The trade-winds, which are the principal currents in the atmosphere, are only a particular case of those very general laws which regulate the motion of the winds depending on the rarefaction of the air combined with the rotation of the earth on its axis. They are permanent currents of wind between the tropics, blowing to the N.E. on the N. side of the equator, and to the S.E. on the S. side.
If currents of air come from the poles, it is clear that equilibrium must be restored by counter-currents from the equator; moreover, winds coming from the poles, where there is no rotation, to the equator, which revolves from W. to E. at the rate of 1000 miles an hour, must of necessity move in a direction resulting from their own progressive motion and that of rotation; hence, in blowing towards the equator the bias is to the E., and in blowing from it the bias is to the W. Thus as N. and S. winds from the poles blow along the surface from the tropics to the equator, in consequence of this composition of motions that from the N. becomes the N.E. trade-wind, and that from the S. the S.E. trade-wind. Now these winds being in contrary directions cross at the equator, balance each other through about 6 degrees of latitude, and produce a belt of calms of that breadth encircling the globe, known as the calms of the equator, or the Variables of seamen. The heat of the sun rarefies the air so much, that the trade-winds, after crossing at the equator, ascend into the higher regions of the atmosphere, where that from the N. goes to the tropic of Capricorn, and that from the S. to the tropic of Cancer. But while travelling in these lofty regions they become cold and heavy, and, sinking to the surface at the tropics, each proceeds to the opposite pole from which it set out. Now, however, they have a greater rotatory motion than the places they successively arrive at, so the bias is to the W., and they become the N.W. and S.W. extra-tropical winds.
If on arriving at the poles the air were to accumulate there, the circulation of the winds would cease; but currents rise into the upper regions, and flow back again to the tropics, where they sink down to fill the vacuum caused by the great precipitation of vapour in these regions, and then flow to the equator as trade-winds (N. 177). So the currents of air cross again at the tropics and produce two belts of calms which surround the globe, named by Lieutenant Maury the Calms of Cancer and the Calms of Capricorn, but generally known to sailors as the Doldrums. Thus the winds go from pole to pole and back again, alternately as under and upper currents. In their circuits the winds cross each other five times, producing regions of calms at the poles, the tropics, and equator. The trade-winds generally extend for about 28° on each side of the equator, but, on account of the greater quantity of land in the northern hemisphere, the N.E. trade-wind is narrower than the S.E.