B31I 1EA2 ALBERT R. MANN LIBRARY New York State Colleges of Agriculture and Home Economics Cornell University Cornell University Library QC 931.N2A2 Meteorology and aeronautics. 3 1924 002 978 322 REPORT No. 13 METEOROLOGY =and — AERONAUTICS a NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS WASHINGTON GOVERNMENT PRINTING OFFICE 1917 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/cu31924002978322 REPORT No. 13 METEOROLOGY and AERONAUTICS ib NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS WASHINGTON GOVERNMENT PRINTING OFFICE 1917 REPORT No. 13 (IN THREE PARTS) METEOROLOGY AND AERONAUTICS A handbook in which are discussed properties and general phenomena of the atmos- phere which aeronauts and aviators should understand. Part I. — Physicial Properties and Dynamics of the Atmosphere. Part II. — Topographic and Climatic Factors in Relation to Aeronautics. Part HI. — Current Meteorology and Its Use. Submitted by the Subcommittee on the Relation of the Atmosphere to Aeronautics of the National Advisory Committee for Aeronautics; CHARLES F. MARVIN, Chairman. Prepared by WILLIAM R. BLAIR, member of Subcommittee on Relation of the Atmos- phere to Aeronautics, in charge Aerological Investigations, U. S. Weather Bureau; later commissioned Major, Signal Corps, IT. S. R. CONTENTS OF REPORT No. 13. Introduction 8 Part I. Physical Properties and Dynamics of the Atmosphere 9 Atmospheric constituents 9 Distribution of constituent gases in the atmosphere 9 Nucleation and condensation •. . . . 9 Pressure and units of measurement 10 Air temperature and vertical motion 10 Local heating 11 Sources of atmospheric heat 11 Conduction and convection of heat 12 Radiation and absorption of heat 12 Air movement 12 Convective systems 13 Ascending air 13 Descending air 14 Extent and rate of air movement 14 Atmospheric equilibrium 14 Variations in atmospheric equilibrium 15 Types of convective systems 16 Degree of reversibility of atmospheric processes 16 Influence of the earth's rotation 17 Pressure distribution 18 The propagation of atmospheric disturbances 18 Part II. Topographic and Climatic Factors in Relation to Aeronautics . 21 Location and use of base stations 21 Disturbances caused by buildings 21 Gustiness caused by topography 21 Vertical motion in gusts 22 The extent of disturbances caused by topography 23 Disturbances caused by local heating 23 Climatic factors 24 Use of climatic data 25 Wind speed and direction 25 Gales 25 Wind direction and gustiness 25 Air temperature and "roughness" 26 Precipitation 26 Fog 26 Humidity 26 Cloudiness 26 Frost and ice formations on aircraft 28 The air pressure 28 Traveling disturbances considered 28 Thunderstorms, hurricaneB, and tornadoes 28^" ^ Frequency of thunderstorms, etc 28 '.. —A thunderstorm described 28 Explanation of the records 30 Warning of thunderstorm's approach 30 ..- Thunderstorm frequency 30 Turbulent conditions indicated by thunderstorms 31 Cyclonic thunderstorms and tornadoes 31 Tropical cyclones or hurricanes 31 Direction of travel of disturbed condition 31 Part III. Current Meteorology and Its Use 33 Current meteorological observations 33 Weather reports and forecasts 33 5 6 CONTENTS. Part III. Cuekbnt Meteorology and Its Use — Continued. P*ge- The simultaneous synoptic weather map 33 Distribution of air temperature and density 34 The rain area of the low 35 Other rain areas 35 Motion of low and high pressure areas 35 Character and intensity of highs and lows 36 Seasonal changes in the highs and lows 36 ... The thunderstorm region 36 Itensity of disturbances and direction of motion 36 The value of upper air observations 37 Pressure distribution and height 37 Winds and pressure distribution 37 Wind change with height in lows 37 Wind change with height in highs 44 General laws of wind change with height 44 Wind speed and height 45 Effect of obstructions on wind 46 Turbulence due to insolation 46 Flying levels and turbulence 46 Temperature change with height 49 LIST OF ILLUSTRATIONS OF REPORT No. 13. Page. Figure 1. Disturbance caused to leeward of a small round tower during a high wind 21 2. Record of wind speed by pressure tube anemometer 22 3. Free air conditions at Drexel Aerological Station during thunder- storm on June 21, 1916 27 4. Surface-pressure changes (inches) at Drexel Aerological Station during thunderstorm on June 21, 1916 29 5. Surface temperature changes (F.) at Drexel Aerological Station during thunderstorm on June 21, 1916 29 6. Surface relative humidity changes (per cent) at Drexel Aerological Station during thunderstorm on June 21, 1916 30 7. Daily weather map, November 28, 1911 34 8. Mean of wind observations in "Highs" at 526 meters above sea level, 1907-1912 38 9. Mean of wind observations in "Lows" at 526 meters above sea level, 1907-1912 38 10. Mean of wind observations in "Highs" at 1,000 meters above sea level, 1907-1912 39 11. Mean of wind observations in "Lows" at 1,000 meters above sea level, 1907-1912 39 12. Mean of wind observations in "Highs" at 2,000 meters above sea level, 1907-1912 40 13. Mean of wind observations in "Lows" at 2,000 meters above sea level, 1907-1912 40 14. Mean of wind observations in "Highs" at 3,000 meters above sea level, 1907-1912 41 15. Mean of wind observations in "Lows" at 3,000 meters above sea level, 1907-1912 41 16. Mean of wind observations in "Highs" at 4,000 meters above sea level, 1907-1912 42 17. Mean of wind observations in "Lows" at 4,000 meters above sea level, 1907-1912 42 18. Mean of wind observations in "Highs" at 5,000 meters above sea level, 1907-1912 43 19. Mean of wind observations in "Lows" at 5,000 meters above sea level, 1907-1912 .». 43 20. Diurnal distribution of temperature for the summer half of the year at different levels above Mount Weather 47 21. Diurnal distribution of temperature for the winter half of the year at different levels above Mount Weather 48 22. Mean free air temperatures above Mount Weather, Va 49 23. Temperature distribution in ' ' Highs " observed at Mount Weather . 49 24. Temperature distribution in "Lows" observed at Mount Weather. 50 7 INTRODUCTION. Present weather conditions and those immediately in prospect, especially those relating to air movement, have always been matters of concern with navigators. In the days of sailing vessels weather conditions were of prune importance, and in the present age, when steam and other engines are used in driving vessels, navigators are still greatly dependent upon weather conditions, and the time is far off, if indeed it ever comes, when the navigation of the oceans will be entirely independent of weather conditions, or when a knowledge of them will not be of advantage to the navigator. In addition to those weather conditions which interest the sailor, the aviator is dependent upon and benefited by a knowledge of the detailed struc- ture of the medium which sustains his craft; and while he may to a degree disregard atmospheric conditions, it is not likely that the time will ever come when a knowledge of the air can not be used by him to advantage. Just what information will be most useful to the aviator is a question that experience and possible development of aircraft must determine. More data than those now being obtained will doubtless be required. The purpose of these brief notes is to show the sort of atmospheric data available and to put the subject in such shape as may make it bear directly on the problems which are met in aviation. The author desires to acknowledge the assistance of Mr. W. R. Gregg, who has reduced the data on which these conclusions are based and read the proof; also that of Profs. C. F. Marvin and J. F. Hayford, who have kindly read and criticized the copy, making helpful sug- gestions thereon. William R. Blair. REPORT No. 13. PART I. By William R. Blaib. PHYSICAL PROPERTIES AND DYNAMICS OF THE ATMOSPHERE. Atmospheric constituents. — Approximately 78 per cent, by volume, of the air near the earth's surface is nitrogen and 21 per cent oxygen. Another diatomic gas, hydrogen, is found uniformly distributed, but in very small quantity. The monatomic gases, argon, neon, helium, krypton, and xenon, are present, their volumes being in the order given. Other gases and vapors of more complex molecular structure and less uniformly distributed are water vapor, carbon dioxide, ammonia, and sulphur dioxide. The last three gases named are hygroscopic and seem to exist in the atmosphere in association with more or less water, depending on the amount of atmospheric moisture available. Other gases, as well as dust of both terrestrial and meteoric origin, are present in varying quantities, depending on the time or location in which observations are made. Distribution of constituent gases in the atmosphere. — In order of their atomic weights the first eight of the gases mentioned are hydro- gen, helium, nitrogen, oxygen, neon, argon, krypton, and xenon. When molecular weights are considered, neon is third in the list, and nitrogen and oxygen fourth and fifth, respectively. It is probable that these gases are sorted by gravity and that, as the distance from the earth's surface increases, the proportion of the lighter gases in- creases until at some height, 150 kilometers or more above sea level, the chief constituent of the air is hydrogen. Convective mixing of the air interferes with this sorting to some extent, especially in the lower 10 kilometers of the atmosphere. Nucleation and condensation. — Recent observations and experi- ments indicate that the hygroscopic gases, rather than dust or ions, furnish the nuclei upon which the water vapor of the air condenses, forming haze and fog or cloud. Strata of air that are hazy are found to be rich in these hygroscopic nuclei and usually a disturb- ance that elevates such strata either in whole or in part results in cloud formation. It may happen that the temperature of the air in the haze layer is lowered without accompanying change of level, with the result that a stratum of cloud replaces the stratum of haze. When air of the haze layer is forced up in places by a relatively strong vertical component in its motion, the resulting type of cloud formation is cumulus; when it is cooled, either by Deing elevated as a whole or by other means, the type of cloud resulting is stratus. 9 10 AERONAUTICS. The formation of cumulus clouds therefore accompanies turbulent atmospheric conditions, and is in turn an indication of these conditions. Pressure and units of measurements. — Atmospherio pressures are most familiarly measured in inches or millimeters of the height of a column that will just balance the static pressure of the air at a given place. Thus, the standard pressure commonly designated one atmosphere is a column of mercury 760 millimeters high having a temperature of 0° C. under gravity at sea level and latitude 45°. The pressure of the atmosphere and local differences in these Eressures are forces capable of causing flow and motions of the air. n order to be able readily to compare these forces with other forces with which we are familiar it is most convenient in all aerological studies to express the atmospheric pressure in dynes per square centimeter, which is the unit commonly employed for the measure- ment of pressure. Meteorologists generally have agreed upon the distinctive name "bar" for the unit for measurement of air and similar gaseous pressures when expressed in absolute units. A pres- sure of one bar is a force of 1,000,000 dynes per square centimeter of area. It is exactly equivalent to the pressure of a mercury column of 750.016 millimeters at 0° C. and reduced to standard gravity at sea level and latitude 45°. The familiar standard atmosphere of 760 milhmeters equals 1.0133 bars. 1 The millibar (mb.), i.e., 1,000 dynes per square centimeter, is the customary subdivision of the absolute unit, and in a barometric measurement is about three- fourths as large as the familiar unit "one millimeter of mercury." Accordingly, pressures will hereafter be expressed in millibars. Sea level pressure will range roughly between 995 and 1,060 millibars. Air temperature and vertical motion. — The ratio of their specific heats at constant pressure, C p , and at constant volume, V) y=-?^> T varies for the different constituent gases of the atmosphere, being about 1.3 for the group in which the molecules are triatomic or more complex, 1.41 for the diatomic group and 1.66 for the monatomic group. For dry air 7 is approximately 1.41. In the well-known equations for adiabatic changes, in which V, T, and p stand for volume of unit mass, absolute temperature and pressure respectively, (wl —-m and \y) =—, suppose the subscripts and 1, respec- tively, represent the conditions, at two stations in the free air on the same vertical line, of an air mass that moves from the lower to the upper station without appreciable loss or gain of heat from out- side sources. If we put 7 = 1.41, v = l,T = 273, 2> = 1000 nib., and T 1 = 272, and solve for pi in millibars, we get )1 .41 It ' ^ = 10001.41/273) =987.5. \V272/ Substituting these values of p g and p x in the standard equation for difference in height, z = Tc log ( — ) it is found that the vertical 1 For derivation of these relations and tables see Monthly Weather Review, April, 1914, p. 239. AERONAUTICS. 11 distance between the stations is 100.5 meters. An air mass cools 1° C, therefore, in ascending approximately 100 meters, and of course warms up 1° C. in descending the same distance. This state- ment is true only if no exchange of heat occurs either to or from the air mass with the outside and if no latent heat of condensation or evaporation enters into consideration. Local heating. — In passing through the atmosphere to the earth's surface the sun's radiant energy is considerably diminished by reflec- tion from the upper surfaces of clouds and from other boundary sur- faces that may exist between strata of different densities or constitu- tion. There is some diminution of this energy because of direct absorption by the gases of the atmosphere, but the amount thus absorbed is relatively small. In general, the sun's rays are some- what refracted by the atmosphere, and the amount of heat reaching the earth by means of them is thus somewhat increased. Locally, the amount of heat received from the sun by the earth's surface may be increased considerably by reflection from the sides of clouds, and it may, of course, be diminished in the cloud's shadow. Insolation, the heating effect of the sun's rays measured at the earth's surface, is considerably greater in the spaces between cumulus clouds, for example, than under "normal" conditions, when there are no clouds present. The air itself when dry absorbs but little of the sun's energy, and is therefore but little heated by the direct rays of the sun. The earth's surface reflects more or less of thesolar radiation reaching it, depending on the nature of the material forming it. Snow, ice, water, and white sand, clay, or rock surfaces reflect more of the sun's radia- tion than do black, brown, or vegetation-covered fields. The color of the surface is determined by the wave lengths it reflects most — a white surface reflects all wave lengths equally, while a black surface absorbs all wave lengths and reflects none. Surfaces that are good absorbers are good radiators, i. e., lose their heat readily by radiation Surfaces that absorb less readily "hold their heat," i. e., are not good radiators. There is, however, less difference in the rapidity with which these different surfaces communicate their heat to the air by conduction. Sources of atmospheric heat. — Altogether, cloudiness and other factors considered, less than half the radiant heat from the sun entering the outer portion of the earth's atmosphere succeeds in penetrating to the lithosphere, where it becomes effective in heating the earth's surface and the air near it. Notwithstanding this absorp- tion, however, the atmosphere as a whole is but little heated by the direct rays of the sun. It is the earth's surface that is the chief source of atmospheric heat, and, as has been shown, this surface locally and in point of time receives varying amounts of heat from the sun. This means that locally and in point of time the tempera- ture of the earth's surface varies. The surface temperature in a field of black earth is somewhat higher at 4 p. m. of a clear day than is that of a green pasture adjoining it and decidedly higher than that of a field of white wheat or wheat stubble in the same vicinity. At 4 a. m. these relations of temperature are reversed. Similar but more marked differences of temperature are found between land and water surfaces. To these must be added the temperature variations accompanying elevation and latitude of the surface considered. 12 AERONAUTICS. Conduction and convection of heat. — The earth's surface heats or cools the air in contact with it largely by conduction. It follows that air temperatures at lowest levels will vary in the same sense as do the temperatures of the earth's surface, but not necessarily to the same extent. Convection processes in the air will then distribute the heat. During insolation the air lying on the earth's surface is likely to be receiving some heat from the earth by conduction; at night it is likely to oe giving up heat to the earth. As water is evaporated from lakes, etc., the heat of evaporation is furnished by the earth's surface; and this heat is supplied to the air at higher levels as heat of condensation when the moisture condenses; e. g., wherever fog or cloud forms or wherever condensation of vapor yields any form of precipitation. Radiation and absorption of Tveat. — The proportions of the con- stituent gases in the atmosphere at any pomt exert some influence on the air temperature at that point. If a given level in the atmos- phere is considered, the inflow through it of energy from the sun and the outflow through it of energy from the earth's surface and from that part of the atmosphere below the level in question are, in the long run, equal. This energy is either transmitted through the atmosphere without heating it or is, to a greater or less degree, {>assed along by the process of absorption and re-radiation. In the atter case the air is heated to such a temperature that equihbrium is established between the rates of absorption and re-radiation. This temperature is different for the different atmospheric constituents, being proportional to their abilities to absorb solar and terrestrial radiation. While the amount of solar radiation absorbed in its passage down through a clear atmosphere to the earth's surface is relatively small, about two-thirds of this amount is absorbed by the water vapor of the air. On the other hand, a large percentage of ter- restrial radiation is absorbed as it flows out through the atmosphere and most of this absorption again is by the water vapor of the air. The earth's surface is therefore to be thought of as by far the most important source of atmospheric heat. When temperature dis- tribution in the atmosphere is thought of from this point of view, the earth's surface temperature, i. e., the earth's potential as a radiator, must be taken into account. Air movement. — The density of adjacent portions of stationary free air in equilibrium under the attraction of gravity varies in a definite way from one surface or stratum to the next higher or lower one, but the density tends to be uniform throughout one and the same surface, which may therefore be called an equigravic surface. An equigravic surface as applied to atmospheric conditions passes through portions of the air having the same gravitational potential. Arrangements of air densities not in conformity with the foregoing requirements are accompanied by motions. The denser portions of air, being pulled down by gravity, spread out, partially mingle and replace the less dense portions, which are pushed up, but are com- monly said to ascend, and may also intermingle and spread out until equilibrium is established. Both vertical and horizontal components of motion are necessary in restoring eqmhbrium. If the composi- tion of the air moisture contents, etc., were exactly the same over a considerable extent, then equigravic surfaces for a state of equilibrium would be horizontal surfaces. AERONAUTICS. 13 The density of air at constant pressure depends largely on its temperature, but to an extent also on its constitution. In the lower strata of the atmosphere moisture is the only constituent whose quantity varies sufficiently to affect air density appreciably. It has been shown above not only that air temperature varies from point to point in the atmosphere but that the moisture content also varies. Usually temperature and moisture content affect air density in the same sense, 1. e., an increase in either makes the density less. Near the earth's surface, where the moisture content and the tempera- ture of the air are rather directly controlled by the nature of that surface, maxima and minima of air density may be found within a few meters, or even a few centimeters, distance. Farther away from the earth's surface these maxima and minima of density are farther apart. Whatever the dimensions of the air mass considered, there- fore, it is likely to contain one or more complete circulatory systems in process of adjusting the differences in density of its parts; and it itself is likely to be moving, in conformity with the need for similar adjustment on a larger scale. Convective systems. — In the process of adjusting these differences in atmospheric density still another factor in the distribution of temperature is introduced. Air masses, with their heat content, are carried bodily from place to place without change of level and conse- quently with comparatively little change in temperature, volume and pressure. Air masses also change level which entails change of pressure and volume. Work is done when the volume of a given mass of air is changed. When its volume increases, the energy for this work must be furnished either by outside sources or by the gas itself; when the volume decreases, energy is given to outside objects or to the gas itself. When the energy is all supplied by or to the air mass itself, the temperature change in the air mass is 1° C. per 100 meters, as has been shown above (p. 11). It is clear that the total heat content of the atmosphere can not be changed by these motions of the air, for under the conditions of our atmosphere as much air must go up as comes down and, if air moves from the point A to the point B, air from B, or air displaced by air from B, etc., moves to the point A. Because quantities of heat are carried about in these circulatory systems and also because temperature differences are the prime causes of the differences in density giving rise to them, they are often called convective systems. Ascending air. — An ascending air mass may have the energy re- quired for its expansion furnished in part by absorbing radiant heat and by the heat of condensation of water vapor that may take place within it. It will lose some heat by conduction to adjacent air masses, by mixture of air near its boundaries with air of these masses, and by radiation. The percentage of the required energy which may be furnished by absorption (conduction and radiation also included) depends on the rate of expansion or of ascent of the air mass. It is always small and probably not considerable, except for rates of ascent well below 1 meter per hour. The percentage of required energy furnished by heat of condensation of water vapor may be as high as 60 or more after condensation begins, depending on the amount of water vapor present. When precipitation occurs, the net result, so far as air temperature is concerned, is the transfer to the air, in the condensation level, of energy originally derived from 14 AERONAUTICS. the earth's surface when the water forming the precipitation was evaporated. Descending air. — A descending air mass may furnish heat to adja- cent air masses by conduction, to objects or air at a distance by radiation, or it may supply heat of evaporation to water suspended in it as mist or cloud. Some radiant heat may be absorbed by the air mass as it descends. The conduction effect is small. The absorp- tion and radiation effects are smaller than in the case of an ascending air mass, because of the relative dryness of the descending air mass. The amount of heat of condensation supplied to ascending air is, of course, far in excess of the heat of evaporation given up by de- scending air. It follows that the adiabatic rate of decrease in tem- perature with height, i. e., 1° C. per 100 meters, is more nearly realized in descending than in ascending air currents. Extent and rate of air movement. — When any convective system is considered, the extent of horizontal motion in the air affected is determined by the horizontal distance apart of the maximum and minimum air densities of the system and on the direction of the motion between these two points. The extent of the vertical motion in any system depends on the condition of equuibrium * of the air mass involved and on the magnitude in any equigravic surface, of the difference between the maximum and minimum of air density. The rate of motion either horizontal or vertical depends on the rate of change of density in any equigravic surface of the system. Com- pared with the rates of horizontal air movement observed, vertical motion is very slow. Horizontal movement of 25 meters per second is frequently recorded, especially at some distance above the earth's surface, and rates of three times this have been observed. It is estimated that over the thermal equator the air is rising at the rate of about one meter per hour. Rates of vertical motion as high as 4 or 5 meters per second may occur locally; e. g., in the deeper cumulus clouds. 2 Atmospheric equilihrium. — The condition of equilibrium of the atmosphere or any part of it depends on the vertical distribution of its density. Since density depends largely on temperature, the vertical distribution of temperature is the best direct indication of the equilibrium. If potential temperature (defined below) rather than actual temperature is considered, the relation of temperature to equuibrium becomes more direct and apparent. The potential temperature at any point in the atmosphere is the temperature, expressed on the absolute scale, which would be assumed by the air about that point, if this air were changed adiabatically to standard pressure. The potential temperature of a gas, like its entropy, remains constant throughout a reversible or strictly adiabatic cycle of changes and increases for an irreversible process. When in any • The condition of equilibrium of any system under the influence of any force whatever is stable if, when slightly disturbed from its position of equilibrium. its_potential energy is increased, neutral if its potential energy remains unchanged, and unstable if its potential energy is decreased. An air mass in equilibrium under the action of gravity is in stable equilibrium if : when any part of it is slightly disturbed in a vertical direction, its potential energy is increased, neutral if its potential energy remains unchanged, and unstable if its potential energy is decreased, i. e. , in addition to overcoming the inertia of the disturbed part of the air mass, the disturbing force must do work on this part of the air mass, in order to effect its change of level, if the air mass be in stable equilibrium, no work is done, either by the disturbing force or by the part of the air mass changing level if the air mass be in neutral equilibrium, and work is done by the part of the air mass disturbed if the condition of equilibrium of the air mass be unstable. In other words, the atmos- phere, in its average condition of equilibrium (see par. 15, following), more or less strongly resists vertical motion, depending on the degree of its stability. 2 C. F. Brooks states (M, W. E., July, 1917, p. 363) he measured 7 m/sec. at Blue Hill, Mass. AERONAUTICS. 15 region of the atmosphere the rate of fall of temperature with altitude is 1° C. per 100 meters, i. e., the adiabatic rate, the potential tempera- ture is constant throughout the region and the state of equilibrium is neutral. If now, in the upper part of this region, the air density increases by an amount such that the additional pull of gravity on account of this increase in density is sufficient to overcome friction, downward motion will result. On the other hand, if air in the lower part of this region decreases in density by a sufficient amount, upward motion will result. In either case a state of unstable equilibrium is brought about. Since in the free air density varies quite directly with temperature, this unstable state is accompanied by a tempera- ture-altitude relation of more than 1° C. per 100 meters or by a decrease of potential temperature with height. When the potential temperature increases with height, i. e., when the temperature- altitude relation is less than the adiabatic rate for dry air, the state of equihbrium is stable, and the degree of stability is indicated by the rate of increase of potential temperature with height. In a region of neutral equilibrium, air in low levels, which becomes warmer, that is, less dense than its environment, will, barring friction, ascend indefinitely to the top of the region in question or until its excess of heat is lost by radiation or exchange with its surrounding air. Similarly, in the higher levels any portion of such air which becomes cooler or denser than surrounding portions will descend to the bottom layers unless stopped by loss of its excess density due to heating from without. When the equihbrium is stable warm light air can rise but a limited amount and then come to rest, because it soon enters strata having the same temperature and density as its own. Similarly, cold heavy air in the higher levels of a stable system can descend but a limited distance before it comes to rest in a stratum of its own temperature and density. 1 Variations in atmospheric eguttihrium. — As a whole, and on the average for any particular region, the atmosphere is in stable equihb- rium. It is only for limited times and regions that neutral or un- stable states of equihbrium exist. The neutral and unstable states are found most frequently during insolation and near the earth's sur- face. They may extend to the 2-kilometer level and in some cases higher. In general, stability increases with latitude. The air is in more stable equilibrium in winter than in summer and at night- time than during insolation. This is especially true of the stratum in which turbulence is most likely to occur. On a clear, quiet summer day it is often possible to observe the process of conversion from stable to neutral and unstable equuibrium of the air in the lower stratum. As insolation heats the earth's surface, the air in contact with that surface becomes heated and to such an extent that its po- tential temperature is higher than that of the air immediately above it. The result is the descent and spreading out of the air of lower potential temperature, and the ascent and spreading out of the air of higher potential temperature. The vertical extent of this cir- culation is but a meter or two at the start and gradually increases with the increase in the temperature of the earth's surface. It often happens that the air within the neutral or unstable region is i It is highly important that the foregoing basic principles governing motions of portions of the atmosphere and its equihbrium be thoroughly comprehended and mastered by the student. Furthermore, in con- sidering circulation in the free air, constant attention must be given to the existing and actual free-air gradient of temperature and its influence on vertical motions under gravitational forces. — C. F. M. 16 aeronautics; hazy compared with the air above it. To one on a hill or mountain top this haze level is distinctly visible by the time it has risen well above the tree and house tops and its upward progress can be noted. The term "haze level" is a natural one because, to the observer, the boundary between the air in stable and that in unstable equilibrium is clearly denned and appears to be very approximately level. The rate of rise of the haze level for given conditions of the earth's sur- face seems to vary inversely with the stability of the air above it, i. e., inversely with the rate of increase of potential temperature with height. Consequently, the height to which turbulence of this sort can attain in a given region, by the time the earth's surface has reached its maximum temperature for the day, varies inversely with the stability of air over this region when insolation began. This height has been found to be about 1 kilometer above the earth's surface for a clear winter day and 1.5 kilometers for a clear summer day, at an inland location and over open farmed land in which are occasional patches of timber. Types of convective systems. — The type of convective system thus far considered is one which has its origin in a difference of air density over a given equigravic surface. Illustrations of this type of con- vective system are the planetary system of convection, the diurnal system, and those currents set up because of differences in the nature or elevation of the earth's surface, such as land and water systems, mountain and valley systems, also of combinations of these. These systems are essentially local, either in point of time or location on the earth's surface. In addition to these, there are traveling con- vective systems, such as those attending maxima and minima of pressure as they pass from west to east, hurricanes, thunderstorms, and tornadoes, the origin of which can not be so directly or com- pletely ascribed to a difference in density over an equigravic surface. Degree of reversibility of atmospheric processes. — Irreversibility in atmospheric processes may result largely from change in the consti- tution of the air concerned in the processes, but also m some measure to the absorption and radiation of heat by this air. So far as the atmosphere has been explored, the constituent most concerned in this change, as well as the one responsible for a large percentage of the absorption and radiation of heat by the air, is water vapor. Great changes in water vapor content occur, as a rule, only in the first 3 or 4 kilometers of the atmosphere. In this region, there- fore, processes are least nearly reversible. Complete reversibility of atmospheric processes seems to be more closely approximated, as distance from the earth's surface increases. If the changes taking place within a convective unit are reversible, the unit is self-contained and independent of adjacent units. If these changes are irreversible, the unit is not independent of similar adjacent units and their airs mix to an extent depending on the degrees of irreversibility in the units concerned. If the processes in the convective unit represented by the trade and antitrade winds were reversible, the air of the antitrades would all have returned to the trades by the time it had reached the northern limit of the trades (confining the illustration to the northern side of the thermal equa- tor). The air of the antitrades is not able to return directly to the trades largely because its constitution differs from that of the air in the trades. Much of it therefore moves on to the north on the AERONAUTICS. 17 earth's surface and either returns to the trades from higher latitudes or replaces air of another convective unit which in turn must supply some air to the trades. This condition results in the passing of more or less oppositely directed air currents on the earth's surface, the air in these passing currents being as a rule quite differ- ently constituted. There are many ways in which the passing of differently constituted airs in nearly oppositely directed currents is brought about. In the Tropics, for example, where insolation is high, a local land and water convective system may create air cur- rents counter to the prevailing trades. _ Influence of the earth's rotation. — Because of the earth's rotation, air (or other matter) in horizontal motion over the earth's surface is acted on by a deflecting force, to the right in the northern hemisphere, / / to the left in the southern hemisphere. This force is 2a> mV sin \y 2?r ' where mT 7 as the poles are approached. If, then, a current of air is flowing along a straight line on the earth's surface (i.e. along a great circle) in a frictionless manner at a uniform speed, this deflective influence due to the earth's rotation must be balanced by an equal and opposite force. This last is due to the pressure gradient in the atmosphere, which is by definition per- pendicular to the isobars. It follows, then, that the current of air must be along a path exactly at right angles to the gradient, that is, parallel to the isobars. It is seen without difficulty that this pressure gradient -y- = 2pwFsin X, where p is the density of the air. Taking X 45°, the pressure gradient across an air current moving at 11 m.p.s. is 1J millibars, or 1 mm. of mercury, per 100 kilometers, or approximately 0.1 inch of mercury per 158 miles. Eleven m.p.s. is the mean wind speed for the year, observed about 300 meters above the surface at Mount Weather, Va. Actual speeds may depart far from this mean in either sense. Summer winds are, as a rule, slower and winter winds faster than this mean. If the current of air has a constant speed V in a circle of radius r, there must be a force acting on each particle of the air in toward the center of curvature equal to mV 2 /r; the deflective force due to the earth's rotation acts in this direction in an anticyclonic system (i. e., a "high") in the northern hemisphere and opposite to this direction in a cyclonic system (i. e. , a " low " ) . It follows at once that the force due to the pressure gradient must be radially out in a high, and radially in in a low, and that therefore the lines of flow of the cur- rent are parallel to the isobars. It is obvious, then, that in order to have a uniform flow at speed V, friction being assumed negligible, in a curve of radius r, V must satisfy the following condition: — = ± (pressure gradient — 2 p&> Fsin X), where the plus sign applies to a cyclonic and the minus sign to an anticyclonic system. The V which satisfies "this equation is called the "gradient velocity." 22417°— 17 2 18 AERONAUTICS. Friction is never absent, however, in the movement of air; and consequently the force due to this should be taken into account in the above equations. The line of flow will never be quite parallel to the isobars; and the angle of inclination will be such that the force due to the pressure-gradient — which is perpendicular to the isobars — will have a component along the line of flow sufficient to overcome the force of friction. Pressure distribution. — Another description of the condition exist- ing owing to the earth's rotation is to say that there is a region of low pressure on the left sides of two oppositely directed passing currents of air, and one of high pressure on their right sides, i. e., two oppositely directed passing currents of air will have low pres- sure between them if each passes on the other's left, but high pres- sure between them if each passes on the other's right. The airs in these currents are usually differently constituted, especially in respect to water vapor, and of different temperature. The pressure gradient is therefore in such a direction as to cause them to mix on their left sides, where the cold, denser air keeps to the earth's surface, forcing the warm and less dense air to rise. This "line type" of convective unit, in which the airs of different densities are mechanically placed, is dominant to a greater or less degree in cyclones, anticyclones, tropical hurricanes, thunderstorms, and tornadoes. It is on the boundary lines between these two oppositely moving air masses, differently constituted and having different temperatures, that the centers of cyclones, tropical hurricanes, and anticyclones are found. It is in the vicinity of these boundary lines, especially to the south of the cyclone center, that thunderstorms and tornadoes most frequently occur. The propagation of atmospheric disturbances. — An air mass that is relatively dense for the level it occupies, may not be able to sink directly to a level appropriate to its density, because of its high adiabatic rate of heatmg compared with the slower rate of cooling of air, latent heat of condensation considered, at a lower level that • must rise to take its place. Such an air mass may, of course, be resting on the earth's surface. This air mass in motion over the earth's surface, or over an "aerial bottom," will tend to conform with the "bottom" over which it flows. If the bottom or supporting surface be of irregular contour, like the earth's surface, this tendency to conform with it will result in aperiodically alternating expansion and contraction of the air in Suestion, with attendant changes in temperature, pressure, and motion, onvective units of this type are called gusts and the wind in which they occur carries them for some distance beyond the irregularity in the bottom which caused them. If this bottom be the solid earth, there is no response by the rigid earth to the traveling gust, which ceases to exist as originally formed. Either equilibrium will be restored in uninterrupted flow over level surface or other irregu- larities in the bottom will superpose new gusts. If the bottom or supporting surface be aerial and the irregularities in its contour be in the nature of pressure variations originating in the lower stratum, there will be reaction in the lower stratum to the gust initially caused by it in the upper stratum. This interaction, while probably not of sufficient magnitude continually to re-create the pressure variation in question in the lower stratum, does seem AERONAUTICS. 19 to be sufficient continually to realign the passing air currents of the lower stratum, which in turn are responsible for the pressure varia- tion. This realignment keeps up with the movement of the air in the upper of the two strata and results in a moving disturbance in the lower of these strata. This mechanism, more or less modified by atmospheric conditions, such as temperature distribution and humidity, in the lower stratum, seems in general to control the motion of traveling disturbances in the surface stratum of the atmosphere and explains the fairly well established fact of observation, viz, that all traveling disturbances in the surface stratum have, in general, the speed and direction of the air movement in the stratum immedi- ately above. Tropical hurricanes, for example, seem to move with the antitrades, the high and low pressure areas of the middle lati- tudes with the upper westerlies. The same mechanism is operative in the "separation" of lows from a semipermanent low and of highs from a semipermanent high. The semipermanent high or low is a surface stratum phenomenon and has its position fixed by local conditions and must itself remain in place, but it does create the disturbance or gust in the moving air of the stratum immediately above, which is carried forward and in its turn interacts with the air of the lower stratum. The result is that a traveling high or low, as the case may be, seems to "separate" from the semipermanent high or low and continue in the general direction of the upper cur- rent. A traveling disturbance is likely to decrease in intensity when there is a southward component in its motion as a whole and to increase in intensity, when there is a northward component in its motion, in accord with the value, 2«p V sin X, which increases with latitude. FIG. 1.— DISTURBANCE CAUSED TO LEEWARD OF A SMALL ROUND TOWER DURING A HIGH WIND. AERONAUTICS. 23 effects of changes in the horizontal speed of air movement. (See part 1, p. 14.) The extent of disturbances caused by topography. — Depending on the contour of the earth's surface and on the speed of the wind, gusts of this sort may extend to a height of 40 or 50 meters (130 to 160 feet) or higher. That part of the Blue Ridge Mountains on which Mount Weather is located rises fully 300 meters (1,000 feet) above the valley floors on either side of the ridge. The disturbing effect of this ridge on a wind at or nearly at right angles to the ridge has been observed to extend from 700 to 900 meters (2,250 to 3,000 feet) above the mountain top. The greater heights were observed in the winds of higher speeds. It is probable that disturbances in the lower air brought about mechanically by such obstacles in the path of the wind as trees, buildings, hills, and mountains seldom extend higher than four times the height of obstacles above the general level of the earth's surface in the vicinity of the obstacles. The nature of the disturbance caused depends on the contour of the obstacle and wind speed. Figure 1 illustrates one type. Another and a more common type has frequently been observed on a large scale at Mount Weather. In the cases above mentioned, where the disturbance extended to a height of 700 to 900 meters (2,250 to 3,000 feet) above the top of the ridge, the effect of the ridge was to deflect the air upward at all levels above the top of the ridge and for some distance to the leeward. The air in the lower part of this deflected current, soon after passing the top of the ridge, had a downward component of motion which brought it to the valley floor at a dis- tance away from the foot of the mountain, depending on the wind speed. The horizontal component of the motion of this air de- creased so that at a distance of sometimes as much as ten times the height of the ridge above the valley floor, the air was descending vertically on the valley floor, part of it returning toward the foot of the mountain and part of it going on out into the valley. That part of the air returning toward the ridge continued up the side of the mountain to near the top, turning here again to join the general current. In one instance a wind of 27 meters per second (60 miles per hour) carried away a kite that was flying at a height of less than 1,000 meters (3,300 feet) above the mountain top. The falling kite was followed by means of a theodolite in order to determine accurately its landing place. After it passed below the level of the mountain top it seemed to descend rapidly into a well-known patch of woods about 3 kilometers (2 miles) from the station; but before reaching the woods it was caught in a fairly strong wind blowing toward the mountain and carried back to within about 2 kilometers (1£ miles) of the station before it finally settled to the earth's surface. Disturbances caused by local heating. — The earth's surface varies considerably with regard to its power to absorb radiation and thus becomes more or less heated locally. (See part 1, pp. 10 and 11.) Abrupt and frequent variations of this sort in the vicinity of an aviation field need close attention. They intensify considerably the condition of "roughness" or "bumpiness" of the air that prevails at certain times and places, sometimes to the height of 1£ kilometers (about 1 mile) above the earth's surface. Assume that the air over- lying a certain area is in stable equilibrium at the time insolation be- gins on any clear day. A part of the sun's heat incident on the outer atmosphere reaches the earth's surface but without affecting to any 24 AERONAUTICS. considerable extent the condition of equilibrium of the air through which it passes. A large part of the sun's heat incident on the earth's surface is absorbed and the surface thereby heated. The air in con- tact with the earth's surface is heated by conduction, decreases in den- sity somewhat and changes places with relatively denser air imme- diately above it. Thus, the process of transformation from a condition of stable to a condition of neutral orof unstableequihbriumisinitiated. It has been shown (see part 1, p. 14) that the rate at which this trans- formation proceeds upward depends on the rate of heating of the earth's surface and on the initial stability of the atmosphere imme- diately above it. Over a plowed field, therefore, which absorbs a cer- tain amount of the sun's heat, the transformation will proceed upward at a given rate. Over a neighboring field of, say, wheat stubble, the surface of which reflects more and absorbs less of the sun's heat than does the surface of the plowed field, the transformation will proceed at a slower rate. At some equigravic surface, below the transforma- tion level over the plowed field, but above this level over the stubble field, the density of the air over the plowed field will be less than that over the stubble field, and, as indicated in paragraph 9, chapter 1, a local convective system the dimensions of which depend on the areas of the fields in question will be set up by way of distributing the heat- ing effects of the two fields. It is clear that small local convective systems of this sort must occur in such a way as to either increase or decrease the general air movement over this part of the earth's surface and that vertical components of air movement will also be added, thus increasing the "roughness" or "bumpiness" of this air to an airplane passing through it. Climatic factors. — When a larger territory is considered, climatic conditions are found to vary greatly from place to place. These climatic conditions determine in a general way what aeronautic work can best be done in any part of the country and the desirability of any E articular place as a location for an aeronautic field and base station. »ata are now in the Weather Bureau files sufficient for the purpose of comparing different places throughout the country with respect to their accessibility by aerial routes for different types of aircraft, i. e., the sort of aerial harbor, ease of entrance and exit, and other things con- sidered which aircraft would find at these places. These datafor any place and the inferences based on them can be made available on short notice. The climatic factors that most need consideration in this connection are outlined as follows: 1. Wind: (a) Direction. (6) Speed. 2. Temperature: (a) Normal. (&) Maximum and minimum. 3. Precipitation: (a) Normal. (6) Excessive. 4. Fog. 5. Relative humidity. 6. Cloudiness. 7. Air pressure. 8. Thunderstorms, tornadoes, and hurricanes. AERONAUTICS. 25 Use of climatic data. — Some discussion of the separate topics in the above outline may serve to indicate the reason for their con- sideration, the form in which the data can be found, and the way in which they can best be used. In general data should be considered by months and the suitability of any location as an aerial harbor determined in this way. A place may be an excellent one for aero- nautic operations during certain months of the year but less suitable during other months. This distribution of suitability throughout the year is of prime importance and the study of the data by months enables it to be determined with sufficient closeness. Wind speed and direction. — The prevailing direction of the wind and the mean speed of the wind from each direction at a place selected for the establishment of an aeronautic station should largely deter- mine the layout of the grounds and the orientation of the buildings. All aircraft, but specially those of the lighter-than-air types, can leave or enter a hangar with greater ease and with less likelihood of damage to the craft when going into the wind than when the wind or a con- siderable component of it blows across the entrance of the hangar. For heavier-than-air machines it is specially important that the field be so laid out as to furnish plenty of clear way for launching and landing parallel to the prevailing wind directions. Prevailing wind direction by months and also by hours for the 24-hour period, together with the mean speed of the wind from each direction, can be furnished by the Weather Bureau for a large number of places well distributed throughout the country. Gales. — Winds of high speed render the launching and landing of aircraft difficult. The limit of wind speed above which it is inadvis- able to attempt launching aircraft varies with the type of machine. Pertinent information on this point is found in the Weather Bureau records of the number of days with gales at its different stations. Wind direction and gustiness. — It is much more difficult to han- dle aircraft in a gusty than in a smoothly flowing wind. Gustiness is usually measured and expressed as the rate of change from second to second in the horizontal speed of the wind. Change of speed in the horizontal direction is, as a rule, accompanied by more or less change of speed in a vertical direction, and may be taken as a fair indication of the turbulence of the air current from the aviator's point of view. As a rule west winds are gustier and therefore "bumpier" than east winds. This difference in the nature of the winds is not based on the wind direction at all but follows largely from the relative densities of the air flowing in these currents. The air in a west wind, because it is moving in the same direction as the surface of the rotating earth, exerts a very slightly greater downward pressure, depending on its rate of movement, than does air of equal density moving from the east. If two such currents met, the tendency would De for the west wind to keep to the earth's surface, forcing the east wind to 'a higher level. It is also true that the air of westerly winds in this country is usually drier and colder and therefore denser than the air of easterly winds. As a result of this difference in density the westerly wind lies close to the bottom over which it is flowing. If this bottom be a topographically rough part of the earth's surface much local expansion ana contraction, or gustiness, will occur in the westerly air current as it adapts itself to the topography. Such gustiness would not occur in a wind that did not he so close to the bottom over which it is flowing. A sea breeze, for example, regardless 26 AERONAUTICS. of its direction, is likely to be much freer from gusts and consequently smoother than the land breeze over the same point. The air of the sea breeze flows over a comparatively smooth, uniform surface, while that of the land breeze flows over a comparatively rough solid surface the nature of which may also vary greatly from place to place. Air temperature and "roughness." — The normal air temperature at a place used as an aeronautic base, also the minimum temperatures likely to be experienced, is of interest in connection with the opera- tion of the engine and the comfort of the pilot and others handling the machinery. The maxima of temperature likely to be experi- enced are of interest from this and also from an entirely different point of view. It is likely that all days with temperatures 32° C. (90° F.) or above will be days on which the surface air is quite rough because of considerable convectional action. The depth of the rough or bumpy air increases during insolation until well into the afternoon. Depending on the length of the diurnal period of insolation the maxi- mum depth of this condition on a clear day is likely to be 1 to 1| kilometers (} to 1J miles). It is turbulence of this type that is specially likely to be increased by sharp variations in the nature of the earth's surface with respect to its ability to absorb or reflect solar radiation. Precipitation. — Precipitation, whatever its form, is not neces- sarily a hindrance to flying, but the added weight on the aircraft, as well as the poor visibility, are great handicaps, and excessive pre- cipitation of any form may render flight impossible. The added weight of rain or snow on the craft may be considerable. A snow cover, especially when the snow has freshly fallen, so changes the appearance of the earth's surface and covers up roads and other lines that the aviator has difficulty in keeping his course. Fog.— Fog is a serious hindrance to aerial navigation. It is prac- tically impossible to keep a course in a given horizontal plane or indeed to know in what plane the craft lies. Humidity. — Whether the relative humidity is high or low is not an important consideration unless the air is so extremely dry that special care is required in seasoning the wood used in the propellers and other parts of the craft, as well as in the construction of these parts and in the glues and varnishes used. The amount of moisture m the air affects the operation of the gasoline engine, but this is a point of secondary consideration. Cloudiness. — While in military operations the aeronaut may use a cloud cover to advantage, the average pilot in times of peace pre- fers a clear sky when he flies. The presence in the sky of certain types of clouds, viz, clouds which form because there is an upward component in the movement of the air below them, indicates a more or less turbulent condition of the air from the earth's surface up to the cloud base as well as within the cloud itself. Such clouds may be scattered cumuli or in the form of a fairly solid cover. In every case the bases of the separate clouds or of the cover appear sharply denned and uniform in height. The disturbed condition of the an- within the cloud may well be thought of separately from that below the cloud base since in the cloud other causes of turbulence are in operation owing to the presence of the cloud particles. These causes produce turbulence within clouds of other types as well as mthose which form in air having a vertical component in its motion While the pilot may expect "rough" air within any cloud, the pres- AEKOKAUTICS. 27 ALT. S - e 10 DEGREES CENTIGRADE 12 14 16. 19 ALT- km. 35 3 2.5 2 !5 » .5 .396 B km. • ' s sw 19.2 in t a n.p.t. -© A -* D SCEN" E5CE 3 2.5 2 1.5 I 5 .396 ] \ - JT \ v V \\ V \ ^ W 1£ 4m.il. . sw 16.5' .p.,„ X* A SW 16.5 n.p.s.^ ^ \ \ _sw 16.5 / > 163 m.p.s. ^ S 17 m.p.t \ \ ssw SJn. )..(. N\ \ v V ^ \ \ T BA SEOF T!0;3 Sl-Cu. N> C .STAT ENTRAL OVER ION AT I2M0 p.m. \ vN / *" M fWNW 13.6 w.pj. \ V L_ \ WNW B 15.3 ISE0F TIK' m.p.s, St.Cu a.m. K S. . 14.2 p. E> A \ \ 4 E 16.; m. P * \ d \ V \* SE. i: 5.ffKp i. ^ \ \ ^ \ \ ^ \ U 7.2 m.p.s. 12:50 p.m- \. \ . ESE. I5.6m.p.s. Nj__L. . 3 3 to 12 " 14 16 " IB DEGREES CENTIGRADE Fig. 3.— Free air conditions at Drexel Aerological Station during thunder- storm on June 21, 1916. 28 AERONAUTICS. ence of the convectional cloud forms, specially of scattered cumulus clouds, leads him to expect "rough" air below the cloud level as well. (See part 1, pp. 9 and 10.) . . Frost and ice formations on aircraft. — Aircraft remaining for some time in a fog or cloud layer, when the latter is at low temper- atures, are likely to collect a considerable weight of moisture in one form or another. This is specially true of the lighter-than-air craft. Cloud and fog particles maintain themselves in liquid form at temperatures far below freezing. If this be because of the sur- face tension of the particle, any influence that breaks down this tension would permit the freezing in one form or another of the water of the particle. Assuming that aircraft within a fog or cloud layer at temperatures well below freezing is electrically charged, surface tension of fog or cloud particles approaching it will be broken down and, depending on the size of the particle and the temper- ature, freeze on the cold surface of the aircraft in crystalline or amorphous form. Similar conditions, but with higher tempera- tures, may result in the collection of liquid moisture on the surface of the craft. The air pressure. — The air pressure, or the height above sea level of an aeronautic base station, affects chiefly the operation of the gasoline engines. Traveling disturbances considered. — While this chapter concerns itself chiefly with the sort of aerial harbor that can be had in any E articular location and not with the laying or pursuit of a course etween two points, it is thought best not to divide the considera- tion of the next topic — thunderstorms, tornadoes, and hurricanes — although it has to do with courses pursued between two stations as well as with the accessibility of either station by aerial routes. Thunderstorms, hurricanes, and tornadoes. — These are disturb- ances in the lower atmosphere of relatively small area and may often be seen approaching in good time for the pilot, if he be in the air, either to fly around them or to land until the disturbance has passed. The aeronaut would benefit greatly by some knowledge of the nature and extent of these disturbances. Frequency of thunderstorms, etc. — The frequency of occurrence of disturbances of these sorts in a given region may be taken as an indication of the turbulence of the air in that region. Consequently data on thunderstorm frequency, for example, would be of value in the consideration of a location for a base station. Such data are in the Weather Bureau records and are readily available. A thunderstorm described. — During a kite flight of June 21, 1916, at the Drexel Aerological Station, 32 kilometers (20 miles) west of Omaha, there occurred a typical thunderstorm. Figure 3 shows the temperature gradient observed in both ascent and descent. Wind directions and speeds, cloud base heights, and other notes are made on the curves showing the temperature-altitude relation. Figures 4, 5, and 6 are the barogram, thermogram, and hygrogram, respec- tively, recorded during the passing of the storm. The observer of surface meteorological conditions makes the following notes: Thunder to the west was heard at 12.15 p. m. A threatening, dark, purple-tinted cloud was observed to the north and west of the station at 12.30 p. in. Lightning flashes were seen in this cloud at about the same time. The wind changed from ESE. toWNW. at 12.40 p. m. The loudest thunder was heard at 12.46 p. m. Light rain began at 12.47 p. m. and heavy rain a minute later. Last thunder was heard to the northeast at 1.05 p. m. Rain ended at 1.54 p. m. AERONAUTICS. 29 The observer at the kite station makes the additional observa- tions: Reeling in at high speed the kites passed directly over the reel house when the wind changed abruptly from ESE. to WNW., i. e., the wind did not swing in a semi- circle but changed by directly reversing its direction. There was evidence of an upward component in the air's motion when the wind change at the surface took place. The surface ESE. wind, which gradually turned to S. and SW. as the altitude increased, gained considerably in speed as the storm approached. Fig. 4. — Surface pressure changes finches) at Drexel Aerological Station during thiiiV t '!\ • i-4 i 1 §^ •T-l I"* G> o to © rf ■£Sfib g Pi 3 1***; H2 9 . >> . SiSft §1.2 £ 8 WM 3M; ill ES sea §*•■£& *"§* lllli 1 &£ A 3_oi_, t* O a ,sa a sot g fl o| i .sa mco do co r-< cj co c» eo ■* »o O) i> to co rfH »0 jH r~ t>. CO to cm o -* co r- oeqQdfO OJ • »H tM t-H 00 ^ CO 00 U5 t- O rHCOlOCOt-QOC^CO OHacqoaoinaoi^cj) rH CO "O «3 CO « CJ O) «# COr-O^'HOOco^f-^-^OOOOCOCOrflO iHor^do6c»c»oieo'^^)''Ot^<©ioco fficooi(M-'*H»0'"-iOsc~ooaoc-iai OSOONNTjiCOiHt^QOOtt5CO(DOOTfrfdN^ddco i ©oddd(o' NNO>OHmM-*cqo-HW»o: ZHZ 46 AERONAUTICS. TURBULENCE, ETC. Effect of obstructions on wind. — The character of the wind with respect to gustiness also varies with height. The height to which fustiness, arising from variations in level of the earth's surface, trees, uildings, and such obstructions, occurs, depends on the abruptness and extent of the change in level and on the wind speed and direction. Over a plain where the changes in level are owing largely to buildings and trees, this kind of gustiness has probably largely disappeared at a height of 50 to 75 meters. Over rolling country, over more broken hilly surface, and over mountainous regions, the atmospheric disturbances caused by the irregularities of the earth's surface ex- tend to greater and greater heights. Limited experience in the ex- ploration of these topographic effects in the atmosphere indicates that they may extend above the general ground level to more than three times the height of the hill or mountain causing them. Turbulence due to insolation. — Turbulence of the lower air resulting from insolation and the variation from place to place of the nature of the earth's surface, its ability to absorb or reflect solar radiation being considered, extends to heights varying with the hour of the day and with the season. On a clear summer day this turbulence may reach a height of 2 kilometers (1J miles); on a clear winter day 1.5 kilometers, or a little less than 1 mile. The temperature dis- tribution in clear weather and at different heights throughout the 24 hours, Figures 20 and 21, indicates these Emits quite clearly. The afternoon maximum of temperature, which owes its existence to the heating of the earth's surface and the accompanying atmos- pheric turbulence in the lower strata at this time, disappears Detween the 1.5 and 2 kilometer levels in the summer months, and between the 1 and 1.5 kilometer levels in the winter months. Figures 20 and 21 are based on data obtained in one year's observations at Mount Weather. These data are more fully discussed in the Bulle- tin of the Mount Weather Observatory, volume 6, part 5. Flying levels and turbulence. — It is probable that an airplane flying at the heights indicated above would not encounter turbulence or "roughness" of air of the types under consideration. It may be added that many storms, such as thunderstorms, tornadoes, hurri- canes, and cyclones of the temperate regions may be safely passed over at a height of 3 -kilometers (2 miles). Most of these storms occur entirely below the 5 kilometer (3 mile) level. AERONAUTICS. 47 A. M. P. M. 12 I .2 3 4 5 6 7 8 9 |0 H 12 I 2 3 4 5 6 7 6 9 10 II 12 Temp •c 30 50 M. TEMP C 4 3 2 7 25 X> M. 6 s =n 20 00 M. : '13 12 11 9 a 17 15 00 M. 15 10 00 M. 12 1 13 20 19 19 » SI RF AC r 17 ie is |4 s 26 M. • O'B. -Diurnal distribution of temperature for the summer half of the year at different levels above Mount Weather. 48 AERONAUTICS. A. M. P. M. 121 2 34 5 6 78 91011121 2 9 4 5 6 7 8 9 10 11 It TEMP. TEMP. •c ^» •c 3 3 X> > W / S \ (■ ■i ' S' V ' \ -s \ 5C ( 1, \ -2 -3 >L \ ~s ^ / -4 i-5 20 30 M. >v -4 15 DO M. -2 3 2 e IC OO M. 1 7 ■» 6 5 4 3 2 SI. RF 26 AC M. E 1 O'B. Fig. 21.— Diurnal distribution of temperature for the winter half of the year at different levels above Mount Weather. AERONAUTICS. 49 Temperature change with height. 1 — Figures 22, 23, and 24 are based on temperature observations at different heights above Mount Weather. Figure 22 shows the variations in temperature at the earth's surface and at higher levels, which may be expected during Fig. 22.— Mean free air temperatures above Mount Weather, Va. (Heights in meters.) f S V V (q P \ s \ \ \ \ \ !£* If^l s \ \ \ \ \ :^\ 1 ■■ ::; \.. ■• £f.V\>> \ V N \ \ \ \ \ \ \ \ \ \ \ \ A - :: I ik\ — 1 I Hi )' Fig. 23.— Temperature distribution in " (Heights in km.) the year. Figures 23 and 24 are a more detailed study of the verti- cal temperature distribution in different quadrants of the high and of the low pressure areas, respectively. They indicate that, even in 1 The data used in paragraphs 15 to 18, inclusive, and in paragraph 22 are based on five years' observa- tions at Mount Weather, va. See Bulletin of the Mount weather Observatory, vol. 6, pt. 4. 22417°— 17 i 50 AERONAUTICS. the mean, the temperature does not always decrease with height. Strong inversions of temperature below the 2-kilometer level are to be expected during the winter half of the year, especially in the high- pressure area. As has been shown in Part 1, pp. 14 and 15, this type of vertical temperature distribution indicates very stable atmos- pheric equilibrium. In tho.summer half of the year the temperature 5: — \ 1 — r~T* -% ^ \ \ \ \ \ U — il :ir a* \ 1 \ \ ^ \ \ \ \ * ' •\ \ \ asv !$t\ ■•ST-Vi " " «»**» -... 1 1 \ \ \ \ \ \ \ , \ \ \ \ \ \ \ \ \ ,»v \ ••» \- aa\ .< " lid I*. L. -»■« ■■ I I. Sfc T(UP(>*f u«( oinilh- \ v_ \ \ ) \ \ ) l'A\- iui\ ■ Si! * ♦»( ... .... ,., .. «. I V \ \ \ \ \ \ \ \ \ \ \ \ \ k «v \ 11 u- • g !\» ?§.,._ •• Fio. 24.— Temperature distribution in " Lows " observed at Mount Weather. (Heights in km.) change with height is more rapid in this region of the atmosphere, especially in the first few hundred meters, than in the winter. Inver- sions of temperature are infrequent and of more or less accidental types. The condition of equiubrium is therefore less stable in the summer than in the winter months, and turbulence is more likely to occur. o •■