How Rotation Influences the Direction of the Winds.—Although the winds that blow out from the subtropical high-pressure area start toward the equator and the poles, the rotation of the earth gradually deflects them. In the northern hemisphere the deflection is toward the right and in the southern toward the left. Thus in the northern hemisphere a wind that starts toward the pole from the northern side of the subtropical belt is gradually deflected to the right until it becomes a southwesterly wind, that is it blows from the southwest, for winds are always named from the direction whence they come. Further deflection causes such a wind to become westerly, as is shown in Fig. 68, and even northwesterly. Thus it happens that on the northern side of the northern subtropical belt of high pressure there is a belt of prevailing " Westerlies." Similarly the air pressed out from the southern subtropical belt toward the south Bole starts as a north wind, but by deflection to its left becomes a northwest and finally a west wind. This gives rise to a belt of " Westerlies " in the southern hemisphere as well as in the northern.
Let us go back now to. the northern hemisphere. On the south ern side of the northern subtropical belt the air starts equatorward as a northerly wind. It is deflected to its right just as in the west - erlies. Hence it blows as a northeast wind. The direction and ness of such winds have caused them to be called the "Northeast Trades." In the southern hemisphere similar winds start equator ward as southerly winds from the subtropical belt, but because of a left-handed deflection become southeast winds. This with their steadiness gives them the name of "Southeast Trades." * The regularity and strength of the trades is so great that for centuries sailing ships from England and France have found it to their advantage to go south to the trade wind zone in order to be blown westward to America. On the return voyage, on the other hand, if a ship started from Florida, for instance, it went north at first in order to get out of the trade winds into the westerlies which would blow it back to Europe. With the increasing use of airships the direction of the wind is becoming of still greater importance. It is worth while for an airship to go hundreds of miles out of its direct course in order to find favorable winds.
The discovery of America was made easier because Columbus hap pened to get into the trade winds which blew him across the ocean. The first airships to cross the ocean, on the contrary, used the westerlies instead of the trades because they wanted to cross where the ocean was narrow. Hence in 1919 when Englishmen were to forestall Americans in being the first to make a trans-Atlantic flight they brought their airships by steamer to Newfoundland in order to fly with the prevailing winds.
The westerlies and the trades together with the regions of calms give rise to nine wind belts. In the center lies the equatorial belt of low pressure, rising air and calms. On the north side of this belt lie the northeast trades and on the south the southeast trades. Next to them come the two subtropical belts of high pressure, descending air, and calms. Still farther toward the poles come the westerlies blowing generally from the southwest in the northern hemisphere and from the northwest in the southern. Finally around the poles we have two areas of irregular winds and calms.
Zones of Rainfall on a Rotating Globe.—On a simplified rotating globe the zones of rainfall would correspond to the belts of tempera ture, pressure, and winds that have just been described. Rain occurs because air is cooled. Everyone knows that warm air can hold more moisture than cold.* That is why wet shoes dry so much better beside the kitchen stove than in the woodshed. When air rises the pressure upon it decreases because there is less air above it. Therefore it expands, and in so doing grows cool. When we fill a tire with a handpump we notice that the pump cylinder gets hot, for the com pression of air develops heat. Conversely when air expands it loses heat and grows cool. Thus rising air becomes cool. It gives up moisture in the form of clouds, much as warm, moist air gives up dew when it touches a pitcher of ice water and thus causes the pitcher to " sweat." Hence clouds are formed and rain falls upon any part of the earth where the air rises sufficiently.
Rainfall of the Equatorial Belt of Low Pressure.—In no other.part of the world does air rise so steadily and so abundantly as in the equatorial belt of low pressure. Therefore this is the region of great est rainfall. Fig. 69 shows what happens there. During the rainy season of the equatorial belt, the mornings are usually sunny; clouds gather toward noon, showers fall in the early afternoon, and then the air clears. So regularly does this happen that people count on it, and plan accordingly. "Let's play tennis an hour after the shower," one man may say to another.
Rainfall of the Trade Wind Belts.—Since the air in the trade wind belts moves horizontally its capacity for moisture suffers prac tically no change because of increase or decrease of pressure. Some change, however, arises from the fact that on its way toward the equator the air moves from cooler to warmer parts of the earth's sur face. Therefore the air gradually grows warm and the capacity of space to hold moisture is increased. Hence the probability of rain grows less and less. That is one reason why the Sahara is so arid.