KITE FLYING FROM A MOVING BOAT ON LAKE CONSTANCE.

The sun is the one great source from which the atmosphere is heated. At its outer limit the atmosphere receives vertically from the sun, on an average, 1.92 calories of heat per square centimeter per minute (Abbot, Proc. Amer. Phil. Soc., 1911). This datum is known as the solar constant -a misnomer, as the actual amount fluctuates by several per cent; i. e., the sun is not actually a constant source of heat.

The atmosphere is mainly heated from below, although the heat originally comes from above. This paradox is explained by the fact that but a small part of the solar heat is absorbed by the atmosphere when passing through it on its way to the earth. Several processes are involved in the disposal of solar heat (more accurately, radiation) by the earth and its atmosphere, and different wave-lengths undergo different effects. This complex subject, involving the study of solar radiation with the aid of the pyrheliometer, bolometer, photometer, polarimeter, etc., forms a border science between meteorology and solar physics, with important applications to biology. It is engaging the attention of a numerous body of investigators, but has not yet received a distinct name.

For the present purpose it may be stated that the earth, heated by the sun's rays, imparts its heat by conduction to a shallow layer of air immediately above it. Conversely, portions of the earth's surface withdrawn from the sun's rays lose their heat by radiation into space, and the air adjacent to them is cooled by conduction.

Inequalities of temperature plus the force of gravity set up air currents, which distribute heat through the atmosphere. The latter process is called convection. As between a land surface and a water surface, the former undergoes much wider fluctuations of temperature from day to night, and from summer to winter, causing correspondingly wider fluctuations in the temperature of the overlying atmosphere. Hence a continental climate is much less equable than a marine climate. Lastly, rising and falling air-masses are adiabatically cooled and heated, respectively, at the rate of 1.6° Fahrenheit per 300 feet of vertical motion. (The cooling process is less rapid than this when densation of moisture is in progress.)

DISTRIBUTION OF TEMPERATURE.

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The earth revolves around the sun, and its axis, which always remains parallel to itself, is inclined to the plane of its orbit. These facts explain the march of the seasons and their opposition in the two hemispheres. The amount of heat (insolation) received at any place at a given moment depends chiefly upon the altitude of the sun. The aggregate amount received at any period of the year depends also upon the length of the day, which varies with latitude, except at the equinoxes. At the summer solstice, the north pole, where the day is then 24 hours long, actually receives a greater daily amount of insolation than any other part of the globe; but this is ineffective in raising the temperature on account of the long oblique path of the solar rays through the atmosphere, and the large amount of snow and ice that must be melted before the overlying air can be warmed. At the winter solstice a still greater amount of insolation is received at the south pole, as the earth is then in perihelion.

If the earth had a smooth homogeneous surface and no atmosphere the horizontal distribution of temperature at any time would depend entirely upon latitude. The theoretical climate resulting from such conditions is called solar climate. Actually, however, this simple distribution is profoundly modified by the thickness of the layer of air through which the sun's rays pass (depending upon the sun's altitude), the different thermal properties of land and water, the presence or absence of snow and ice, the configuration of the earth's

surface, the prevailing winds, the distribution of water vapor, etc.

How widely the actual distribution of temperature differs from that of the solar climate may be seen from an annual isothermal chart of the globe. An isotherm is a line on such a chart drawn through places having the same temperature. The first isotherms of the mean annual temperature of the whole world were drawn by Humboldt in 1817, and introduced into meteorology the valuable idea of the isogram-i. e.. a line on a chart connecting places at which equality of some physical condition exists. An isogram of barometric pressure is called an isobar; of rainfall, an isohyet; of cloudiness, an isoneph; of duration of sunshine, an isohel; etc. Upwards of eighty meteorological isograms have been given special names."

on the

The lowest temperatures earth occur in winter in the northeastern part of Siberia, the somewhat indefinite center of greatest cold being known as the "cold pole." At Verkhoyansk, in this region, a temperature of 90.4° below zero Fahrenheit was recorded on Jan. 15, 1885-the lowest ever reported at a regular meteorolog

See "The Meteorological Isograms." Scientific American Supplement, Nov. 12, 1910.

ical station. The highest temperatures occur in the deserts of both the temperate and the torrid zones. At Wargla (French Ouargla), in the Algerian Sahara, a temperature of 127.4° Fahrenheit was recorded on July 17, 1879. Much higher temperatures have been reported-as high as 167° in the desert of Gobi-but the records in these cases are not entirely trustworthy. Of course these are all

shade temperatures.

In the upper atmosphere the lowest temperatures occur at great heights over the equatorial regions, where the troposphere is thicker than in higher latitudes, and hence the ordinary fall of temperature with ascent proceeds to a greater height before the isothermal layer is reached. The lowest temperature ever registered by a sounding-balloon was 119° below zero Fahrenheit, over Victoria Nyanza, in the heart of Africa.

GENERAL CIRCULATION OF THE ATMOS

PHERE.

In the equatorial regions the surface air is heated more than elsewhere, and tends to rise and overflow at high levels, toward the poles; while the relatively cold air of the polar regions tends to flow equatorward, near the earth's surface, to replace it. A simple circulation between the equator and

the poles could, however, only occur if the earth did not rotate on its axis.

The deflective force of the earth's rotation causes a particle of air moving in any direction over the earth's surface to deviate to the right in the northern hemisphere and to the left in the southern.

At about latitude 30° the winds coming from the equator have been so much deflected that they move almost due eastwardly. The result is a great whirl around the pole, occupying most of the temperate zone in each hemisphere, with prevailing winds from west to east at all levels. The centrifugal force of this whirl causes the air to bank up at about latitude 30°, producing a belt of high pressure in that region, which is known as the horse latitudes. Between this belt and the equator there is a regular circulation of air equatorward below (the trade winds) and poleward above (the antitrades); both systems being given an oblique direction by the earth's rotation. Near the equator, between the two trade wind systems, is a region of calms or variable winds, with abundant clouds and rains, known as the doldrums. Trades and doldrums shift north and south in the course of the year, following the sun, and give to regions which come alternately under their control successive dry and rainy

seasons.

The prevailing westerly winds of middle latitudes are stronger in the southern hemisphere, where they blow mainly over the ocean and are little impeded by friction, than in the northern hemisphere; hence the violence of the winds known to mariners as the "brave west winds" in the region called the "roaring forties" (about 40° south latitude).

Within the polar circles the low temperatures increase the density of the air, which flows radially away from the poles near the earth's surface; an effect that appears to be reenforced by the drainage of air down the glacier slopes of the two polar continents (Greenland and Antarctica).

From north to south the main wind systems of the globe run in the following sequence:

1. Arctic calms and outflowing winds, deflected westwardly (with poleward winds overhead).

2. Westerly (i. e., eastwardly) winds of middle latitudes.

3. Horse latitudes ("calms of Cancer").

Northeast trade winds (with

southwest antitrades overhead).

5. Doldrums or equatorial calms (with east winds overhead).

(with

6. Southeast trade winds northwest antitrades overhead). 7. Horse latitudes ("calms of Capricorn").

8. Westerly (i. e., eastwardly) winds of middle latitudes.

9. Antarctic calms and outflowing winds, deflected westwardly (with poleward winds overhead).

These prevailing wind systems are, however, greatly disturbed by the periodic winds due to the different thermal effects of land and water surfaces; by the surface configuration of the land; and, in middle latitudes, by the continual passage of cyclonic and anticyclonic areas.

PERIODIC WINDS.

Comparing day and night, summer and winter, the land is alternately warmer and colder than the ocean. Hence there is an annual seesaw of the winds on a vast scale between land and sea (the monsoons), and a daily seesaw on a smaller scale between coasts and the adjacent waters (land and sea breezes; land and lake breezes).

Another class of alternating winds occurs in valleys, where warm air flows up the slopes by day, and cold air drains downward by night (mountain and valley breezes). This phenomenon has always strongly impressed the popular imagination; and scores of winds of this class have been given individual local names. Such are the pontias, vésine and solore of the French Alps; the joran of Lake Geneva; the breva and the tivano of Lake Como, etc.

CYCLONES AND RELATED PHENOMENA.

A cyclone, barometric depression, or low is a system of winds blowing around a center of low barometric pressure. Near the earth's surface the wind is drawn spirally inward toward the center of the system, the direction of rotation being always counterclockwise in the northern hemisphere and clockwise in the southern. Hence we have Buys Ballot's law: Stand with your back to the wind and the barometer will be lowest on your left hand in the northern hemisphere, and on your right hand in the southern. The

air drawn into the vortex of the system rises and tends to flow spirally outward, though its actual direction is much modified by the prevailing drift of the atmosphere (west-east in middle latitudes). Besides its rotary motion, the cyclone as a whole has usually a more or less rapid translatory motion. The two motions may be compared with those of the earth, which rotates on its axis and at the same time revolves in its orbit around the

sun.

Extratropical cyclones, which are responsible for the very changeable weather of the temperate zones, cover hundreds or thousands of square miles and have a translatory movement averaging 600 or 700 miles a day, usually in an eastwardly direction. They ap pear to be carried around the globe in the general circumpolar whirl described above. They are typically accompanied by cloudy weather, with rain or snow and rising temperature on their east and equatorward sides; and by clearing weather, with falling temperature, on their west and poleward sides.

The term anticyclone, or high, is somewhat loosely applied to any region of high barometric pressure. The typical anticyclone has a system of winds just the reverse of that found in the cyclone, outflowing below and inflowing above; and such a system is commonly assumed to be characterized by clear, cool and settled weather. In fact, however, all kinds of weather occur in anticyclones, which appear to be essentially somewhat inert masses of air which are not partaking of the circulation going on around them.

The tropical cyclone (hurricane of the West Indies; typhoon of the China Sea; baguio of the Philippines), is a relatively violent whirl, which originates in the stagnant air of the doldrums, and usually moves in an oblique and curved path toward higher latitudes, sometimes passing into the temperate zone and becoming an extratropical cyclone. These disturbances (which are always "storms,' while extratropical cyclones frequently are not) are confined to certain relatively small regions of the globe, and to certain seasons. West India hurricanes are most common from July to October (the "hurricane season"). They frequently cause frightful devastation in the Caribbean Sea and the Gulf of Mexico, and on the southeastern coasts of the United

States (as at Galveston, Sept. 8, 1900, when 6,000 lives and $30,000,000 in property were destroyed). The amount of shipping exposed to these storms will be much increased with the opening of the Panama Canal. Their movements are now closely watched by the U. S. Weather Bureau, which maintains observing stations in the West Indies during the hurricane season, and receives regular wireless weather reports from vessels plying in that region.

The spout is a vortex in the atmosphere, usually not over a few hundred feet in diameter, which begins in the upper air and is propagated downward. Its position is marked by a funnel-shaped cloud. Spouts are distinguished, according to their place of occurrence, as landspouts and waterspouts, and the more violent landspouts are called tornadoes. The tornado is popularly miscalled a "cyclone." These disturbances appear to be secondary phenomena of the true cyclone, and (in the northern hemisphere) occur chiefly in a region southeast of the cyclone center.

Thunderstorms are sometimes scattered phenomena, of local origin, and sometimes occur in a long line extending radially from center to border of a cyclone. In the latter case they constitute a line-squall. Their winds tend to rotate about a horizontal axis. Their electrical phenomena are probably the result, not the cause, of the atmospheric movements.

A wind blowing from a warm region toward a cyclonic center is called a sirocco, and its attendant weather is often called, in the United States, a warm wave. Winds blowing in winter from a cold region toward such a center bring us cold waves, or blizzards (the latter term implying the presence of driving snow as well as a low temperature).

A wind of cyclonic origin blowing down a mountain slope constitutes a fallwind. Such a wind, dried by the precipitation of its moisture on the windward slope, and further dried and heated by compression in its descent, is called a foehn (chinook in the northwestern United States); its effects are most striking in winter, when it sometimes raises the temperature on the lee side of the mountains 30° or 40° in a few minutes, causing the snow to disappear with astonishing rapidity. The bora of the Adriatic and the mistral of the French Riviera