General Circulation of the Atmosphere ONE OF A SERIES OF PAMPHLETS ON Important Topics in Geography By DOUGLAS C. RIDGLEY Professor of Geography , Illinois State Normal University Normal , Illinois SiOLQQY published by Mcknight & Mcknight Normal, Illinois 9 5 S\ ( S\ ' ' o r / U: NOTE This pamphlet is the first of a series contem- plated by the writer dealing with Important Topics in Geography which can not receive adequate treat- ment in the usual text books because of the limited space which must necessarily be given to them. The topic treated in this pamphlet — General Circu- lation of the Atmosphere — is fundamental to any causal study of the geography of the world so far as it deals with the amount and distribution of rainfall and its consequences; namely, vegetation, animal life, distribution of population, etc. To know the wind belts of the earth as to loca- tion and direction of air movement is to have the key by which to interpret the rainfall of a region. To know the temperature and rainfall of a region enables the student to interpret its vegetation. To know the natural vegetation of a region gives a basis for the study of the settlement and industrial development of a country. Other pamphlets under consideration at the pres- ent time are: Rainfall of the Earth; Vegetation Zones of the earth ; and Population of the Earth. These are intended for use in the geography classes of the upper grades and in the physical and commercial geography classes of the high school as supplementary to the regular text book treatment of the same topics. This pamphlet on General Circulation of the Atmosphere covers fully the topic as outlined in the Illinois State Course of Study. Douglas C. Ridgley. Department of Geography, 1 \ Illinois State Normal University, ' ^ *. Normal, Illinois. ° 4 ''048 Copyright 1910 By Douglas C. Ridgley GENERAL CIRCULATION OF THE ATMOSPHERE Introduction. To understand the geography of the earth we need to know much about the surface of the land, the temperature, rainfall, vegetation, animal life, and the people of the various regions of the earth. We also need to know the reasons for these things as we find them in different continents or countries. We all know that the air, or atmos- phere, is almost always in motion. When the air is not in motion we say that it is calm. Moving air is called wind. The winds may be light, moderate, brisk, or high. The regions of the world having calm air at any time are very small in comparison to the regions having winds. The general circulation of the atmosphere in- cludes the usual condition of the air as found in large areas of calms or winds on the earth’s surface. Its study will teach us the location and extent of these regions of calms and winds in general, and their shifting north and south with the change of seasons. It will also give us some understanding of the rea- sons for rainfall and for the distribution of plants, animals, and people in the various regions of the earth. Some Facts to be Remembered. There are many important things about the atmosphere which have been learned by the careful study of scholars, and which boys and girls may know and use in their study 4 IMPORTANT TOPICS IN GEOGRAPHY of geography, and in the observation of many things in their daily experiences. Some of them are the following : 1. Air has weight. Fill a glass jar, or a bottle, with water. Invert it with the open end just under the water in a pan or pail. Does the water run out of the jar or bottle? Why? Try with the longest glass vessel or tube you can get. At sea level air pressure is about 15 pounds to the square inch. This means that a column of air one inch square and extending to the top of the atmosphere (more than 100 miles) weighs 15 pounds. The air in a box or a tank 10 ft. by 10 ft. by 5 ft. at freezing temperature weighs about 38 pounds. 2. Air, when warmed, expands; when cooled, contracts. This means that as air grows warmer, the same air occupies more space than before it was warmed. If it grows cooler, it occupies less space than before it was cooled. Think of a space 10 ft. by 10 ft. by 5 ft. filled with air at a temperature of 32 degrees, freezing temperature. You have 500 cu. ft. of air. If this air is warmed 1 degree, to 33 degrees, it will expand and occupy 501 cu. ft. of space. If cooled 1 degree, to 31 degrees, it will contract, and occupy 499 cu. ft. of space. If warmed to 52 degrees it will occupy 520 cu. ft., if cooled to 12 degrees it will occupy 480 cu. ft. 3. Warm air is lighter than cold air, other con- ditions being the same. The 500 cu. ft. of air at freezing weighs about' 38 pounds. Warmed to 52 de- GENERAL CIRCULATION OR THE ATMOSPHERE 5 grees, it occupies 520 cu. ft., but still weighs only 38 pounds as before; 500 cu. ft. of the warmer air weighs less than 3 6% pounds. Cold air on a mountain top is lighter than the warmer air at the base of the mountain because the conditions are not the same. The air at the base of the mountain is under greater pressure than the air higher up. 4. Warm air can hold more water vapor, or moisture, than cold air. At 32 degrees air cannot hold very much water vapor, at 50 degrees it can hold about twice as much as at 32 degrees; at 70 degrees nearly four times as much as at 32 degrees ; and at 90 degrees nearly eight times as much as at 32 degrees. 5. Moist air is lighter than dry air, other con- ditions being the same. We usually speak of the “heavy” air of a warm, damp, cloudy, sultry day; and of the “light” air of a cool, clear, dry day. This is because of our feeling, not because of the weight of the air. The barometer, which is an instrument for measuring the pressure of the air, shows clearly that the pressure is greater, therefore, the air heavier, on clear, dry days, than on damp, rainy days. Find a description and a picture of a barome- ter in your school geography or in a physical geog- raphy, and learn how it records the pressure or weight of the air. If you have a barometer at school, watch it each day for a month or more and learn for yourself what it tells about air pressure on different kinds of days. 6 IMPORTANT TOPICS IN GEOGRAPHY Circulation of Air on a Small Scale. Hold a smoking match below the burner of a lighted lamp. The smoke can be seen passing upward through the chimney. Hold the hand above the chimney of the lighted lamp. The current of warm air moving upward can he plainly felt. The air around the flame of the lamp is heated; it expands; it becomes lighter ; the cool air around the lamp is heavier than the warm air inside the lamp chimney; it enters through the openings on the under side of the burner, and pushes the warmer, lighter air upward through the chimney. The air which entered through the burner is now heated, and pushed up- ward by additional air entering, and so the move- ment of the air about the lamp goes on as long as the flame continues to burn. The smoke, rising from the chimney of a house, tells the same story of the air movement as the lamp. State it. The leaping flames and flying cinders from a bonfire tell of the warm, expanded air near the fire being pushed rapidly upward by the colder, heavier air moving inward near the ground. The warm, quivering air seen over a hot stove on a winter day illustrates the same thing as the lamp or house chimney. The air near the stove is heated rapidly by the hot stove. It expands, grows lighter, and the colder, heavier air on all sides pushes the hot air rapidly upward. It carries heat upward and outward to all parts of the room. Let us suppose that our school room is heated by a steam pipe running entirely across the middle GENERAL CIRCULATION OP THE ATMOSPHERE 7 of the room from east to west near the floor. Let us have the north and south windows open a few inches at the top. After a cold night, all parts of the room have been cooled to the same temperature and there is no circulation of the air. When the hot steam passes into the pipe in the morning let us see what takes place. 1. The heat of the steam passes through the solid iron. Heat passes through a solid body in this way by the process of conduction. 2. The heat passes from the pipe into the air surrounding it. The air in contact with the pipe is heated by conduction. The heat radiates or passes out into the air in all directions, and we say that the air is heated by radiation also. 3. As the air near the pipe is warmed, it ex- pands and grows lighter. The colder, heavier air on the north and south sides of the pipe pushes against the warm, light air near the pipe, and it rises. This is the beginning of the circulation of the atmosphere of the room. The air moves inward along the floor from north and south toward the hot steam pipe; the air near the pipe moves upward, carrying heat to other parts of the room. This process of carrying heat by currents of air is called convection. The air then moves outward to the north and south along the upper part of the room, and downward in the north and south parts of the room, to move inward again along the lower part of the room. Some air passes out through the north and south windows which are partly opened, accord- 8 IMPORTANT TOPICS IN GEOGRAPHY ing to our supposition, and some air passes in through the windows. Get the above conditions clearly in mind so that you readily see in imagination the air movements described. Picture in your mind the circulation of air in the north half of the room, and in the south half of the room. Circulation of the Air on the Earth Without Eotation. If the earth did not rotate, winds would move along north-south lines as the air moved in the school room heated by a steam pipe. Let us apply the illustration of the school room to the earth’s surface. A warm belt of air several hundred miles wide extends east-west around the earth near the equator. This belt corresponds to the warm, rising air near the steam pipe. It is called the belt of equatorial calms, or the doldrums. The cooler, heavier air on the north and south sides of the doldrums moves towards the doldrums as the air moves along the floor of the school room towards the steam pipe. These are the trade winds. As the air of the doldrums rises, it flows off to the north and sonth as the air moves outward along the upper part of the school room. These are the antitrade winds. The name means against the trades. Between 30 and 35 degrees north and south lati- tudes much of the air of the antitrades settles to the earth. This downward moving air produces calms, which are called the horse latitudes. They GENERAL CIRCULATION OF THE ATMOSPHERE 9 correspond to the downward moving air in the north and south parts of. the school room. Some of the downward moying air of the horse latitudes turns toward the equator and supplies the trades. Some moves on toward the poles. These winds moving from the horse latitudes towards the poles are the prevailing westerlies, but if the earth were not in rotation they would move along north- south lines. They correspond, in some measure, to the air which passed through the windows at the north and south ends of the school room. The Naming of Winds. It is important to be able to name winds accurately and quickly, to tell wind directions by weather vane, or other means, and to represent winds on charts and maps. Winds are always named by the direction from which they blow. Thus a wind moving from the south toward the north is a south wind, from the northeast to the southwest a northeast wind. The arrow of the weather vane points against the wind and not with the wind. See if you can tell why. When the weather vane points west, we have a west wind, because the wind is blowing from the west. Winds are represented on charts and maps by means of arrows. The arrows are drawn as if flying with the wind. An arrow on a chart or map pointing to the east represents a west wind. Name the wind when the arrow of the weather vane points north- west. Represent the same wind by an arrow on the blackboard. A flag is floating northeast from the flagstaff. Name the wind. Represent it by an 10 IMPORTANT TOPICS IN GEOGRAPHY arrow. Which direction would the weather vane point for this same wind! Answer the same points when you see smoke from a chimney floating away to the south. Are you sure that you can name winds accurately and quickly, and can you represent them correctly on a chart or map? Circulation of the Air on a Rotating Earth. If the earth did not rotate, all winds in the general circulation of the atmosphere would be either north winds or south winds. Our earth is a sphere 8,000 miles in diameter, 25,000 miles in circumference, rotating from west to east once in 24 hours. The reason for the change of wind direction because of the earth’s rotation is too difficult to he understood by boys and girls, hut they can learn to what extent the winds are changed by the earth’s rotation. Memorize the following law of winds: Because of the earth’s daily rotation from west to east , winds in the northern hemisphere are turned , or deflected, to the right hand; and winds in the southern hemi- sphere are turned, or defected, to the left hand. Suppose you were to start at the north horse latitudes, 30 degrees north latitude, and go toward the doldrums, turning somewhat to the right as you go. In what direction would you he going? You would be traveling from what direction? A wind following the same line would receive what name? This is the region of the northeast trade winds. Suppose you were to start at the south horse lati- tudes, 30 degrees south latitude, and go toward the doldrums, turning somewhat to the left as you go. GENERAL CIRCULATION OP THE ATMOSPHERE 11 In wliat direction would you be going? You would be traveling from wbat direction? A wind following the same line would receive wbat name? This is the region of the southeast trade winds. The air which rises at the doldrums spreads north and south over the trade winds several miles above the surface of the earth, and moves from the dol- drums toward the poles. In the northern hemi- sphere these winds move northward hut are deflected to the right by the earth’s rotation, and thus become the southwest antitrades. In the southern hemi- sphere they move southward but are deflected to the left by the earth’s rotation, and thus become the northwest antitrades. Between 30 and 35 degrees north latitude, and between 30 and 35 degrees south latitude the air of the antitrades settles downward in the horse latitudes. Some of this downward moving air turns hack toward the equator and supplies the northeast trades and the southeast trades. Some continues toward the poles in the same general di- rection as the antitrades. These winds are the prevailing westerlies. Since the westerlies are sur- face winds moving in the same direction as the antitrades, the ivesterlies of the northern hemisphere are southwest winds, and the westerlies of the south- ern hemisphere are northwest winds. The law of winds given above may be stated in other words as follows: Because of the earth’s daily rotation from west to east: winds moving equatorward are turned, or deflected, to the west; 12 IMPORTANT TOPICS IN GEOGRAPHY and winds moving poleward are turned, or deflected, to the east. The Wind Belts of the Earth and Their An- nual Movement. One ray of the sun meets the Figure 1. WIND BELTS OF THE EARTH IN AVERAGE POSITION. From Figure 1 learn the names of the nine wind belts of the earth. Give the name, latitude and direction of air movement in each of the nine wind belts. If you learn these facts first about the dol- drums and the horse latitudes, you can learn the latitudes of the other wind belts more easily. Two of the nine wind belts do not touch the surface of the earth. Which are they? How are they represented? curved surface of the earth so as to make a right angle with the earth’s surface at that point. This is the vertical ray of the sun. All other rays are GENERAL CIRCULATION OF THE ATMOSPHERE 13 oblique rays. If the vertical ray of the sun were always on the equator the wind belts of the earth would remain in the same position throughout the year. But the vertical ray of the sun moves, or migrates, northward to 23% degrees north latitude on June 21, and southward to 23% degrees south latitude on December 21. When the vertical ray is No™ 11 b j t t-. dlfc- Figure 2. VERTICAL SECTION OF WIND BELTS IN AVERAGE POSITION. Turn the page so that “North” in the figure is actually north. Compare figure 2 with figure 1. Imagine yourself looking at a section of the air, as shown in the figure, extending upward ten or twelve miles above the earth’s surface, and from pole to pole. In this figure point out and nam£ the nine wind belts and give latitude and direc- tion of air movement in each as in figure 1. The arrows pointing equatorward in the middle of the westerlies do not represent a distinct wind belt. They simply indicate that the air which has been carried poleward by the westerlies must return equatorward. Just how this is done is not fully known. north of the equator, the northern hemisphere has longer days and steeper rays than the southern hemi- sphere, and so receives most of the sun’s heat. Make a similar statement for the southern hemi- sphere when the vertical ray is north of the equator. Make similar statements for both hemispheres when the vertical ray is south of the equator. The doldrum belt moves north and south, but not so far, nor so rapidly as the vertical ray of the sun. The doldrums move farther from the equator over 14 IMPORTANT TOPICS IN GEOGRAPHY land than over the water. Wherever the doldrums pass there is heavy rainfall. The amount of their migration to the north and the south of the equator on land is best determined by the distance north Figure 3. WIND BELTS FOR JULY— NORTHERN SUMMER, SOUTHERN WINTER. Compare figure 3 with figure 1. Give the names, latitude and direction of air movement in the nine wind belts for July. Give first the doldrums and horse latitudes. and south of the equator that we find considerable rainfall due to this annual migration of the dol- drums. Your school geography probably contains a good rainfall map of the world and of each of the conti- GENERAL CIRCULATION OF THE ATMOSPHERE 15 nents. Find these maps and make frequent use of them as you study geography. In this pamphlet reference will be made to maps of Longmans’ New School Atlas, price $1.50. This Atlas should be in Figure 4. WIND BELTS IN JANUARY— NORTHERN WINTER, SOUTHERN SUMMER. Compare figure 4 with figures 1 and 3. Give the names, latitude and direction of air movement in the nine wind belts for January. Make a table of four columns. In column 1, place names of wind belts; in column 2, the average latitude; in column 3, latitude for July; in column 4, latitude for January. every school. Obtain it through your local book dealer. By examining the Bainfall Map of the world in your text book or in Longmans’ Atlas we find that that part of Africa lying between 20 de- 16 IMPORTANT TOPICS IN GEOGRAPHY grees north latitude and 20 degrees south latitude receives much more rainfall than the regions farther north or south. The same is true for the West Indies and for those parts of North America and South America lying between and somewhat beyond the same parallels, but the influence of the doldrums is not so clearly marked as in Africa. This fact of heavy rainfall is shown in the East Indies and in those portions of Asia, and Australia lying between, and in some cases beyond, the same parallels. It will be helpful in our study of wind belts and their influence on rainfall to consider that the dol- drums migrate northward so that the northern edge reaches about 20 degrees north latitude, and south- ward so that the southern edge reaches about 20 degrees south latitude. The width of the doldrums differs in different places and in different seasons. It will be helpful to consider it as having a width of about 10 degrees of latitude or about 700 miles. The average position of the doldrums may then be considered from 5 degrees north latitude to 5 de- grees south latitude; their most northern position in northern summer (southern winter) from 10 de- grees north latitude to 20 degrees north latitude, and their most southern position in northern win- ter (southern summer) from 10 degrees south lati- tude to 20 degrees south latitude. The doldrums form the center of the entire sys- tem of wind belts. As the doldrums migrate north and south, all other wind belts migrate in the same direction as the doldrums, but not so far. The GENERAL CIRCULATION OP THE ATMOSPHERE 17 average latitude of the horse latitudes may be con- sidered 30 to 35 degrees both north and south of the equator. In the northward migration the north horse latitudes occupy 35 to 40 degrees north lati- tude and the south horse latitudes 25 to 30 degrees south latitude. In the southward migration the north horse latitudes occupy 25 to 30 degrees north latitude and the south horse latitudes occupy 35 to 40 degrees south latitude. These latitudes give us only a general notion of the shifting or migration of the wind belts. The boundaries of the wind belts are not sharply defined, and these boundaries do not follow exactly lines of latitude. The two regions between the doldrums and horse latitudes are always occupied by the trades as surface winds and by the antitrades as upper winds. The two regions be- tween the horse latitudes and the polar areas are always occupied by the prevailing westerlies. Learn the general location of the doldrums and horse lati- tudes and you then know the location of all other wind belts. Make a study of Figs. 1, 2, 3, and 4. In each figure name and locate by latitude the nine wind belts described above, and give the direction of air movement in each. Give your description in com- plete correct sentences. Give latitude fully. Al- ways using the words “degrees” after the number, and state whether it is “north latitude” or “south latitude.” The latitude nearer the equator should be given first. A Map Exercise. 1 . On an outline map of the 18 IMPORTANT TOPICS IN GEOGRAPHY world trace with pencil the equator. Name the con- tinents, oceans, and the groups of islands crossed. Note what part of each continent is crossed by the equator. Name the countries of South America crossed by the equator; of Africa; three or more islands. To find these names turn to the political maps of the different continents in your school geography. 2. On the same map draw 20 degrees north latitude and 20 degrees south latitude. From maps in your geography name the continents, oceans, other water bodies, islands, and the countries crossed by each parallel. Parts of five continents lie between these parallels. Write the names of these five con- tinents and under the name of each continent write the names of the countries which lie wholly or partly between these two parallels. These are the coun- tries of the world which are influenced by the dol- drums in their annual migration. During a part of the year these same regions are influenced by the trade winds. 3. On the same map draw 30 degrees north lati- tude and 30 degrees south latitude. Name the coun- tries crossed by each. The horse latitudes and the trade winds usually bring little rainfall. The countries between 20 degrees and 30 degrees of lati- tude both north and south of the equator lie almost wholly in the trade winds and horse latitudes throughout the entire year. Therefore, they have little rainfall as a rule, and contain extensive deserts. Examine a vegetation map in your geography or GENERAL CIRCULATION OF THE ATMOSPHERE 19 in Longmans’ Atlas and note the regions of desert land between 20 degrees and 30 degrees latitude in both hemispheres. Write a list of countries between these latitudes in the northern hemisphere, another list for the southern hemisphere. If your school is supplied with large wall maps of the continents, study them and compare them with the maps in your geography. 4. The countries between the latitude of 40 de- grees and the poles are in the prevailing westerlies for all of the year. Countries between 30 degrees and 40 degrees latitude are influenced for a part of the year (winter) by the westerlies, and for the rest of the year (summer) by the horse latitudes and trades. Each of the six continents contains regions influenced by the westerlies. Write the names of the six continents, and under the name of each con- tinent write the names of the countries which lie between 30 degrees and the poles. These countries are in the westerly winds for all or a part of the year. In southern Europe and western Asia the influence of the dry horse latitudes is carried some- what farther north than 40 degrees north latitude in northern summer, probably as far as 43 degrees. Test your imagination to see whether you can think of the earth as a whole, spherical in shape, 8,000 miles in diameter, 25,000 miles in circumfer- ence, surrounded by an atmosphere 100 miles or more in thickness. One-half of the atmosphere, by weight, is within 3.6 miles, and nearly all of it is 20 IMPORTANT TOPICS IN GEOGRAPHY within 10 miles, of the earth’s surface. The higher atmosphere is very rare. Think of the wind belts we have named in their average latitudes and the air moving up in the dol- drums, down in the horse latitudes, equatorward in the trades, and poleward in the antitrades and west- erlies. Think of the vertical ray of the sun moving northward at the rate of about 18 miles per day, the doldrums migrating only about half as fast, and the horse latitudes shifting still more slowly. Then think of the reverse movement to the south. Think of all these things as they really are on the earth. Learn your home latitude, and think how far each wind belt is away from you in degrees of latitude, and in miles. One degree of latitude should always he counted 70 miles in computing distances in geog- raphy. Degrees of longitude differ in length from 70 miles at the equator to zero at the poles. One degree of longitude at 30 degrees latitude is 60 miles; at 40 degrees 53 miles; at 50 degrees 45 miles; at 60 degrees 35 miles; at 70 degrees 24 miles; and at 80 degrees 12 miles. Monsoons. The foregoing description of wind belts has not taken into account the influence of large areas of land and water in modifying the winds. In India and elsewhere we have winds known as monsoons. India furnishes the best ex- ample of monsoons in the world. Monsoons are seasonal winds, that is, they change their direction in summer and winter. They blow from the ocean toward the land in summer, these are the summer GENERAL CIRCULATION OF THE ATMOSPHERE 21 monsoons; and from the land toward the ocean in the winter, these are the winter monsoons. Monsoons are caused by the difference in tem- perature between the ocean and large land areas in summer and winter. With the long days and steep rays of northern summer the great land mass of Central Asia becomes much warmer than the water of the Indian Ocean, because land heats more rap- idly than water. The warm air over the land ex- pands and grows lighter. The cooler, heavier air over the ocean blows strongly toward the land, car- rying great quantities of water vapor over India, producing abundant rainfall. With the shorter days and more slanting rays of northern winter the great land mass of Asia be- comes much colder than the water of the Indian Ocean, because land cools more rapidly than water. The heavier, colder air of the land then blows toward the ocean. This winter monsoon is a cool, dry wind. Some school geographies have maps showing the monsoons of India in opposite seasons. Such maps should be studied and fully understood. Monsoons also occur in southeastern Asia, in Australia, on the west coast of Africa north of the equator, in Spain, and to some extent in other regions. Cyclonic Storms of the Prevailing Westerlies. The prevailing movement of the westerlies from the southwest in the northern hemisphere and from the northwest in the southern hemisphere is interrupted by the passage of cyclonic storms. The cyclone is 22 IMPORTANT TOPICS IN GEOGRAPHY a storm of vast extent, in which the winds blow spirally about a calm center. The winds blow from all directions toward the center, which is a region of lighter air than that surrounding it. The move- ment of these cyclonic storms from west to east across the continent brings to us the ordinary changes of weather from day to day. Since the ivinds blow from all directions, they may carry moisture from any direction in which a large body of warm water is found. Thus the cyclonic storms of Europe carry moisture from the Atlantic Ocean which lies on the west, and distribute it far inland over the great central plain of Europe. The cyclonic storms of North America bring rain to the west coast by westerly winds from the Pacific. The mountains are so close to the west coast of North America, however, that abundant rainfall is found only along a narrow strip of our western states, Canada and Alaska. Examine rainfall map of North America and United States in Longmans’ Atlas, maps 6 and 15, or in your geography. In the eastern half of the United States and Canada the west winds are dry winds because they blow from large land areas to the west. Yet, the eastern half of both countries is much better watered than the western half. Examine a rainfall map of North America and the United States. This is be- cause the cyclonic storms are made up of winds blowing from all directions. The moist, rain-bear- ing winds come from the east, southeast, and south, bringing abundant moisture from the Atlantic GENERAL CIRCULATION OF THE ATMOSPHERE 23 Ocean and the Gulf of Mexico, to fall as rain over the United States and Canada as far west as the Rocky Mountains. The amount of rainfall decreases gradually from the Atlantic and Gulf of Mexico to the northwest, until we reach the desert regions beyond the mountains. See rainfall map. If it were not for the cyclonic storms of the west- erlies, with their ability to carry moisture from the east and south, that part of the United States and Canada lying between the Rocky Mountains and the Atlantic Ocean would he one vast desert, as dry and barren as the Sahara. Tornado is the name applied to a violent whirl- ing narrow storm of the Mississippi Valley. Tor- nadoes occur in the southeast quarter of a cyclone. The tornado is often called a cyclone, hut incor- rectly. In the southern hemisphere the cyclonic storms of the westerlies influence those parts of South America, Africa, and Australia lying south of the vicinity of 30 degrees south latitude. Examine rainfall maps of these continents. Tropical Cyclones. These are large, violent, and destructive storms starting in the doldrums over the oceans at certain times of the year. They occur in the North Atlantic, occasionally doing great destruction in the West Indies and along our Gulf and Atlantic coasts. They also occur in the Indian and Pacific Oceans, causing damage in the East Indies and along the coasts of India and China. Hurricane is the name given to the tropical 24 IMPORTANT TOPICS IN GEOGRAPHY cyclones of the North Atlantic; they are called typhoons in the North Pacific. Maps. The outline maps of the world necessary for the map exercise given on p. 17 may be obtained from McKnight & McKnight, Normal, Illinois, for lc each if taken in lots of 10 or more. Each pupil should have at least two outline maps for the exer- cise. On request, McKnight & McKnight will send a pamphlet, very helpful to teachers, on “Outline Maps and Their Use.” Pictures. The following pictures from the ‘ ‘ 600 set” of lantern slides and stereographs of the Key- stone View Company, Meadville, Pennsylvania, are selected for the purpose of showing characteristic scenes in the different wind belts of the earth. These pictures will aid greatly in giving concrete- ness and reality to the study of a somewhat abstract problem in geography. Each picture should be studied with reference to the wind belt in which it occurs. Each view should become familiar to the pupils as to title, country, approximate latitude, and life forms, whether plant, animal, or human life. Pupils should be taught to study the splendid cata- log of the “600 set” of Keystone Views, and learn to make individual use of the stereographs in geog- raphy and other studies. The numbers are the serial numbers in the list of 600, and are instantly available in any school provided with the 600 set of stereographs or lan- tern slides. These pictures may be examined early GENERAL CIRCULATION OF THE ATMOSPHERE 25 in the study of this pamphlet, during its study, or at the close, as a review of the work. Doldrums. 232, 237, 242, 243, 249, 304, 307, 310, 329, 333. Trade Wind Eegious at some distance from the sea. 288, 292, 294, 576, 577, 578, 579, 584, 593, 596. Trade Wind Coasts and Islands. 233, 234, 235, 301, 314, 315, 325, 327. Monsoon Regions. 492, 493, 497, 500. Horse Latitudes. 196, 197, 199, 200, 201, 476, 482, 486, 488. Westerlies, west coast. 214, 215, 216. 217, 218, 219, 221, 340, 341. Westerlies, interior, or toward east coast. 17, 41, 43, 44, 108, 132, 151, 152, 153, 154, 208, 220, 223, 224, 225, 255, 258, 342, 345, 359, 416, 431, 433, 519, 530. TOPICAL ANALYSIS GENERAL CIRCULATION OF THE ATMOSPHERE I. Introduction. II. Some facts to be remembered. 1. Air has weight. 2. Air when warmed, expands; when cooled, contracts. 3. Warm air is lighter than cold air. 4. Warm air can hold more water vapor, or moisture, than cold air. 5. Moist air is lighter than dry air. 26 IMPOBTANT TOPICS IN GEOGBAPHY III. Circulation of air on a small scale. 1. Around a lighted lamp. 2. Smoke from the house chimney. 3. The bonfire. 4. The hot stove. 5. In school room heated by steam pipe. IV. Circulation of the air on the earth without rotation. 1. Equatorial calms, or doldrums. 2. Trade winds. 3. Antitrade winds. 4. Horse latitudes. 5. Prevailing westerlies. V. The Naming of Winds. VI. Circulation of the air on a rotating earth. 1. The equatorial calms, or doldrums. 2. Northeast trade winds. 3. Southeast trade winds. 4. Southwest antitrades. 5. Northwest antitrades. 6. Horse latitudes of the northern hemi- sphere. 7. Horse latitudes of the southern hemi- sphere. 8. Westerlies of the northern hemisphere. 9. Westerlies of the southern hemisphere. VII. The wind belts of the earth and their annual movement. GENERAL CIRCULATION OF THE ATMOSPHERE 27 1. Migration of the sun’s vertical ray. 2. Migration of the doldrum belt. 3. Migration of the horse latitudes. 4. Average location of wind belts. Figs. 1 and 2. 5. Location of wind belts in northern sum- mer. Fig. 3. 6. Location of wind belts in northern win- ter. Fig. 4. "VTII. A Map Exercise. 1. The relation of equator to continents and countries. 2. Countries between 20 degrees north latitude and 20 degrees south latitude. 3. Countries between 20 degrees and 30 degrees latitude in both hemispheres. 4. Countries between 30 degrees and the poles. IX. Monsoons. X. Cyclonic Storms of the prevailing westerlies. 1. Tornado. XI. Tropical Cyclones. 1. Hurricanes. 2. Typhoons. XII. Maps. XIII. Pictures. Use the foregoing Topical Outline for Summary and Eeview. Normal School Series of Outline Maps and Observation Blanks These Maps and Blanks have been prepared under the direction of Douglas C. Ridgley, of the Department of Geography, of the Illinois State Normal University Published by McKNIGHT & McKNIGHT, Normal , Illinois The List of Maps and Blanks, Thus Far Issued, Includes the Following: 1. Illinois. 2. United States. 3. United States (without drainage.) 4. North America. 5. North America (without drainage.) 6. South America. 7. Europe. 8. Asia. 9. Africa. 10. Australia. 11. The World. 12. Normal University Campus (Topographic Map.) 13. Eurasia. 101. Southern United States and Mexico. 102. New England and New Netherlands. 14. Missouri. 15. Iowa. 16. Wisconsin. 17. Indiana. 51. Ruled Blank for Recording Observations of the Sun. 52. Ruled Blank for Recording Observations of the Moon. 53. Ruled Blank for Recording Weather Observations without instruments. 54. Ruled Blank for Recording Weather Observations without instruments. 55. Ruled Blank for Recording Observations of Planets and Stars. 103. Eastern United States and Canada. 104. NortheasternUnited States. 105. Southeastern United States. History Maps New Maps are being issued as the demand arises PRICES Ten or more maps or Blanks assorted, 1 cent each; per hundred, 80 cents; per thousand, $6.50, except numbers 12 and 101, which are double size and count each as two maps. These prices are postpaid, to any address in the United States. McKNIGHT & McKNIGHT, Publishers NORMAL, ILLINOIS Normal School Series of Outline Maps and Observation Blanks A Z a sT The maps and blanks, in sets, bound in strong manila covers, arranged with special reference to the Illinois State Course of Study in Geography and History. Price per set made on a basis of 1 cent for each map or blank. pHce Per Sef Fourth Year Geography .... 10c Fifth Year Geography .... 25c Seventh Year Geography .... 25c Eighth Year Geography .... 35c Physical Geography ..... 35c Commercial Geography .... 35c Sixth Year History . . . . . 15c Seventh Year History ..... 20c Eighth Year History ..... 20c Map-Nets . . . . . . 10c The following sets have been arranged for schools having a Course of Study of their own: Observation Blanks ..... 10c Elementary Geography .... 25c Trip Around the World .... 15c Outline Maps of North America . . . 20c Outline Maps of South America . . . 10c Outline Maps of Europe .... 15c Outline Maps of Asia ..... 10c Outline Maps of Africa .... 10c Outline Maps of Australia .... 10c Map-Nets of the World, with notes . . . 10c Sets of Maps, combining any numbers of the above list, will be put up specially on request, providing there are not less than ten maps in a set. In ordering a number of sets in which the aggregate number of maps is 100 or 1000, the price will be the same as for the single maps or blanks, notwithstanding they are bound in substantial covers. For further information address McKNIGHT & McKNIGHT, Publishers NORMAL, ILLINOIS List of Supplementary Readers and Books for Teachers Selected Readings for First and Second Grades Rimes and Stories ....... 35c Classic Stories for the Little Ones Teachers’ edition ...... 40c Pupils’ edition ...... 35c Songs of the Tree Top and Meadow .... 40c Stories of Indian Children ..... 40c For Primary and Intermediate Grades The Little Cliff-Dwellers ..... 35c Robinson Crusoe for Boys and Girls Teachers’ edition ...... 40c Pupils’ edition ...... 35c The Story of Ulysses ...... 50c Wagner Opera Stories ...... 50c The Lolami Books (2 vols.) each .... 50c Tales of Troy ....... 35c History and Nature Stories for Intermediate and Grammar Grades The Fly-Aways and Other Seed Travelers . . . 60c Stories of Indian Chiefs ...... 40c The Story of Lincoln ...... 35c Pioneers of the Revolution ..... 35c The King and His Wonderful Castle .... 35c Jack and Nell in Field and Forest .... 50c Out Door Studies in Geography (2 parts), each part . . 60c Bound together . . . . . $1.00 The Story of Our English Grandfathers, introductory, 75c; mail 1.00 Books for Teachers Jean Mitchell’s School .... $1.00 Glimpses of Child Nature ... .50 The Point of View of Modern Education .60 Suggestions on Teaching Geography . .50 School and Home Education, per year . 1.25 (A Magazine all teachers will find of interest) For the High School Lampe’s Latin Drill Book . . . $0.25 Lampe’s German Drill Book . . . .25 Music Elements of Music and Notation . . .25 By F. W. Westhoff, Director of Music, Illinois State Normal University Normal, Illinois Dealers and Teachers ordering a number of copies at one time will be allowed the usual trade discount For further information address Mcknight & Mcknight NORMAL:ILLINOIS 3 0112 072839480 Geographical Pamphlet Series Dealing with Important Topics in Geography General Circulation of the Atmosphere . . . 15c *■ By Douglas C. Ridgley Rainfall of the Earth 15c By Douglas C. Ridgley Vegetation Zones of the Earth 15c By Douglas C. Ridgley Population of the Earth (now being prepared) . . 15c By Douglas C. Ridgley A Trip Around the World on the Fortieth Parallel of North Latitude 10c By Douglas C. Ridgley Normal School Series of Outline Maps and Observation Blanks, each . lc (These are also put up in sets in strong manila covers, each set in an envelope, for Geography and History work in the grades and high school.) McKnight & McKnight Publishers NORMAL, ILLINOIS