when it blows

In part, global wind patterns are caused by the way in which the sun’s rays warm the
earth. The part of the earth that receives most of the sun’s heat is the tropics—the region
about the equator that lies between the tropics of Cancer and Capricorn. In the tropics the
sun’s rays hit the earth almost at right angles, concentrating their heat energy on the
smallest surface area. They also pass through the least depth of atmosphere, with minimal
scattering and spreading, before they reach the surface. Farther north and south from the
equator the sun’s rays approach the earth at a slanting angle and their heat energy covers
a correspondingly larger area. They also have to pass through a much greater depth of
atmosphere, causing their heating effects to be spread and dissipated.
The greater heat at and near the equator means that the air there becomes hot and rises,
allowing cooler air to flow in. The rising hot air might be expected to move away, due
north and south. That this does not quite happen is due to the Coriolis effect, or force (see
panel), named after the French civil engineer Gaspard Coriolis, who first noted the
phenomenon in the 19th century. Under its influence the hot equatorial winds blowing
north and south are deflected from west to east, while cooler winds drawn back to the
equator are deflected from east to west. These masses of air are known as the trade winds,
which blow steadily for much of the year from the northeast north of the equator and
from the southeast to its south, roughly between latitudes 0 and 30°. (The term “trade”
used to describe the winds derives from an obsolete meaning of the word—“in a regular
course or direction”—but also reflects the winds’ importance for merchant shipping.)