Page:The American Cyclopædia (1879) Volume XVI.djvu/682

 658 WIND ment of the atmosphere. The resultants for the United States are given in plates ix. and x. of the "Statistical Atlas" of the census bureau (Washington, 1874). But as the num- ber of observations of the actual wind veloci- ties is insufficient, an attempt has been made, first by Lambert, but principally by Ooffin, to make use of the estimated wind force, and even to deduce resultants based on the assumption of an equal average velocity for each wind. The result may be stated in general terms as fol- lows : Between the latitudes 80 N. and 80 S. the lowest stratum of air moves westward ; be- tween these parallels and the respective poles the resultant motion of the lowest stratum of air is eastward. 2. The movements of the high- er strata of the atmosphere are of equal impor- tance to a proper understanding of the whole subject, but are much more difficult of investi- gation. In general, the observations of the motions of the clouds, of the conrses of bal- loons, and of the winds on mountains show that between the tropics the upper current has a resultant motion toward the east, while be- yond the tropics the resultant is toward the east only for the highest stratum of cirrus clouds, and is for the lower clouds occasional- ly toward the west, depending apparently on some special causes. A few observations of the truils loft by shooting stars show that at heights of from 5 to 100 m. a great diversity of currents prevails, but that no one direction predominates. 8. The most important of the general and periodical currents are the trade winds, which prevail between the parallels of 80 N. and 80 S. latitude, but attain their full breadth and force only in mid-ocean, and are very materially circumscribed near the eastern and western continents. The breadth of the trade-wind zone of the Pacific ocean is not so well determined as in the Atlantic, and the trades of the Indian ocean offer notable irregu- larities depending on the seasons. The whole system of trade winds is divided by a belt of calms in the neighborhood of the equator, sep- arating the N. . winds of the northern from the S. . winds of the southern hemisphere. Although the position of this calm belt, and also of the exterior limits of the trade winds, lies a little northward in August and Septem- ber and southward in March of its mean posi- tion, yet these slight seasonal changes do not deprive the whole system of winds of their permanent character. In the equatorial belt of calms heavy rains constantly occur ; but in the trade-wind region proper few clouds and light showers are observed. 4. The term an- ti-trades designates the currents that prevail quite uniformly above the trade winds, at a height of 8,000 ft. or more above the earth's surface ; this current, known also as the "up- per trade " or " return trade," seems to be mere- ly the return to the temperate zones of the air that had flowed at the surface toward the equator. 5. The term monsoons, from a Ma- laysian or Arabic word signifying " seasonal," is applicable to all winds that with the season change their character from being land winds to sea winds. Thus, on the W. coast of Africa in summer, the regular N. E. winds are denY< t ol decidedly toward the continent ; in Australia and in North America similar phenomena are noted ; but the most striking case is that of India, where the N. E. trade wind of the north- ern portion of the Indian ocean is completely reversed during summer, but in winter is greatly refinforced by the land and sea winds that thus alternate between N. E. and S. W. (See Blanford's " Winds of Northern India," London " Philosophical Transactions," 1874.) The ancient Greeks designated certain winds that came and went with the season as Etesian winds. 6. Similar to the monsoons, but less decidedly pronounced, are the day breeze and night breeze, or the land wind and sea wind. These winds follow each other as do the diurnal changes of temperature on which they depend ; they are generally felt most distinctly but a few miles (rarely 60) from mountain ranges or coast lines, and are much feebler than the mon- soons. 7. There are numerous special winds, known usually by special local names. Thus we have in Italy the following terms for the respective winds : for the N. wind, tramon- tana ; the N. E., greco ; the E., letante; the S. E., sirocco; the S., ostro; the S. W., libeccio ; the W., poncnte; the N. W., maestro or maes- trale. The last is known as the mistral in France, and at Nice blows from the north, but at Toulon from the northeast; it is in fact simply the strongest of the winds, as the ety- mology of the name implies. In Greece the mistral blows from the northwest, and brings cool moist air from the Adriatic after a season of rainy weather. A similar latitude is cus- tomary in the appellations of the other winds ; thus at Athens the warm damp S. or S. W. wind is termed the sirocco, and brings the heavy rains of autumn and winter ; in Madeira, on the other hand, the sirocco is a very dry hot wind from E. S. E. ; in Sicily the sirocco wind is so oppressive by reason of the heat and dampness that both men and animals suf- fer extremely. In Spain the same wind is known as the solano, and in Turkey as the samiel. The fi>hn wind, as it is called in the Alps, was until lately usually described as a continuation of the sirocco ; but as now more philosophically explained, it may be described as a warm or hot dry wind with hazy weath- er; it blows down the mountain sides and valleys, and, although generally spoken of as a S. wind, is often a N. wind. On the wind- ward side of a mountain range the fohn is moist like the sirocco ; on the leeward side it is drier. It is found on a careful examination that winds similar to the fohn exist in every mountainous region ; indeed, in the Rocky mountains and the Himalaya its distinctive characters are even more deci- ded. Of the hot winds, none is more famous than the simoom of northern Africa and Ara-