The Weather

The atmosphere, at the bottom of which we live, may be compared to a great ocean of air, about two hundred miles deep, resting upon the earth. The changes and movements that take place in this ocean of air, the storms that invade it, the clouds that float in it, the sunshine, the rain, the dew, the sleet, the frost, the snow, and the hail are termed "weather." We live in it; we partake of its moods; we reflect its sunshine and shadows; it invades the everyday affairs of life, influences every business and social activity, and molds the character of nations; and yet nearly everything we know about the weather has been learned within the lifetime of the present generation. Not that the weather did not interest men of early times, but the problem appeared to be so complicated and so complex that it baffled their utmost endeavors.

Temple of the winds at Athens.  Photo by J. H. Comstock.

Boreas, the cold north wind, is represented by the figure of an old man wearing a thick mantle, high buskins (boots) and blowing on a "weathered horn." The northeast wind, which brought, and still brings to Athens, cold, snow, sleet and hail, is symbolized by a man with a severe countenance who is rattling slingstones in a shield, thus expressing the noise made by the falling hail and sleet.

The east wind, which brought weather favorable to the growth of vegetation, is shown by the figure of a beautiful youth bearing fruit and flowers in his tucked-up mantle.

Natos, the warm south wind, brought rain, and he is about to pour the water over the earth from the jar which he carries.

Lips, the southwest wind, beloved of the Greek sailors, drives a ship before him, while Zephros, the gentle west wind, is represented by a youth lightly clad, scattering flowers as he goes.

The northwest wind, which brought dry and sometimes hot weather to Athens, is symbolized in the figure of a man holding a vessel of charcoal in his hands. Thus, the character of the weather brought by each separate wind is fixed in stone, and from this record we learn that, even with the lapse of twenty centuries, there has come no material change.

Snow crystal.  Photomicrograph by W. A. Bentley.

It might be interesting to learn how Franklin made this discovery. Franklin, being interested at that time in astronomy, had arranged with a friend in Boston to take observations of a lunar eclipse at the same time that he, himself, was to take observations at Philadelphia. On the night of the eclipse a terrific northeast wind and rain storm set in at Philadelphia, and Franklin was unable to make any observations. He reasoned, that as the wind blew from the northeast, the storm must have been experienced in Boston before it reached Philadelphia. But imagine his surprise, when he heard from his friend in Boston that the night had been clear and favorable for observation, but that a fierce wind and rain storm set in on the following morning. Franklin determined to investigate. He sent out letters of inquiry to all surrounding mail stations, asking for the time of the beginning and ending of the storm, the direction and strength of the wind, etc. When the information contained in the replies was charted on a map it showed that, at all places to the southwest of Philadelphia, the beginning of the storm was earlier than at Philadelphia, while at all places to the northeast of Philadelphia the beginning of the storm was later than at Philadelphia. Likewise, the ending was earlier to the southwest and later to the northeast of Philadelphia than at Philadelphia. He also found that the winds in every instance passed through a regular sequence, setting in from some easterly point and veering to the south as the storm progressed, then to the southeast and finally to the west or northwest as the storm passed away and the weather cleared.

A further study of these facts convinced Franklin that the storm was an eddy in the atmosphere, and that the eddy moved as a whole from the southwest toward the northeast, and that the winds blew from all directions toward the center of the eddy, impelled by what he termed suction.

Franklin was so far in advance of his time that his ideas about storms made little impression on his contemporaries, and so it remained for Redfield, Espy, Loomis, Henry and Maury and other American meteorologists, a hundred years later, to show that Franklin had gained the first essentially correct and adequate conception of the structure and movement of storms.

Snow crystal.  Photomicrograph by W. A. Bentley.

During the first half of the nineteenth century, considerable progress was made in the study of storms, principally by American meteorologists, among whom was William Redfield of New York, who first demonstrated that storms had both a rotary and progressive movement. James Espy followed Redfield in the construction of weather maps, although he had already published much on meteorological subjects before the latter entered the field.

Professor Joseph Henry, Secretary of the Smithsonian Institution at Washington, was the first to prepare a daily weather map from observations collected by telegraph. He made no attempt to make forecasts, but used his weather map to demonstrate to members of Congress the feasibility of a national weather service.

An incident occurred during the Crimean War that gave meteorology a great impetus, especially in Europe. On November 10th, 1854, while the French fleet was at anchor in the Black Sea, a storm of great intensity occurred which practically destroyed its effectiveness against the enemy. The investigation that followed showed that the storm came from western Europe, and had there been adequate means of communication and its character and direction of progress been known, it would have been possible to have warned the fleet of its approach and thus afforded an opportunity for its protection.

This report created a profound impression among scientific men and active measures were taken at once that resulted in the organization of weather services in the principal countries of Europe between 1855 and 1860.

The work of Professor Henry, Abbe, and others in this country would, doubtless, have resulted in such an organization in the United States in the early 60's, had not the Civil War intervened, absorbing public attention to the exclusion of other matters. It was not until 1870, that Dr. Increase A. Lapham of Milwaukee, in conjunction with Representative Paine of that city, was able so to present the claims for a national weather service that the act was finally passed that gave birth to the present meteorological bureau in the United States. Dr. Lapham issued from Chicago on November 10, 1871, the first official forecast of the weather made in this country.

Snow crystal.  Photomicrograph by W. A. Bentley.

During the past few years a concerted effort has been made in various parts of the world to explore the upper atmosphere by means of kites and balloons, carrying meteorological instruments that automatically record the temperature, pressure, humidity, velocity and direction of the wind, etc. In this country this work has been carried on principally at the Mount Weather Observatory, which is located in Loudon County, Virginia, and is under the direction of the United States Weather Bureau and at Blue Hill Observatory, a private institution located near Boston and supported by Professor Lawrence Rotch. From observations thus obtained much has been learned about the upper atmosphere that was not even suspected before. Some theories have been confirmed and some destroyed, but this line of research is gradually bringing us nearer the truth.

The approximate proportion, by volume, is as follows: Nitrogen 78 parts, oxygen 21 parts, argon 1 part, carbon-dioxide .03 parts, and krepton, helium and xenon a trace. Pure dry air does not exist in nature, so there is always present in natural air a variable amount of water vapor, depending upon the temperature and the source of supply. Besides these, which may be termed the permanent constituents of the atmosphere, many other substances are occasionally met with. Lightning produces minute quantities of ammonia, nitrous acid and ozone. Dust comes from the earth, salt from the sea, while innumerable micro-organisms, most of which are harmless, besides the pollen and spores of plants, are frequently found floating in the atmosphere. Recent investigations in atmospheric electricity lead to the conclusion that electric ions are also present, and perform important functions, especially with respect to precipitation.

Although carbon dioxide forms but a small proportion of the atmosphere, it is a very important element in plant life. Animals consume oxygen and exhale carbon dioxide, while plants take in carbon dioxide and give off oxygen; thus, the amount of these gases in the atmosphere is maintained at an equilibrium. Plants, through their leaves, absorb the carbon dioxide, which is decomposed by the sunlight, returning the oxygen free into the air, while the carbon is used to build up plant tissue.

The dewpoint is the temperature at which saturation occurs. If the air is saturated, the temperature of the air and the dewpoint will be the same, but if the air is not saturated the dewpoint will be below that of the air.

Relative humidity is expressed in percentages of the amount necessary to saturate. If the air contains one-half enough vapor to saturate it, the relative humidity will be 50%; if one-fourth, enough to saturate, 25%; if saturated 100% etc.

The absolute humidity is the actual amount of water in the form of vapor in the air, and is usually expressed by weight in grains per cubic foot or in inches of mercury, the weight of which would counterbalance the weight of the vapor in the air. The conditions present in a volume of saturated air at a temperature of 32° may be expressed as follows: Relative humidity 100%; dewpoint 32°; absolute humidity 2.11 grains per cu. ft. or .18 inch.

Snow crystal.  Photomicrograph by W. A. Bentley.

The air near the surface is a mixture of eight different gases, and each individual gas arranges itself so as to form an atmosphere just as it would if no other gases were present. Thus, the earth is surrounded by an atmosphere of oxygen, an atmosphere of nitrogen, one of carbon dioxide, one of water vapor, one each of argon, krypton, neon, and xenon, while hydrogen and helium are believed to exist at great elevations above the earth's surface.

These gases are kept from flying off into space by the force of gravity, just as a piece of iron, stone, or a building is held fast to the earth by the same force. Gravity acts with greater force on some things than on others. For example, a piece of iron is pulled down by gravity with greater force than is a piece of wood of the same size; likewise, a piece of lead is pulled down with greater force than a piece of iron. We, therefore, say that iron is heavier than wood and that lead is heavier than iron, simply because gravity acts with greater force on the one than on the other. The weight of gases differ just as the weight of different solids, such as lead, wood or iron differ. For instance, nitrogen is 14 and oxygen 16 times heavier than hydrogen.

Gases having the least weight extend upward the farthest, because the lighter the gas the greater its expansive force. Every boy who rides a bicycle takes advantage of the expansive force of air when he pumps his tires. The air is compressed by the pump into the tube and the expansive force exerted by the air in trying to expand makes the tire "stand up." If it requires 10 pounds pressure to compress the gas into the tube, the expansive force will be just ten pounds.

There are two forces in constant operation on each gas that surrounds the earth, viz., expansive force and gravity. Expansive force pushes the gas up and gravity pulls it down, but the force of gravity decreases as the distance from the center of the earth increases, so there is a point at a certain distance above the earth where the two forces just balance each other, and each gas will expand upward to that point but will not rise beyond it. Therefore, if we know the expansive force of a gas and the rate at which gravity decreases, it is possible to calculate the height to which the different gases that compose the air will rise.

In this way it has been determined that carbon dioxide, which is one of the heavier gases, extends upward about ten miles, water vapor about 12 miles, oxygen about 30 miles and nitrogen about 35 miles while hydrogen and helium, the lightest gases known, do not appear at the surface at all, but probably exist at a height of from 30 miles to possibly 200 miles.

The zone of twilight in midwinter.  From Todd's New Astronomy.

There are other ways in which we are able to gain some idea of the approximate height at which there is an appreciable atmosphere. When the rays of light from the sun enter our atmosphere they are broken up or scattered—diffracted—so that the atmosphere is partially lighted for some time before sunrise and after sunset. This is called twilight. If there were no atmosphere, there would be no twilight, and darkness would fall the instant the sun passed below the horizon. Twilight, which is caused by the sun shining on the upper atmosphere, is perceptible until the sun is about 16° below the horizon. From this it is calculated that the atmosphere has sufficient density at a height of 40 miles to scatter, or diffract, sunlight.

Observations of meteors, commonly called shooting stars, indicate that there is an appreciable atmosphere at a height of nearly 200 miles. Meteors are solid bodies flying with great velocity through space. Occasionally they enter our atmosphere. Their velocity is so great that the slight resistance offered by the air generates enough heat by friction, or by the compression of the air in the path of the meteor, to make it red hot or to burn it up before it reaches the bottom of the atmosphere. Only the largest meteors reach the earth.

When a meteor is observed by two or more persons at a known distance from each other, and the angle which the line of vision makes with the horizon is noted by each, it is a simple matter to calculate the distance from the earth where the lines of vision intersect, and thus determine the height of the meteor. In this way, reliable observations have given the height at which there is sufficient density in the atmosphere to render meteors luminous as 188 miles.

The Fahrenheit scale was the invention of a German by that name, but it is worthy of note that this scale is used principally by English speaking nations and is not in common use in Germany. Fahrenheit found that by mixing snow and salt he was able to obtain a very low temperature, and believing that the temperature thus obtained was the lowest possible he started his scale at that point, which he called zero. He then fixed the freezing temperature of water 32 degrees above this zero, and the boiling point of water at 212 degrees. There are, therefore, 180 divisions or degrees between the freezing and boiling point of water on the Fahrenheit scale.

The Centigrade scale starts with zero at the freezing point of water and makes the boiling point 100. Thus 180 degrees on the Fahrenheit scale equal 100 degrees on the Centigrade. The Fahrenheit degree is, therefore, only a little more than half as large, to be exact five-ninths of a degree, as a degree on the Centigrade scale. The Centigrade scale is in common use in France and is used almost exclusively in all scientific work throughout the world.

The Reaumur scale is used generally in Russia and quite commonly in some parts of Europe, especially in Germany. On this scale the zero is placed at the freezing point of water and the boiling point at 80 degrees. The divisions are, therefore, larger than those of the Centigrade scale and more than twice as large as the Fahrenheit. The general use of these different scales has led to endless confusion and made the comparison of records difficult, so that even at the present time when making a temperature record it is necessary to indicate the scale in use.

Furthermore, the fact that the path of the earth about the sun is not a circle but an ellipse, and that the axis of the earth is not perpendicular to the plane of its orbit, result in an unequal distribution of heat over the surface. It is always warmer near the equator than at the poles, and warmer in summer than in winter. All these differences in temperature cause corresponding differences in density, which, in turn, cause differences in weight or pressure over various parts of the earth's surface. These changes are, in no way, the result of chance but are determined by the operation of fixed natural laws, and with this in mind we may now take up the study of the winds of the world.

Snow crystal.  Photomicrograph by W. A. Bentley.

There are greater rivers of air in the atmosphere than any in the oceans, and they move on their courses with equally systematic precision and in obedience to fixed laws, which we may in a measure understand.

The river, at the bottom of which we live, is broad and deep, extending in width from Florida northward nearly to the north pole. It flows from west to east circling the globe and its name is The Prevailing Westerlies. The other river in this hemisphere extends southward from latitude about 35° nearly to the equator. Its name is The Northeast Trade Winds.

In the southern hemisphere are two similar rivers, one extending southward from latitude about 30° nearly to the south pole with its current, like its counterpart in the northern hemisphere, flowing from west to east, circling the globe. It is also called The Prevailing Westerlies. The other river in the southern hemisphere extends from about latitude 30° northward nearly to the equator and flows from the southeast toward the northwest, hence the name Southeast Trade Winds. The dividing line, or bank, between the rivers in each hemisphere belts the earth at about 35° north and 30° south of the equator. Why does the air move and why does it move in such a regular, systematic manner? To answer these questions we will rely upon gravity, the heat from the sun and the effect of the rotation of the earth on moving wind currents.

Everyone knows that water flows down hill because of the force of gravity. Gravity is nature's great peacemaker. It is always trying to settle disturbances, even things up, smooth them over. If there were no winds to bring rain to the land or to stir up the ocean, gravity would soon run all the water into the lakes and the seas, and then smooth them out like sheets of glass; and if there were nothing to stir up the winds, gravity would soon settle all differences in the atmosphere and the air would become perfectly quiet. So gravity is kept busy trying to smooth out the water which the wind stirs up, at the same time trying to quiet the winds which are stirred up by the heat of the sun.

Tyndall says that heat is a mode of motion, that when heat is imparted to a substance the molecules of which it is composed are set into very rapid vibration. They are continually trying to get away from each other and usually succeed in getting more space, and thus increase the size or volume of the substance, or, in other words, expand it. Iron, brass, copper, water and many other substances expand under heat. Air is a gas and expands very rapidly when heated. One cubic foot of cold air becomes two cubic feet when heated. Now gravity pulls things down toward the center of the earth in accordance with their weight-density, and a cubic foot of cold air, being more dense and thus heavier than an equal volume of warm air, is pulled down with greater force. We, therefore, say that warm air is lighter than cold air, and if lighter it will rise. What it actually does is to press equally in all directions and when a place is found where there is less resistance than elsewhere it moves in that direction. So when heat causes air to expand and become lighter than the surrounding cool air, it moves, and air in motion is wind. 

Figure 1.   Diagram showing air currents set up by sun's heat.

This diagram represents a section of the atmosphere over a broad, level plain with the air at rest and pressing down equally on every part of the surface. The dotted line H represents the top of the quiet atmosphere. Such a condition occurs frequently at night after the heat from the sun is withdrawn and gravity has settled the atmosphere. When the rays of the sun fall upon the earth upon which this quiet air rests they warm the earth first, then the layer of air immediately in contact with the surface, so the atmosphere is heated from the bottom upward. We will assume that the layer of air between the earth and the dotted line, G, is thus heated to a higher temperature than the air above it. It will, therefore, expand. It cannot expand downward because of the earth. It cannot expand much laterally because it is pressed upon by air that is also seeking more space. It, therefore, expands upward as represented by the line A B C. Now in expanding upward it lifts all the air above it and the line H, representing the top of the atmosphere, will become bowed upward also as indicated by the line A'B'C'. As a result, the air at the top of the atmosphere over the warm center slides down the slopes on either side toward the cool margins. As soon as the flow of air away from the warm center begins, just that instant the pressure upon the heated layer at the surface is relieved and the warm air rushes upward (is pushed upward) and the whole circulation, as indicated by the arrows, begins. It must be remembered that gravity is the really active force in maintaining this movement, because it pulls down the denser, heavier air at the cool margins with greater force than the warm, expanded, light air at the warm center. The descent of the cool air actually lifts the warm air.

The normal pressure, or weight, of the atmosphere at sea level is about 14.7 pounds on each square inch of surface. It is customary, however, to express the weight of the atmosphere in terms of inches of mercury instead of in pounds and ounces. A column of air one inch square from sea level to the top of the atmosphere will just counterbalance a column of mercury 30.00 inches high in a barometer tube of the same size. We, therefore, say that the normal pressure of the atmosphere at sea level is about 30.00 inches. If, for any reason, the atmosphere becomes heavier than normal, it will raise the column of mercury above the 30 inch mark, and we say that the pressure is "high." If the atmosphere becomes lighter than normal, we say that the pressure is "low." So high pressure means a heavy atmosphere and low pressure a light atmosphere.

At the beginning we assumed that the atmosphere over the broad, level plain was quiet and that it pressed down equally on every part of the surface. We will now assume that the pressure was normal, or 30.00 inches, and note the changes in pressure that result from the interchange of air between the warm center and the cool margins. So long as none of the air raised by the expanding layer at the surface, moved away toward the cool margins, no change in pressure occurred; but the instant the air began to glide down the slopes away from the warm center, then the pressure at the surface decreased, because, some air having moved away, there was less to press down than before. The pressure at the warm center, therefore, became less than 30.00 inches, or in other words, low. Likewise, the air as it moved away from the warm center, having lost much of its heat during its ascent, was gradually pulled down by gravity because of its greater density, thus increasing the pressure over the cool margins. We, therefore, have low pressure at the warm center, 29.90 inches and high pressure, 30.10 inches, at the cool margins. From this illustration we obtain the six principles of convectional circulation, viz.:

1. Low pressure at warm center.

2. High pressure at cool margins.

3. Ascending currents at warm center.

4. Descending currents at cool margins.

5. Surface winds from high pressure to low pressure.

6. Upper currents from low pressure to high pressure.

Figure 2.   Isobars of the world.

Now, we all know that the temperature of air is much higher at the equator than at the poles and we may, therefore, let Figure 1 represent a section of the atmosphere along any meridian from the north to the south pole. The equator would then become the warm center and the poles the cool margins. We would then expect to find a belt of low pressure around the world near the equator because of the high temperature, and high pressure at the poles because of the low temperature. We would, also, expect to find ascending currents at the equator; upper currents flowing from the equator toward the poles; descending currents at the poles, and surface winds blowing from the poles toward the equator. Let us now test our theory by actual facts and see how far they are in accord.

The chart, Figure 2, represents the normal, or average, pressure at sea level for the world, and if our theory is in accord with the facts, we should find a belt of low pressure all around the world near the equator, with areas of high pressure at the poles. Let us examine the chart. Beginning at the equator, and bearing in mind that the normal pressure is about 30.00 inches, we find irregular lines, representing pressures of 29.90 inches—slightly below normal—around the world on both sides of the equator. Between these lines we find pressure as low as 29.80. It is, therefore, evident that there is a belt of low pressure around the world near the equator, as anticipated. Let us look for the high pressure at the poles. We have comparatively few observations near the poles, but the line nearest the south pole is marked 29.30 inches, a surprisingly low pressure, much lower even than the low belt at the equator, and just the reverse of what we expected to find. When we look at the north pole we find that the pressure is not so low as at the south pole, but still below normal and about as low as at the equator. Going north and south from the equator we find that the pressure increases gradually up to about latitude 35° in the northern hemisphere and to about latitude 30° in the southern, after which it decreases toward the poles. So there are two well marked belts of high pressure circling the globe; the one about 35° north, and the other about 30°, south of the equator. May it not be significant that these belts of high pressure coincide so nearly with the margins, or banks, of the air rivers mentioned on page 867?

Thus far our theory does not accord very well with the facts. True, we found the low pressure at the equator as anticipated; but we also found low pressure at the poles, where the reverse was expected; and the high pressure that we anticipated at the poles, we found not far north and south of the equator. We will, therefore, have to discard our theory, or reconstruct it to accord with the facts. Let us reconstruct Figure 1, and mark the pressure on the line representing the earth's surface along any meridian to accord with the facts as they appear on Figure 2.

Figure 3.   Diagram showing air currents along any meridian.

The above diagram now represents the true pressure along any meridian, as determined by actual observations, and we cannot escape the conviction that the requirements as to temperature and pressure at the warm center are fulfilled by the high temperature and low pressure found at the equator. Furthermore, the temperature decreases north and south from the equator, and thus the belts of high pressure near the tropics may be taken to represent the conditions at the cool margins. The first and second principles of a convectional circulation, viz., low pressure at the warm center and a high pressure at the cool margins, are thus fulfilled. To satisfy the remaining conditions, we should find ascending currents near the equator, upper currents flowing from the equator toward the tropical belts of high pressure, descending currents at the tropics, and surface winds blowing from the tropics toward the equator. Let us now examine the surface winds of the world as illustrated by the diagram on page 867.

On either side of the equator and blowing toward it, we find the famous trade winds—the most constant and steady winds of the world. Their northern and southern margins coincide with the tropical belts of high pressure. They blow from high pressure to low pressure and we cannot doubt that they act in obedience to the fifth principle of convectional circulation. From observation of the lofty, cirrus clouds in the trade wind belts, we have abundant evidence of upper currents, flowing away from the equator toward the tropical belts of high pressure; thus the sixth principle is satisfied. The torrential rains and violent thunderstorms, characteristic of the equatorial regions, bear evidence to the rapid cooling of the ascending currents near the equator; while the clear, cool weather and light winds of the Horse Latitudes clearly indicate the presence of descending currents at the tropics. Thus, the six principles of a convectional circulation are satisfied, and the evidence is conclusive that the trade winds form a part of a convectional circulation between the tropical belts of high pressure and the equatorial belt of low pressure.

You have doubtless observed that the trade winds do not blow directly toward the equator but are turned to the west so that they blow from the northeast in the northern hemisphere, and from the southeast in the southern. This peculiarity is not in strict accord with our ideas of a simple convectional circulation and suggests, at least, the presence of some outside influence. If we turn to Ferre's treatise on the winds, we find a demonstration of the following principle: a free moving body, such as air, in moving over the surface of a rotating globe, such as the earth, describes a path on the surface that turns to the right of the direction of motion in the northern hemisphere and to the left in the southern. The curvature of the path increases with the latitude, being zero at the equator and greatest at the poles, and is independent of direction. With this in mind, if we take position at the northern limit of the trade winds in the northern hemisphere and face the equator, (see p. 867), we find that the winds moving toward the equator turn to our right; likewise, if we face the equator from the southern limit of the southeast trades, we find them turning to our left. Observations of upper clouds in the trade wind belts show that the upper currents also turn to the right in the northern hemisphere, and to the left in the southern. It is, therefore, clear that the systematic turning of the trade winds from the meridian is due to the rotation of the earth. The value of a force at various latitudes and for various velocities that would cause a body to turn away from a straight line, is purely a problem in mathematics, and for the benefit of those versed in the science the formula is given. The amount of such a force is expressed by 2 MVW sin D, where M is the mass, V the velocity, W the angular rotation of the earth, and D the latitude.

Not all of us may be able to solve the problem, but we may understand something of the effect of the rotation of the earth on moving wind currents. It is a well-known principle of physics that if a body be given a motion in any direction, it will continue to move in a straight line by reason of its inertia, without reference to north, south, east or west. A personal experience of this principle may be gained in a street car while it is rounding a curve.

Diagram showing effect of earth's rotation on the atmosphere.

In this diagram, we have a view of the northern hemisphere. The direction of the rotation is indicated by the curved arrows outside the circle representing the equator. Suppose that a wind starts from the equator, moving along the meridian A directly toward the north pole. It is clear that it cannot continue to move along the meridian, because the direction of the meridian with reference to space, is continually changing, and the inertia of the wind compels it to move in a straight line without reference to the points of the compass. So when the meridian A has been moved to B by the rotation of the earth, the wind, although it maintains its original direction, no longer points toward the pole but to the right of the pole. Likewise, a wind starting from the pole toward the equator also turns to the right of the meridians and becomes a northeast wind as it approaches the equator. A wind moving east or west, also turns to the right of the parallels for the same reason. So a wind starting out from the equator with the best possible intention of hitting the pole, and all the while continuing in the same straight line, will miss the pole by many miles, and always on the right side in the northern and on the left side in the southern hemisphere. Thus, the oblique movement of both the trade winds and the prevailing westerlies is accounted for.

It now remains to consider the cause of the unexpected low pressure found at the poles, and the reason for the belts of high pressure at the tropics. If we refer to Figure 2, it is evident that not all the air that ascends at the equator descends at the tropics, else there would be an absence of air at the higher latitudes, which is manifestly not the case. On the other hand, it is equally impossible that all the air ascending at the equator should move to the poles, because the space it could occupy decreases rapidly from a maximum at the equator to zero at the poles. Only a part of the air that ascends at the equator is, therefore, involved in the trade wind circulation and a part passes over the tropics and moves on toward the low pressure at the poles. Furthermore, some of the air that descends at the tropics moves along the surface toward the poles, obeying the law that impels air to move from high pressure to low pressure. Now every particle of air that passes over the tropics, every particle that moves northward along the surface, turns to the right in the northern and to the left in the southern hemisphere. All, therefore, miss the poles—on the right side in the northern and on the left side in the southern hemisphere. The result is that two great whirlpools develop in the atmosphere; one whirling about the north and the other whirling about the south pole. The outer margins of these whirlpools coincide with the tropical belts of high pressure.

As an example of a whirlpool we may take a basin having a vent at the center of the bottom. If the basin is filled with water, the plug withdrawn and the water given a slight rotary motion, its velocity will increase as it approaches the center and the rapid whirling will develop sufficient centrifugal force to open an empty core. Those who have visited the great whirlpool at Niagara, undoubtedly noticed that the whirling waters are held away from the center and piled up around the margins by the centrifugal force developed. Let us suppose that air starting from the equator, moves without friction or other resistances toward the pole. Its velocity must increase as its radius shortens, because the law of the conservation of areas requires that the radius must always sweep over equal areas in a given unit of time. (See law of conservation of areas.) At the equator, the air has an easterly motion equal to the eastward motion of the earth, which is 1,000 miles per hour. At latitude 60° the radius will have decreased one-half and the velocity, therefore, doubled; but at latitude 60° the eastward motion of the earth is only 500 miles per hour, so the air would be moving 1500 miles per hour faster than the earth. At a distance of 40 miles from the pole the wind would attain an easterly velocity of 100,000 miles per hour, and moving on so short a radius would develop sufficient centrifugal force to hold all the air away from the pole and thus form a vacuum. That the supposed case of no friction is far from the truth is evidenced by the fact that the pressure at the north pole is but little less than at the equator; but the centrifugal force developed by the gyration winds, in thus withdrawing the air from the poles and piling it up at the tropics, may be fairly taken as sufficient cause for the low pressure found at the poles and the belts of high pressure at the tropics.

The questions that remain to be considered are: (1) the low pressure at the south pole as compared with the pressure at the north pole and (2) the unequal distance of the tropical belts of high pressure from the equator. These questions may be considered together.

It is to be remembered that the southern hemisphere is the water hemisphere, and that the prevailing westerlies, in gliding over the smooth water surface, are but little retarded by friction and, therefore, attain a higher velocity than the corresponding winds of the northern hemisphere, where the rougher surface materially retards their movement. As a consequence, the circumpolar whirl of the southern hemisphere is stronger, and develops a greater centrifugal force, thus holding a larger quantity of air away from the south pole and reducing the pressure to a greater degree than is brought about by the weaker winds of the northern hemisphere.

Since the circumpolar whirl of the southern hemisphere is the stronger of the two, it withdraws the air to a greater distance from the pole than does its weaker counterpart of the northern hemisphere, and piles it up in the tropical belt of high pressure about five degrees nearer the equator than does the weaker forces of the northern hemisphere.

Snow crystal.  Photomicrograph by W. A. Bentley.

Storms are simply eddies in the atmosphere. They may be compared to the eddies that are often seen floating along with the current of a river or creek. In these eddies the water is seen to move rapidly around a central vertex, developing sufficient centrifugal force to hold some of the water away from the center, thus forming a well marked depression, frequently of considerable depth. The whole circulation of the eddy is quite independent of the current of the stream which carries it along its course, and while its general direction and velocity of movement coincide with that of the current, there are times when it will be seen to move quickly from side to side and again when it will remain nearly stationary for a time or take on a rapid movement.

The eddies or storms in the atmosphere act in much the same way. They are carried along by the general currents of the river of air in which they exist. Their general direction coincides with the direction of the current in which they are floating, and their rate of movement conforms in a general way to its velocity; but like the eddies in the river, they do not always move in straight lines nor at a uniform rate of speed.

There is one important respect in which the eddies in the air differ from eddies in water. The water eddy may revolve in either direction, depending upon the direction in which the initial force was applied, but the storm eddies in the atmosphere always revolve counter-clockwise in the northern hemisphere, and clockwise in the southern.

This is due to the deflecting force of the earth's rotation, which is fully explained on page 872.

Snow crystal.  Photomicrograph by W. A. Bentley.

Snow crystal.  Photomicrograph by W. A. Bentley.

The forecasts that accompany the maps are simply an expression on the part of the official forecaster as to the weather changes he expects to occur in various parts of the country within the time specified, usually within 36 to 48 hours. His opinion is based upon the conditions shown by the map. He has no secret source of information. You may accept his conclusions, or, if in your opinion they are not justified, you have all the information necessary to make a forecast for yourself. Weather maps are published so extensively with a view to thus stimulating an intelligent interest in the problem of weather forecasting, and also that one may see at a glance what the temperature, rainfall, wind and weather is in any part of the country in which he may be interested. The friends of the weather service are those who best understand its work.

Snow crystal.  Photomicrograph by W. A. Bentley.

The value of the weather service to the agriculture and commerce of the United States cannot be questioned seriously. That the appropriations for its support have been increased year by year from $1,500 in 1871 to nearly $1,500,000 in 1910 is evidence of its value and efficiency. A conservative estimate places the value of property saved by the warnings issued by the Weather Bureau at $30,000,000 annually.