Trade Winds

trade winds, movement of air toward the equator, from the NE in the Northern Hemisphere and from the SE in the Southern Hemisphere. The trade winds originate on the equatorial sides of the horse latitudes, which are two belts of high air pressure, one lying between 25° and 30° north of the equator and the other lying between 25° and 30° south of it. The high air pressure in these belts forces air to move toward a belt of low air pressure along the equator called the doldrums. The air converging at the doldrums rises high over the earth, recirculates poleward, and sinks back toward the earth's surface in the region of the horse latitudes, thus completing a cycle. The air does not move directly north or south because it is deflected by the rotation of the earth. See wind; Coriolis effect.

---

Trade Winds 

winds in the tropical latitudes of the oceans. Comparatively stable throughout the year, the trade winds are part of the general circulation of the atmosphere. Their general direction is from east to west. In the lower part of the troposphere (at elevations of 1–2 km), the trade winds are supplemented by meridional components that are usually directed toward the equator. As a result, the trade winds in the northern hemisphere are usually northeasterly winds, while in the southern hemisphere they are southeasterly. The trade winds are closely related to the subtropical oceanic anticyclones, moving along the anticyclones equatorial peripheries. Above the continents in the tropics, where the wind patterns are more variable, they are less defined, and in some regions they are replaced by monsoons.

In the northern hemisphere, trade winds cover 11 percent of the ocean surface, and in the southern hemisphere 20 percent. Their directional stability reaches 90 percent for the year in some regions of the ocean. This makes the trade winds the most extensive and most stable winds in the total system of atmospheric circulation. Trade winds are ordinarily moderate in velocity (5–8 m per sec, occasionally up to 15 m per sec). The depth, or thickness, of the trade-wind zone is several kilometers and increases as one moves from the subtropics toward the equator. Above this is the prevailing westerly transfer of air typical of the overall circulation in the upper troposphere and lower stratosphere above the whole globe. This transfer above the trade winds is traditionally called the antitrades. Near the equator, especially during the summer, the prevailing easterly transfer of air persists in the lower stratosphere to very high elevations.

The trade winds and the antitrades form a circulatory system between the equator and the subtropics. The air of the trade winds rises in the equatorial zone and in part returns in the stream of the antitrades to the subtropics, where it descends and again moves toward the low latitudes. But trade-wind circulation is not a closed cycle. The trade winds are constantly being replenished by intruding masses of colder air from the moderate latitudes; at the same time, part of the air departs into the moderate latitudes along the western peripheries of the subtropical anticyclones.

At the ocean surface, the air of the trade winds, which is flowing into the lower latitudes, is almost always colder than the water. Thus it heats up on the bottom, causing a stable temperature stratification of the atmosphere to arise and convection and cumulus clouds to develop; but the anticyclonal temperature inversions that are typical at elevations of 1–2 km restrict the development of clouds to the lowest layer of the troposphere only. Precipitation is rare and the weather is dry in regions where the trade winds originate, with the exception of mountainous coasts, where the steep ascent of the trade-wind air up the slopes causes rain. The tropical deserts are continental areas corresponding to the areas where trade winds develop. In the border zone between the trade winds of the northern and southern hemispheres (the intertropical convergence zone), however, the intensity and vertical reach of convection increase sharply; vast cloud formations arise, reaching up to great elevations, and showers fall from them. On land, this zone is usually the zone of humid subequatorial and equatorial forests. Where the intertropical convergence zone shifts significantly in the course of the year, the trade-wind circulation of air masses is replaced by the monsoon circulation; this is especially common in the Indian Ocean basin.

Trade winds have been known to Europeans since the first voyage of Columbus (1492–93), whose crew members were amazed at the steadiness of the northeasterly winds that carried their ships from the shores of Europe across the tropical regions of the Atlantic. Consideration of the trade winds was extremely important for navigation in the age of sailing ships.

REFERENCES

Khromov, S. P. Osnovy sinopticheskoi meteorologii. Leningrad, 1948.
Riehl, H. Tropicheskaia meteorologiia. Moscow, 1963. (Translated from English.)

S. P. KHROMOV