The new air world

by Willis L. Moore

Chart 2.—High and Low Centers of Action and Prevailing Winds of the Globe for January (Buchan).

Trade Winds. But to return to the current that we left as it divided above the equator ([Figure 12]) and descended on an inclined plane to latitudes 30° north and south. It is cooler and dryer and heavier than when it started to ascend and it has lost the thousand miles per hour and more easterly velocity that it had at the equator and now only has the velocity that belongs to latitude 30°; therefore as it moves toward the equator from either side it lags behind latitudes whose easterly velocity is greater, and it takes up a direction partly toward the west, which, relative to the earth, makes it a northeast wind in the Northern Hemisphere and a southeast wind in the Southern Hemisphere. And thus is established a circulation the lower part of which is known as the “trade winds.” ([Figure 13].)

Navigators profit largely by availing themselves of the west winds in the middle latitudes and of the east winds in the tropics. To the daring and persistence of Columbus, and the force and constancy of the trade winds which blew him westward, we owe the discovery of America.

Fig. 13.—Average surface winds and pressure of the globe.

Winds of Middle Latitudes. Now study [Figure 12] and associate the information it conveys with that of [Figure 13], and observe that from the two belts of high pressure the air is pushed outward on both sides. In each case it starts as a true north or south wind, but, due to the rotation of the earth, is always and everywhere deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, and this deflection increases until what started as a poleward wind in the middle latitudes soon becomes almost a due west wind. In this region of west winds cyclonic storms are more frequent than in any other part of the globe. Now get clear in the mind the fact that no matter what may be the direction of the wind inside a cyclonic or anti-cyclonic whirl (often one thousand miles in diameter), the whirl is carried toward the east by the general drift from the west of the winds between latitudes 30° and 60°, and toward the west in the region of the trade winds.

Low Barometer at the Poles. Even though the air is contracted and rendered denser by the great cold of the Arctic regions, the pressure remains low because of the quantity of air driven equatorward by the centrifugal force both of the earth and of the winds themselves as they rim ahead of the earth and encircle the globe in the middle latitudes.

Data too Meager to Show Full Circulation Aloft of the Atmosphere of the Globe. Many charts have been published in the attempt to show how the atmosphere circulates below and aloft through the whole world. They only have speculative value, as our knowledge is too limited to permit us to unravel the complexities of all the upper movements.

Rain Winds of the Tropics. The trade winds, mostly moving over water surfaces, are laden with moisture, but, gaining temperature as they move towards the equator, their capacity to hold water vapor steadily increases, and therefore they do not become rain winds unless forced to ascend by the interposition of mountains, or until cooled by ascension at the equator. In no part of the world does the air rise so steadily and in such great volume as in the equatorial belt of calms and low pressure. Hence this is the region of greatest rainfall. During the two rainy seasons, spring and fall, the day opens clear; near midday the clouds gather and rain falls early in the afternoon; after which it quickly clears. This is so regular a program that one lays his plans accordingly. There is almost no rain in December and January; this is because the belt of calms and the inflowing trade winds move northward and southward with the migrations of the sun, and in December and January, the sun being far south, the northern trades, with their rainless winds, cover the equator and the region formerly occupied by the belt of calms. In midsummer the sun is far north and then the southern trades move up and give dryness to the equator. In the northern trades, of the moderate amount of rain that falls, the greater quantity falls in summer; in the southern trades the order is reversed.