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THE TRADE WINDS
19. The trade winds belong to the first-named class of winds. They blow unceasingly from northeast to southwest in the northern hemisphere and from southeast to northwest in the southern hemisphere, their area of operation extending from about 30° N to 30° S, with a belt of calms between, commonly known as the doldrums. The name trade winds was given these winds on account of their constancy in force and direction, as well as for the great service they render to commerce and navigation.
20. Cause of the Trade Winds.-For centuries the trade winds were a puzzle both to the meteorologist and the
navigator. The astronomer Halley was the first to suggest an explanation of the cause of these winds, and his theory, with a slight modification, is now accepted as correct. This explanation, briefly told, is as follows: It is well known that warm air is lighter than cold air. Therefore, the atmosphere at the equatorial regions of the earth being heated to a considerable degree will accordingly rise and in its place will flow cold air from the direction of the poles. A circulation of air is thus established, one current flowing from the equator toward the poles in the upper regions of the atmosphere and another current flowing toward the equator from the poles along the surface, as indicated by the arrows in Fig. 8 (a). Now, if the earth were at rest, a northerly surface wind would consequently prevail in the northern hemisphere and a southerly surface wind in the southern hemisphere. But these directions are modified by the earth's rotation. During their movement from the poles the surface currents pass gradually by the latitude parallels, the diameters, and, consequently, the rotary speed of which progressively increase; therefore, if their absolute velocity does not diminish, they will apparently move toward the west, as indicated in Fig. 8 (6), and their seeming direction will be from northeast to southwest, which is, in fact, the general direction of the trade winds in the northern hemisphere. A similar result follows in the southern hemisphere; the wind there, coming from the south, is influenced by two forces—one drawing it north, the other drawing it west—and will by the law of the composition of forces take an intermediate direction and blow from southeast toward the northwest. All observations confirm this reasoning.
21. The regions occupied by the trade winds are seldom invaded by storms; they are marked by the most pleasant of weather conditions for which a navigator can possibly wish; they bear only small clouds by day, and the nights in these regions are nearly cloudless, being admirably adapted for star observations. There are times when the trade winds have a tendency to weaken or shift—this probably being caused by disturbing counter currents of air outside or near their limits—but, as a general rule they blow with a remarkable constancy, both in direction and velocity. 2. The Doldrums.-The doldrums, or calm regions, already mentioned, extend across the Atlantic and Pacific oceans, their general directions being parallel to the equator. They occupy very different positions at the close of the winter months than they do at the end of the summer months. They never cross the equator in the Atlantic Ocean. In the spring the centers of these regions are only 1° or 2° north of the equator, while in the summer they frequently rise to latitude 9° or 10° N. These changes are directly influenced by the sun, advancing with that luminary to the northward during the summer and retreating with it during the early winter months. The doldrums with their calm, sultry air, occasional baffling breezes, and frequent rains, are always dreaded by the crew of a sailing ship about to cross the equator. In many instances ships have been detained in these calm regions for weeks in a state of painful helplessness, the crew being unable to do anything but wait patiently for a breeze to fill their flapping sails. The water all around them resembles a waste sheet of glossy, smooth ice, slowly rising and falling with the monotonous motion of the sea. Thanks to the efficiency of the Hydrographic Office of the Navy Department, Sailing Directions and Pilot Charts are now issued regularly, showing the best route to take in different months of the year in order to avoid these calm regions, or at least to cross the belt of doldrums where its extent is smallest. The study of these directions and charts should never be neglected by a navigator about to cross this zone. 24. The Horse Latitudes.—The regions of light variable winds and occasional calms prevailing at the outside borders of the trade winds both in north and south latitudes are commonly known among mariners as the horse latitudes. Unlike the doldrums these regions are marked for comparatively clear and fresh weather.
23. The following table shows the approximate limits of the trade winds and calm regions during the months of March and September in the Atlantic and Pacific oceans:
25. Regions of Westerly Winds.-Outside the horse latitudes, and all across the temperate zones, westerly winds are predominating, although frequently interrupted by storms and occasional shiftings. For instance, between 40° and 60° S the wind blows almost continuously from some westerly point. Exceptions, of course, occur, and sometimes the wind is reversed to easterly, but she duration of easterly winds in these regions is very brief and seldom violent. For this reason the passage around Cape Horn to the westward, in a sailing ship, is always considered as a “rough passage,’’ the wind always being from the west and accompanied with rainy, cold, and stormy weather. Sailing vessels from Northern Europe and the United States, bound for Australia, New Zealand, etc., therefore take an outward course by Cape of Good Hope, but return across the South Pacific Ocean, passing Cape Horn to the eastward, thus carrying comparatively fair winds nearly all around the world. These westerly winds just mentioned are sometimes termed anti-trade winds.
26. Of the periodical winds the monsoons are the most noteworthy and important. Like the trade winds, the monsoons are caused by the inequality of heat at different regions as well as by the rotation of the earth. The monsoons of the Indian Ocean and China Sea are the most famous winds of their class. Throughout the whole of this region as far as 30° N in India and 20° S, between Madagascar and the coast of Australia, the wind is reversed every