As the rotation of the earth and the heating of the air near the equator are conditions that do not change, among the most permanent things about our planet are the belts into which the wind circulation is divided. The change in the position of the heat equator, -the belt of highest temperature, due to the apparent movement of the sun north and south, modifies the conditions in these wind belts during the year. The planetary winds thus modified are sometimes called terrestrial winds.
74.Wind Belts of the Earth. -Near the heat equator where theair is rising there is a belt of calms and light breezes called the dol- drums. As the air here is rising and cooling, thus having its capacity to hold moisture decreased, this is acloudy rainy belt of high temperature in which much of the land is marshy and the vegetation so rank and luxu- riant that agriculture is exceedingly difficult.
Extending north and south of the doldrums to about 28° of latitude are belts in which constant winds blowtoward the doldrum belt and supply
WIND BELTS OF THE EARTH.
the air for the upward current there. In the northern hemisphere these winds have a northeast to southwest direction and in the southern hemisphere a southeast to northwest direction. They are the most constant winds on the globe in their intensity and direction, and are called trade winds. Since they blow from a cold region to a warmer region, their power to hold moisture is constantly increasing and clouds and rains are not usual. The places where they blow are dry belts and in them are found the great dry deserts of the world.
On the poleward sides of the trade-wind belts lie the areas of high pressure already referred to. These are called the horse latitudes or belts of tropical calms and are rather ill-defined. The air is here descending and the surface movements are light and irregular. These,like the doldrums, are regions of calms. But unlike the doldrums, they are dry belts, since the temperature of the descending air is increasing, owing to adiabatic heating (section 59), and thus its power to hold moisture is increasing. Therefore the tendency in these belts is to take up moisture rather than to deposit it.
In the middle latitudes there is a belt of irregular winds which have a prevailing tendency to move from west to east or northeast. This general eastward drift of the air is constantly being interrupted by great rotary air movements having a diameter of from 500 to 1000 miles. These are called cyclones and anti-cyclones. In this region of the "westerlies," since the air tends to move from lower to higher latitudes, an abundance of moisture is usually supplied.
In the anti-cyclone the air movement is slowly downward and outward from the center and in the cyclone it is inward toward thecenter, and upward. The center of the anti-cyclone is a place of clear sky and high pressure, while that of the cyclone is a place of cloudy sky and low pressure. The anti-cyclones, or high-pressure areas, have dry, cold, light winds, while those of the cyclones, or lowpressure areas, are usually strong and wet.
75.Land and Water Winds. -As the land is much more rapidlyheated by the rays of the sun than is the water, the land during the day- time becomes hotter than the water near it. On this account the cool air over the water flows in over the land and displaces the lighter warm air. Therefore near large bodies of water when the temperature is high there is often in the daytime a wind blowing from the water to the land. At night, as the land loses its heat more rapidly than the water, the wind blows in the opposite direction. These water winds temper the climate of the tropics near the coasts and also render seaside resorts popular in summer.
76.Monsoons. -Over the interior of the great continent of Asiathe temperature becomes so high in the summer months that the air above it is greatly expanded and decreases in weight. This causes aFig. 72.
Fig. 73.
strong indraft from the colder ocean. The high temperature also brings the heat equator far north of the earth"s equator and causes the south- east trade winds to cross the earth"s equator. These swing to the right on the north side of the equator and proceed as southwest winds (Fig. 72), thus greatly strengthening the air movement toward the heated continental interior.
The winds, being heavily loaded with moisture from their passage over the tropical seas, are forced to rise when they come upon the high lands of India near the coast. There they become cooled and deposit a great amount of rain, making this southern part of Asia the place of greatest rainfall on the earth.
In the winter, when the heat equator has moved south and when the continental interior has become exceedingly cold, there is a strong movement of air out from it toward the warmer ocean (Fig. 73). This strengthens the northeast trade winds over the Indian Ocean. It thus happens that near the southern coast of Asia there are strong seasonal winds that blow toward the northeast in summer and toward the southwest in winter. These winds are called monsoons. In the early sailing voyages to India they were very important, the trip to India being made so as to utilize the summer monsoons and that from India so as to utilize the winter monsoons. On this account these winds had much to do with the conquest of India by the nations of Europe.
77.Rainfall and its Measurement.
Experiment 81. -Place a dish with vertical sides in a large open space so that the rim is horizontal and at a height of about one foot above the ground. Fasten the dish so that it cannot be overturned by the wind. After a rain, measure the water that has collected in the dish to the smallest fraction of an inch possible. This will be the amount of rainfall for this storm.
