cooler than the water. The air over the land thus becoming the colder, the pressure is seaward. This of course does not apply to all localities. DIFFERENT KINDS OF WINDS. In a similar manner, mountain breezes are caused by heating and cooling of the hills and valleys. Avalanche wind is the rush of air produced in front of a landslide. Volcanic wind is the outrush of air with volcanic eruptions. A "squall" is a local rush of air to restore the normal condition when disturbed by local causes. A "monsoon" blows in one direction for six months and in another for the next six months. The "simoon" is a hot wind which blows over the deserts of Asia and Africa. It is known under the name of "sirocco" when blowing over the Sahara. The reason the velocity of wind is less on land than on water is that on land the wind is continually re- tarded by obstacles and has the very great friction against the earth. THE FORCE OF WIND PER SQUARE FOOT. Force, Force, Miles. Lbs. Oz. Miles. Lbs. 3 X ! 50 13 18 iK 75 28 35 6 90 40 Periodic winds are those which blow regularly in the same direction at the same seasons and at the same hours of the day. 136 WEATHER The velocity of the wind is determined by the use of an instrument called the anemometer. Among the earliest of many different forms of wind gauges, was one in- vented by Dr. Lind. This consisted of a glass syphon tube, half filled with water, one end bent outwards at right angles. The ap- paratus is supported on a vane so that the wind blows in the open end of the tube, forcing the water higher into the opposite end of the tube, which is closed. This is graduated with the zero point at the level of the water. Lind's Anemometer ROBINSON S ANEMOMETER. The type illustrated Fig. 1 (invented by Dr. Rob- inson, of Armagh, in 1846, and known as Robinson's Anemometer) has become the standard pattern. It consists of four arms, revolving horizontally. At the end of each is fitted a hemispherical cup, three inches in diameter. These vanes are connected to the mech- anism by a' steel jrod through the central pillar of the instrument. The dials of Robinson's Anemometer register 500 miles, showing the velocity of the air. As the hands can be readily set to zero, it obviates the necessity of taking a reading before each observation. Regular winds are those which blow all the year through in a virtually constant direction. ANEMOMETER i37 A useful improvement is an attachment of a weather vane, with magnetic direction points, by means of which the direction of the wind can be ob- tained as well as the velocity. Another recent improvement (ball bearings on the central shaft) reduces the friction and wear to a mini- mum. Fig. i. Robinson's Anemometer Another style, known as the "Birams" (Fig. 2) is used in registering and regulating the velocity of currents of air in mines, tunnels and sewers, also for "When the wind shifts against the sun Trust it not, for back it will run." 138 WEATHER ventilation of public buildings, schools, etc. It records velocities up to about 3,000 feet per minute; beyond this, it is liable to derangement. This anemometer has a circular brass collar about two inches deep, in the center of which is a very deli- cately poised fan wheel extending from the center outwards. The revolutions of this vane or fan wheel are recorded on a dial in the center of the instrument. Some instruments have dials capable of taking readings to 1,000 feet, others to 10,000,000 feet, but the latter are not generally used. An excessive monthly rainfall of nearly 42 inches occurred in Northern California and one of 37 inches in Louisiana. THE "BIRAMS" 139 Fig. 3 Fig. 3 shows an "airmeter" — an instrument arranged so that the dial is at right angles to the fan wheel. It is used for the registration of currents of air (in pressure and velocity) in mines, tunnels, sewers, and the ventilators, etc., of public buildings. This form of instrument is supplied with a universal jointed socket holder, which enables the operator to hold it by means of a staff at any angle. A disconnector projecting from the band of the instrument serves to throw the mechanism out of gear, and arrest its action when required. The most recent and valuable addition to air- meters is a patent zero setting attachment, a patented plan by which all the indices can be set to zero, or the starting point, at the will of the user. 140 WEATHER In using instruments of this character, great care should be exercised, as the slightest bend in the vane or vane arm will make the reading inaccurate. The following table may be found useful in deter- mining the effect of recorded winds. Name. Miles per hour. Apparent effect. Calm No visible horizontal motion to inanimate matter. Light I to 2 Causes smoke to move from the vertical. Gentle 3 to 5 Moves leaves of trees. Fresh 6 to 14 Moves small branches of trees and blows up dust. Brisk 15 to 24 Good sailing breeze, makes whitecaps. High 25 to 39 Sways trees and breaks small branches. Gale 40 to 59 Dangerous for sailing vessels. Storm 60 to 79 Prostrates exposed trees and frail houses. Hurricane 80 upwards Prostrates everything. Feet Miles Pressure in lbs. Feet Miles Pressure in lbs. per minute per hour per square foot per minute per hour per square foot 20 .227 .0002 1,500 17.405 1.4375 30 .340 .0006 2,000 22 . 727 2.5553 40 .454 .0010 2,500 28 407 3.9918 50 .568 .0016 3,000 34.090 5.7500 60 .681 .0023 3,500 39 . 772 7.8265 70 .795 .0031 4,000 45.454 10.2202 SO .909 .0041 4,500 51.131 12.9375 90 1.022 .0051 5,000 56.818 15.9709 100 1.136 .0063 5,500 62.499 19.2982 150 1.704 .0143 6,000 68.181 22.9954 200 2.272 .0255 6,500 73.861 26.9764 300 3.409 .0576 7,000 79.545 31.3020 400 4.545 .1021 7,600 85.225 35.9375 500 5.681 .1596 8,000 90.909 40.8868 600 6.818 .2300 8,600 96.589 46.1554 700 7.954 .3125 9,000 102.272 51.7500 800 9.090 .4087 9,500 107.952 57.7447 900 10 227 .5175 10,000 113.636 63.8837 1,000 11 363 .6384 An anemogram is the record as taken by a recording anemometer. VELOCITY OF AIR 141 VELOCITY OF AIR. To determine the velocity of air in an opening, it is necessary to take a number of readings in different positions. Add the readings and divide by the num- ber taken. The result will be an average velocity in lineal feet. Allowances must then be made for fric- tional and momentum errors, as provided for on cor- rection chart. Multiply the number of feet recorded by the area of the opening to obtain the number of cubic feet of air passing through the opening per unit of measured time. As an example, suppose an average of 100 feet of air is registered in one minute in an opening 6X3. Then — yi square feet x 100/1 = 100/8, or 12J4 cubic feet per minute. This reading having been taken at 100 feet per minute, to find the velocity of air in the passage we proceed as follows: 100 divided by 88 equals 1.136 miles per hour, as being i/6oth of a mile. FORCE OF AIR. To ascertain the force of the air current, multiply the square of velocity of the air in feet per second by .0023. Compasses EOGRAPHICAL meridian of a place is the imaginary plane passing through this place and through the two terres- trial poles, and the meridian is the out- line of this plane upon the surface of the globe. The magnetic meridian is the verti- cal plane passing at this place through the two poles of a compass needle. The magnetic meridian does not co- incide with the geographical meridian and the angle which exists between these two meridians is called declination, or variation of the magnetic needle. In certain parts of the earth the two meridians co- incide. This "line of no variation" is called the "Agnoic line." Such a line cuts the east of South America near Cape Hatteras, and traverses Hudson's Bay. Thence it passes through the arctic regions, en- tering the Old World east of the White Sea, traverses the Caspian, cuts the east of Arabia, turns then towards Australia, and passes across the Atlantic circle to complete the circuit. There are places where the declination of the com- pass changes most rapidly. The most remarkable of these is the coast of Newfoundland, the Gulf of St. Natural iron magnets are exceedingly rare, but a large quantity of mag- netic iron is found in Sweden and the states of New York and New Jersey. VARIATION OF COMPASS 143 Lawrence, the seaboard of North America and the English Channel and its approaches. The magnetism of the earth is subject (within cer- tain limitations) to almost continual changes, both in direction and intensity. The magnetic needle is hardly ever absolutely stationary, but exhibits almost contin- ually very minute variations. TRUE NORTH AND SOUTH. The earth being a magnet, a free needle at any place should assume a definite direction, but it does not follow that this direction must be true north and south, as the magnetic poles of the earth do not natur- ally coincide with the geographical poles. If a compass be at a place in the same meridian with the two poles, the needle will point to true north. But if the magnetic pole lie either west or east of the meridian of the given place, the north end of the needle will deviate either east or west of the true north, and the declination (or variation of the needle) will thus be shown in degrees. CHANGE IN COMPASS VARIATION. In the region between San Francisco and Honolulu recent charts gave systematically too small a value of easterly variation (magnetic declination), so that the compass actually pointed 1° to 2° farther east than shown by the charts used in directing the course of a vessel between these ports. Since the distance is about 2,000 miles, and assuming an average systematic error of but 1°, it might transpire during a cloudy or foggy passage, when no sun or stars would be visible and The end of the needle pointing south contains northern magnetism be- cause (according to the law of magnetism) like poles repel, while unlike poles attract. i44 WEATHER sole dependence would have to be put upon the com- pass and the log, that the vessel at the end of her 2,000 mile voyage would find herself too far north by about i-6oth of the distance traveled (roughly, 35 miles). CHANGES IN DECLINATION OF COMPASS. Illustrations of the difference in magnetic varia- tions are well shown by the following: In London, in 1576, the declination was 11°, 15' east of true north. Eighty years later, it pointed due north, and in 1760, there is a record of it pointing 19°, 13' west of north. The westerly declination attained its maximum about 1819, when its reading was 24 , 40'. Since then the needle has been traveling slightly eastward, the pres- ent annual rate of decrease being more than 8'. In 1904, it was 16 , 15' THE DIP OF THE COMPASS. If we imagine the earth as a huge round magnet (with the north and south poles opposite one another) and hold a magnetic needle which is accurately bal- anced (at the equator of that sphere) , it will not only point north and south but assume a perfectly horizon- tal position. If it is moved nearer to the north end of the sphere, that end of the needle will dip, and the same thing takes place if it is held towards the south. At either pole it would point to the earth (at an angle of 90 P.). This is called the inclination or dip of the compass. Robert Normas is credited with the discovery of the dip of the compass as far back as 1576. Between 5 and 7 130 a. m. the positive electricity is at its minimum. It reaches its first maximum about 9:30 a. m., when it again decreases — from 2 130 to 4 :3c It increases, reaching its second maximum from 6 130 to 9:30. COMPASSES WHERE MAGNETISM IS GREATEST. 145 By counting the vibrations of a delicate dipping needle, it will be found that the strength of the earth's magnetism increases- as we go from the equator towards either of the poles. COMPASSES. A compass is probably best described as a mag- netized needle pivoted upon its center to swing freely Fig. 1 Lightning often reverses the poles of compass needles. 1 46 WEATHER on a hardened point, used to indicate the magnetic meridian and, by the means of a graduated dial or circle, the azimuths of bearing of objects from this meridian. EARLY COMPASSES. It is difficult to determine who first put magnetism to practical use, but the early Chinese appear to have been acquainted with the polarity property of loadstone (magnetic iron ore) and used it as a com- pass by floating it in water upon a piece of cork. INVENTED. Flavio Goija, of Amalfi. (early part of the fourteenth century) is said to have been the first to have invented the magnetic needle. Dr. Gilbert (1600) states that the compass was brought to Italy from China by Marco Polo about 1295. There is evidence of its having been used in France about the year 1150, in Syria about the same period and in Norway previous to 1266. There are many kinds of compasses, each adapted for a certain purpose when used by surveyors, hunts- men, mariners and for the military, et al. PRISMATIC COMPASS. The prismatic compass (Fig. 1) is used for survey- ing, more especially for military purposes. . . It consists of a brass box about 3 inches in diameter. Upon the pivot is; balanced the magnetic needle, to the top of which is fixed a card correctly divided into degrees. In the best quality compasses, a divided aluminum ring is substituted in place of the card dial, as it makes a far more satisfactory instrument and less liable to derangement. The greatest amount of electricity is observed when barometric pressure is highest. COMPASSES 147 OBSERVATION OF ANGLES. As horizontal angles can be observed with great rapidity, it is a very valuable instrument to the mili- tary surveyor, who can make observations (holding the compass in his hands) with all the accuracy neces- sary for an observation or sketch; to obtain absolute accuracy the use of a tripod stand is necessary. METHOD OF USING COMPASS FIG. I. The sight vane and prism box must be turned up so that the instrument appears as illustrated, then set or hold the instrument as nearly horizontal as possible so that the dial may revolve freely. The divisions on the dial can be finely focused by either raising or lowering the prism box in its socket. Look at the object being sighted (through the slit in the prism box) until the metal line in the sight vane cuts through the object. Then, by looking through the prism box at the dial, a certain number can be read. That degree number is the magnetic bearing of the object from the point of observation. Should the observer wish to take an angle from that object to another repeat the operation by sighting the second object (being careful to revolve the compass box on its center), and after that reading has been noted, the value of the angle is the difference between the two readings taken. If the first reading were 249°, 30', and the second reading 319 , 30', the value of the angle would be 70°. AZIMUTH SHADES AND MIRROR ATTACHMENTS. For the purpose of taking the bearing of objects considerable above or below the level of the observer, mirrors and sun glasses ("azimuth shades and mirror The blue of the sky is attributable to the reflection of sunshine from minute particles of dust in the air. 148 WEATHER pig. 2. Lord Kelvin's Standard Compass SHIPS' COMPASSES 149 attachments") are applied to a certain type of pris- matic compasses. The mirror slides up and down the sight vane with sufficient friction to remain at any desired part of the vane. It can be put on with its face either above or below the horizontal plane of the eye. If the instru- ment is used for obtaining the magnetic azimuth of the sun, the dark glasses must be placed between the sun's image and the eye. ships' compasses. The ship compass, which Sir Wm. Thompson (Lord Kelvin) invented (Fig. 2), has been taken as the standard for the marine world. The mariner's compass consists of a copper or brass bowl, hemispherical in shape, into which is mounted a compass card fitted upon a delicate point, the dial re- volving upon an agate cap to insure its working easily. As the roll and pitch of a vessel would be liable to unsettle the ordinary compasses, these bowls are usu- ally filled with some alcoholic liquid to keep the card steady. CONSTRUCTION. The bowls of the compasses are supported in a ring by two pivots projecting from the opposite sides of the box. This ring is swung by two pivots at right angles to the first. This arrangement (called "gim- balling") keeps the pivot of the compass always verti- cal, the bowl being weighted at the bottom, so that its center of gravity is considerably below the points of suspension. The dial card (with its attachments) is constructed as light as possible to make the compass very sensitive. The Peruvians, in order to preserve the shoots of young plants from freezing, light great fires, the smoke of which, producing an artificial cloud, hinders the cooling produced by radiation. i 5 o WEATHER It consists of a thin aluminum circular rim, at- tached by silk strings to a small aluminum disc, in the center of which is an agate cap. To the strings is gummed a thin paper annulus, on which is marked the points of the compass. The pivot upon which this dial rests is made of platinum-iridiurn, to insure hardness and freedom from oxidization. There are eight magnets (about as thick as knitting needles, and from two to three inches long), placed symmetrically on each side of the center. These lie in a plane about i}i inches below the card, being sup- ported from the aluminum ring by silk strings. Since the weight of the card (magnets and all) is not more than iiK grams, and since the needles are some way below the point of suspension, the card remains hori- zontal even when there is considerable tendency of the needles to dip. The bowl of Lord Kelvin's compass has a com- partment at its base, partially filled with castor oil to prevent oscillations. A MAGNETIC EXPERIMENT. To induce a small amount of natural magnetism into a bar of soft iron (such as an ordinary poker), tie a silk string around the center, holding it so that it points due north and south at the proper angle of dip. By lightly tapping the iron with a piece of metal, the molecules will arrange themselves by the induction of natural magnetism which is constantly passing around us. It will not retain its power for any length of time and will lose it instantly if dropped or put into a fire SHIPS ARE MAGNETS. In this manner, ships become huge magnets, as the hammering of plates, rivets, etc., in the construction In some parts of the world nearly all the moisture which the earth ever receives comes in the form of dew. This is particularly true of some parts of Egypt and Arabia. SHIPS ARE SWUNG iSi induces natural magnetism. A great amount of it is generally lost on the first voyage, due to the buffeting of the waves and the vibration of the machinery and engines. The magnetism which is left is called perma- nent magnetism, as it undergoes very little subsequent loss of power. Fig- 3 Magnetizing a Bar of Iron WHY SHIPS ARE "SWUNG." Before leaving port, ships are "swung" for the adjustment of the compass to compensate for the local attraction of iron and steel in the ship. A sufficient number of hard steel magnets are then placed in the binnacle (under the compass) in such a manner as to exactly counterbalance the permanent magnetism of the ship. Other influences are corrected by a bar of soft iron (Flinders Bar) placed immediately forward or abaft the binnacle. It must be of the proper length A fall of one foot of snow may be roughly taken as equal to an inch of rain. !52 WEATHER to produce exact compensation when its upper end is on a level with the needles of the compass. HOW SHIPS ARE SWUNG. The process consists in observing the direction of the Standard Compass on board the vessel, as the ship's head points N., N.E.,E.,S.E., etc., and compar- ing it with that of an undisturbed compass on shore. In this way the error of the compass on each point is ascertained, and from it the table of errors is drawn off. By examining this table, the navigating officer as- certains how much of the errors are due to the perma- nent magnetism in the ship, and how much to tempor- ary induction in the vertical and horizontal iron. METHODS OF COMPENSATION. These several errors are compensated for, first, by permanent magnets in the binnacle; second, by the Flinders Bar; third, by the port or starboard iron spheres. Pig. 4 Heavy dews in hot weather indicate a continuance of fair weather PROTECTION OF JEWELS iS3 Many types of compasses are used by travelers, tourists and sportsmen, the most popular styles being mounted in hunter watch cases (Fig. 4) or contained in brass boxes with lifting covers (Fig. 5). Most of Hg. s- these have the dial (fixed in the base of the compass) graduated 0° to 90 between N. and E., E. and S., S. and W., and W. and N. The "bar needle," which is usually employed in these compasses, has a jeweled center, the whole revolving upon a delicate steel point. PROTECTION OF JEWELS. An automatic stop is fitted to the better styles, the spring of the lid, when closed, coming in contact with a lifter, which "throws" the needle and jewel cap off the point, preventing friction and wear. In August, 1851, hailstones weighing 18 ounces — diameter 4 inches, cir- cumference 12^2 inches— fell in New Hampshire. Hailstones weighing 16 ounces fell in Pittsburgh. 154 WEATHER Fig. 6. Luminous Dial Compass. Fig. 6 illustrates a luminous dial compass, which has great advantage over the ordinary kind, as by ex- posure to daylight, the dial becomes luminous (can be seen throughout the night). In the lid is inserted a small glass having a vertical line etched upon it. By means of the small sight hole in the ring or bow of the compass and the line on the glass, it is quite a simple matter to readily ascertain the magnetic direc- tion of any place. MILITARY MARCHING. In military marching, all magnetic directions are given from o° to 360°, counting from right to left. HOW TO SET COMPASSES 155 It is necessary that all military compasses, having fixed dials of degrees, should be figured from right to left and all compasses having movable or floating dials of degrees should be figured from left to right. This will be apparent by the following examples: TO SET A FIXED DIAL COMPASS. To set a compass (having fixed dial) to a given magnetic bearing, say 45 , the compass should be turned until the magnetic needle stands directly over the point at 45°, and the march made in the direction of the north point on the dial. Fig. 7. Compass with Floating Dial 156 WEATHER TO SET A FLOATING DIAL COMPASS To set a compass (having a floating dial of de- grees) to the same magnetic bearing, the compass should be turned until the central or luminous line in the lid of the compass is directly over the point at 45 , and the march made in the direction of the central line in lid of the case. INDEX . . 98 Adjustment of Barometers 48 Aerial Meteors Agonic Line . . . . . . 142 Airey's Table of Altitude . • • • 55 Air, Height of . . . . . . . . . 3 Pressure of 4 Weight at Sea Level . . . 4 Airmeters . . 139 Alcohol Thermometers, how made ... 86 Altitude Barometers 5i. 52 Hand, use on Aneroids . . • 41.42 . . . . . , 105 Scales, method of reading .51. 52. 53 55 Anemometers, Biram's . . . ■ • 138 Lind's. ... . . . 136 • • ■ 136, 137 Aneroid Barometers, how compensated 49 " how to adjust to standard read- ing . . . . . . 4 s ' construction of . . • • ■ • 35 ' definition of ... . • • 5 ' for Surveying 56 how to reduce to sea level ... 38, 41, 42, 46 " " set . . 14 ' observation of ■ • • 13, 14 Recording ... . 62 Surveying, how to read . . 56 ' useof "C._&T." Altitude Har d . 41,42,46 ' vacuums, illustration of 35 Watch and Pocket Styles . .... 51, 52 Aqueous Meteors . . • • 13 Are; i of Earth s Surface .... 4 158 INDEX Atmosphere 3 weight of . . . .... 3 Atmospheric Pressures, how to trace by winds 17 Avalanche Wind 135 Barographs 62 advantage of . . 64 " and Thermographs, combined 69 Barometer, discovery of 9 first use of . . . 9 how compensated 49 how to set 48 indications . 18 living 21 reading, what indicates 28 Recording . . . 62 reduced to sea level 38 Spider and Frog . . . 21 stationary, what indicates 28 what falling shows . 15, 17. 28 what rising shows 15, 17,28 words on dial of 15 Barometers, adjustment of 48 " Mercurial 94 Barometric inches, value of . 46 Blowing Bulbs on Thermometers 82 Boiling Point Apparatus . . 60 Boyle's Scale on Thermometers 77 Biram's Anemometers . . 138 Bulbs on Thermometers . . . . 82 Calibration of Thermometers 84 Cause of Cyclones . . . 47 " " Fogs 109 " " Frost 114. "5 " " Rain 123 " " Wind . . ... 133 Celsius Thermometers . ... 78 Centigrade Themometers 78 Change of Aneroid . 17 Charts, Weather 22 Cloudbursts 125 Clouds .... . . 103, 106 " altitude of 105 color of 20 formation of 103 nature of 106 Thunder 118 " various kinds 106 INDEX 159 Clouds, velocity of 105 Color and Nature of Clouds 106 Combined Barograph and Thermograph 69 Compass, declination of . 144 Compasses . . 142, 156 Compasses, compensation of Marine 149, 152 first use of 146 for Ships at Sea . 148, 149 Inventor of . . . . . . . 146 Luminous . . . 154 Mariners' 148 Military . 154, 156 Prismatic . . 146 " examples of 147 reading . 147 Ship 148, 149 Compensation of Barographs 63 1 Barometers 49, 5° " Marine Compasses ... . 152 Construction of Aneroid 35 " Barographs 62 "C. & T." Hand on Aneroids 41, 42, 46 Cyclones . . . 47 cause of . 47 Cyclonic Storms, cause of 47, 122 Declination of Compass . . 144 Definition of Aneroid 5 " Isobars . 23, 29 " Isotherms . . 29 Delance Thermometer 77 Depth of Ocean . 93 Determine Velocity of Wind, how to . . . 135, 136 Dew . .... 114 Dew Point 114 Diameter of Fog Particles ... . 109 Different Forms of Lightning . 117 " Thunder .... . 119 Kinds of Rain Gauges .... 125 Dip of Compass, discoverer of . . . ; 144 Direction of Thunderstorms ... 19 " Wind 133 Discovery of Barometer . 9 Dry Fogs no Duration of Lightning Flash 117 Dust Storms .... in Earliest Records of Weather . . . 6 Earth, area of surface 4 160 INDEX Earth Magnetism, change of . . ... • 144 Effect of Altitude . . 40 Low Temperature on Lakes • • 93 " Pressure on Temperature . . 90 " Sun on Weather . . . . 12 " Weather on Humans 8 " Wind on Thermometer . . . , . 18 " Winds at Different Velocities . 140 Errors in Poor Exposure of Rain Gauges ■ • 130, 131 . . 102 Example for Taking Relative Humidity . . 101 53 Exposure of Rain Gauges • 13°. 131 Fahrenheit's Thermometer . . . 75 Falling Barometer .14, 17 Ferdinand II. Thermometer 76 Filling Thermometer Tubes 82 Flinders Bar, use of in Compasses . . ... 152 Fogs 103, 109 " cause of 109 diameter of particles 109 ' ' difference in Clouds 103 Force of Wind per Square Foot 135 Forecasting . ... ... 13, 22, 26, 33, 107 Formation of Clouds 106 " Snow 115 Freezing . . . . • • 93 Frog as Barometer . . ... 21 Frost .... 114, US Geographical Meridian 142 Graduating Scales on Thermometers ... 80 no " temperature of no Hail no Height of Air . . ... 3 Highs and Lows on Weather Map, use of 30 Hooke's Scale on Thermometer .... • • 77 How to Read Altitude Scales • 51, 53 Humidity • 95, 102 Hygrodeik 100 Hygrometers . 96, 102 Hypsometers 60 Illustration of Weather Map ... 32 28 ... . 146 Isobars, definition of • 23, 29 Isotherms " " ■ • • 29, 31 INDEX 161 Kelvin's Compass Kircher's Thermometer . . . Lakes, effect of cold in . . . . Lightning different forms of duration of flash . Living Barometers Lind's Anemometer . . "Lows" and "Highs" on Weather Map, use of Luminous Compasses Luminous Meteors ... Magnetic Meridian . . Magnetism . . natural . . .... permanent ... ... Manufacture of Thermometers Map, Use of "Highs" and "Lows" on Weather Marine Compasses construction of Maximum Temperature ... Thermometers Mercurial Barometers Meridian, geographical . . . magnetic Meteorological Stations Altitude of . . . . Meteors, aerial, aqueous, luminous .... Method of Surveying Ships for Adjustment of Com- passes . . . . Military Compasses ... . ... Minimum Temperature . : . . . Thermometers Moisture Monsoon, definition of Movement of Barometer, how constructed . ... Natural Magnetism ... ... Observation of Aneroid . Ocean, Depth of . . . Permanent Magnetism .... Pluviometers . . . . . Pocket Aneroids . . . Pointing Thermometer Tube . . Precipitation, when it occurs ... Predictions, Weather, how made Pressure of Air .... .... " Wind Prismatic Compasses Process of Calibrating Thermometers 148, 149 76,77 93 117 148, 149 118 117 117 21 136 3" 154 13 142 U3 i5<> 151 75,90 30 148 152 91 92 94 142 142 43, 45 13 ■ 149- 151 • 154. 155 91 92 95 124, 135 35 i5<> 13 93 150 124 52 71,88 104 11, 13 4 135 145, 147 73 1 62 INDEX Process of Filling Tube of Thermometer " " Graduating Thermometers " " Making Thermometers ' ' Seasoning Thermometers " " Testing Thermometers . Psychrometers, Sling Rainbow ... Rainfall Rain Drawn From Hills .... " Gauges, different kinds of . . " exposure of . . . "Red " Winds Reading of Aneroid . . Reaumur Thermometers Recording Barometers ... " Thermometers ... . . and Barometers Combined Records, Earliest of Weather Reduce Aneroids to Sea Level . ... Red Rain Relative Humidity Robinson's Anemometers Rules for Converting One Thermometer Scale Another Scales, Altitude, how to read ... " Fahrenheit's Thermometer. " Graduating on Thermometers Sea Level, how to reduce Barometers to " " weight of air at Sea Wind Seasoning Thermometer Tubes Set Aneroids, how to ..... . . . Ship's Compass " construction of . . . . " how swung ... Signs, Weather Simoon Simplified Sea Level Barometer ... Sky. 80, 130 95 136 82 83,85 75- 90 85 87, 88 101 116 123 124 125 J 32 III 124 13, 14 78 62 67 69 6 38,41 in 102 137 tj Sleet Sling Hygrometer Snow " Gauges . . . "' to measure fall of Sound, rate of travel . Spider as Barometer. 78 51, 52, 53 75 8o, 83, 85 38, 41, 46 4 134 85 14 148 • 148, 149 151 ■6, 15, 18 135 38 106 III 101 126 [2d 126 119 21 II 4 INDEX 163 Spirit Thermometers, how made . . 86 Squalls 135 Stationary Barometer 19 Storms, causes of ... . 47 Dust III motion of 9 when occur . . 11 Sun . . ... n> 74 effect of on weather . . 12, 104 Surveying Aneroids 56 5° Table Giving Values of Altitudes in Inches 53 " of Altitudes, Prof. Airey . . . 55 Temperature .... • 70, 94 Maximum, when reached 91 Minimum " 9 1 of Gulf Stream no Terms Used in Weather 33. 98 Testing Thermometers. 71.87 Thermographs . 67, 92 Thermometer, effect of wind on 18 Thermometers ... ... 70. 7-' and Barometers, Recording, combined 69 Boyle 77 Bulbs . 82 Calibration of 73 Celsius 78 Centigrade 78 Delance . 77 Drebbels . . . 76 Essay on (1738-39) 7