trade wind (tradwind)

[ Over the land the greater friction, irregular temperature gradients, and local disturbances of all kinds combine to interrupt their uniformity. The trade-winds form a part of the general system of atmospheric circulation arising from the permanent difference in temperature between equatorial and polar regions. By the greater heating of the torrid zone the air is expanded, occasioning a diminished density of the surface-layer and an increase of pre ure at high levels, which produce a tendency for the air to flow off toward the poles on either side. This overflow reduces the atmospheric pre ure near the equator, and increases it in the higher latitudes to which the current flows. These conditions, therefore, give rise to two permanent currents in each hemisphere-a lower one, the trade - wind, blowing from near the tropics to the thermal equator, and an upper one, the anti - trade, flowing from the equator to about the thirtieth parallel of latitude, where it descends, producing there the calms of Cancer and Capricorn, and continues northward or southward, according to the hemisphere, as a surface-current with a component of motion to the eastward, arising from the earth's rotation. In the northern hemisphere these anti-trades are much interrupted by irregular temperature-gradients over the great and by cyclonic storms; but in the southern hemisphere, where these disturbances are le , the anti-trades attain such a force as to give the name of "the roaring forties" to the belt of latitude where they are chiefly felt. On their equatorial side the trade-winds die out in a belt of calms, which varies in breadth, in different seasons and different longitudes, from 150 to 600 miles. In March the center of the calm-belt is approximately at the equator, while in summer it rises in some longitudes to 8° or 9° N. latitude. The trade-wind zones in all oceans change their March to midsummer, and southward from September March, the range of oscillation being from 200 to 600 miles. During the first nine months of the year the equatorial limit of the northeast trade in the Atlantic lies in a higher latitude near the west coast of Africa than it does further to the westward until the fortieth meridian is pa ed, where the limit again recedes from the equator. From October to December, however, the North Atlantic trade-wind extends to its lowest latitude on the African position with the season, moving to the northward from coast. On the eastern side of each ocean the polar limit of the trade-wind extends furthest from the equator, and blows most directly toward it: thus, on the coast of Portugal and on the coast of California, the trade-wind reaches far north of the tropics, the extension of it being often felt as far north as latitude 40°, and it is frequently felt as a north wind. Toward the western part of each ocean the trade-wind becomes more easterly, often prevailing due east for many days. The trade-wind attains its greatest strength in the South Indian ocean, which is called the "heart of the trades"; in the Pacific it does not blow with either the strength or the constancy that it has in the Atlantic; and in parts of the South Pacific it is frequently interrupted by westerly winds, which prevail through the summer, and sometimes through the greater part of the year. The region of high pre ure at the tropics is in the form of great anticyclones extending in an east and west direction, and having shifting boundaries and variable gradients. As a consequence, the strength, and in some regions the direction, of the trades are subject to considerable variations. In general, the regions of the trade winds have a scanty rainfall, for cyclones do not occur except in limited areas and at definite seasons; and convection-currents, although frequently covering the sky with a small detached cloud known as trade cumulus, are generally insufficient to produce rain. Thus to the Eastern wealth through storms we go, But now, the Cape once doubled, fear no more; A constant trade - wind will securely blow, And gently lay us on the spicy shore. Dryden, Annus Mirabilis, st. 304.