The most remarkable of the regular winds are the trade-winds. The
atmosphere at the surface between the tropics is much warmer than in the
higher latitudes; and since air expands when heated, the light warm air
of intertropical regions perpetually rises, and its place is as
perpetually supplied by the colder air from the north and the south. If
it were not for the Earth’s rotation, these would be merely north and
south winds; but like the equinoctial water-currents, these cool
currents of air coming from regions which have not an equal velocity of
rotation with the air at the equator, pause and hang back, and thus
these aerial currents acquire a westerly direction, forming
north-easterly constant winds in the northern hemisphere, and
south-easterly in the southern hemisphere.