Fig. 47.—Normal Wind Direction and Velocity for July and August. (Köppen.)
Fig. 48.—Trade and Counter Trade-winds.
The prevailing westerlies are high-latitude surface winds of the permanent circulation. In the southern hemisphere they are particularly strong and steady owing to the comparatively unbroken stretch of ocean. In the north also they are strong and persistent, but variable in direction because of disturbances by local winds due to unequal heating of tracts of land and sea. These features are well illustrated in charts 47 and 48. Of particular interest in aëronautics is the prevailing wind blowing from the United States to Europe, which has been considered a suitable current for transoceanic balloon voyages.[64]
The periodic winds are those whose gradient alternates annually or daily, due to annual or daily fluctuations of temperature on sloping or on heterogeneous parts of the globe. The annually fluctuating winds due to alternate heating and cooling of continents, or large land areas, bear the general name of monsoon. Among diurnal winds the most prominent are the land-and-sea breezes, and the mountain-and-valley breezes. Both kinds are practically available in aëronautics; the monsoons for long-distance travel, the diurnal winds for local use.
The general motive cause is the same for all periodic winds. When any portion of the earth’s surface is periodically more heated above its normal temperature, or average for the year, than the neighboring region, the resulting abnormal temperature gradient causes a periodic surface wind tending toward the excessively heated place, and a counter wind above. That is, the cooler and heavier column of air sinking and uplifting the lighter, results in a lowering of the common center of gravity of the two columns of air, and thus furnishes the driving power of the wind. For example, an island or a peninsula may be considerably hotter by day and cooler by night than the surrounding water; a continent may be much hotter in summer and much colder in winter than the bordering ocean. Thus during the hot period a moist wind blows landward; during the cold period a dry wind blows seaward. If the land has vast and lofty slopes the uprush of air during the hot period and the downrush during the cool period may be very powerful. The currents so produced by the aggregate of local agencies, including the deviation caused by the earth’s rotation, combine with the general circulation of the atmosphere to form the actual wind of the place. Thus the periodic current may conspire with the general circulation, or oppose it; may intensify, weaken or obliterate it; may overmaster, reverse or mask it completely.
Of the various continental monsoons of the globe the most powerful spring from the annual flux and reflux of the atmosphere over the vast declivities and table-lands of Asia. Here the conditions are especially favorable. As the sun approaches Cancer, the burning deserts and high plateaus, combining their force with the draft on the mountain sides, generate a continental uprush that sucks in all the aërial currents of the surrounding seas, hurling them aloft to the isothermal layer whence they radiate as the four winds of heaven; for here at this season the planetary circulation is disrupted, obliterated or reversed, appearing merely as a perturbation of the monsoon at its height. In India the force is particularly effective. Along the north the Himalayas stretch 1,300 miles in latitude, with an average height of 18,000 feet and with sunburned areas on either side. North of this range are the lofty plateaus of Thibet and Cashmere, south of it the desert of Gobi and the borders of the Indian Ocean. Over this watery tract from beyond the equatorial line, from the isles of Oceanica and from the wintry plains of Australia, the air flows in with accumulated strength, sweeping the Bay of Bengal and the Arabian Sea in a continuous gale bearing up the mountain slopes incredible floods of water. Over the Arabian Sea in summer the gale is so steady and swift that no ordinary ship can force a passage from Bombay to the Gulf of Aden. Above the Bay of Bengal the moist south winds, converging between the coast and headlands, pour cloud laden up the Himalayan slopes, precipitating their whole vapor in prodigious torrents seldom seen elsewhere. Khasia at this season sustains a Noachian deluge, the rain at times falling nearly a yard deep in one day and night.[65] Quite appropriately, therefore, the summer monsoon over India, especially its component southwest wind from the Arabian Sea, and southerly wind from the Bengal Bay and farther east, is called the wet monsoon.
The winter monsoon of Asia, is the reverse of the summer one, both in direction of gradient and in physical character. It is a cold flood of air pouring from the frigid table-lands and wintry depths of the desert, down the mountains and valleys in continual overflow on all sides of the continent, and then far out over the sea, where it reascends to complete its long cycle. In its descent all moisture vanishes by heating, and no intensive temperature gradient occurs, as in summer, to accelerate its gently modulated tide. In India the winds from Cashmere and Thibet pour down the Himalayas toward the Arabian Sea a clear current of air which unites with the trade-wind, increasing its force, and forming the moderate winter monsoon of that region, or as it is commonly called, from its lack of moisture, the dry monsoon.
The kinematic character, and the extent of both summer and winter currents, are well portrayed in charts 47 and 48 for all the south and southeast of Asia. Across the islands of Japan, it will be observed, the winds blow in opposite directions summer and winter. In Siberia the monsoon winds trend along her great rivers and valleys, generally northward in the winter and the reverse in the summer, combining in both seasons with the prevailing westerlies, due to the rotation of the earth.
All the other continents have their monsoons, though less powerful than those of Asia. In the great desert of Sahara, for example, there is an ascending hot current in the summer, causing a strong indraught from the Atlantic and the Mediterranean; but this is far less intense than if its action were fortified by lofty slopes and table-lands. In winter when the Sahara cools to nearly the oceanic temperature, little monsoon effect is perceptible, and the general circulation continues unperturbed. In Australia the monsoon influence is still feebler, owing to the limited extent of the country and to the general lowness and flatness of the land. Over parts of South America, the annual ebb and flow of the atmosphere is considerable, particularly along the northeastern coast, and in the whole Amazon Valley, whose aërial currents in general conspire with the trade-winds, strengthening them materially in the southern summer, though it is less in winter when the continental temperature more nearly approximates that of the ocean. The monsoons of North America have been described in some detail by Ferrel as follows: