WIND, air in motion; the distributer of heat and moisture over the earth's surface; thus constituting a principal factor of what is called the weather. Any cause which disturbs the equilib rium of pressure of the atmosphere will give rise to aerial currents. The most potent causes are variations in tempera ture and in amount of aqueous vapor. Winds will always blow or tend to blow, from the regions of higher to regions of lower pressure, and the greater this difference, or, as the meteorologist ex presses it, the steeper the gradient, the greater will be their force or intensity. The study of the action of the wind thus takes into account the differential pres sure producing it, its direction, and its velocity; and for the sake of convenience of record and discussion winds are classi fied according to their character as permanent, periodical, and variable; with respect to direction, we distinguish between horizontal, vertical, inclined, and spiral movements. Various scales are in use; the seaman is generally sat isfied with eight divisions, viz., the directions of the cardinal points and the four intermediate points. On land, when greater precision can be had, as many as 32 equidistant compass directions may be employed, but in more refined dis cussions the ordinary divison of the circle into degrees has been used. With respect to velocity, the scales adopted in different countries, and used on land or sea, vary likewise according to the de gree of precision desired.
Storm warnings, as dangerous to navi gation, are given when the wind velocity reaches or surpasses 35 miles an hour. During cyclonic storms or tornadoes the velocity is often very much greater, and may approach the upper limit of our scale. Generally the greater the eleva tion of a place above the sea-level the greater the velocity of the wind, and winds blow stronger at sea than over land.
Of the periodical winds, blowing half a year in one, and the other half in the other direction, the monsoons are the most noteworthy, in particular those of southern Asia, where from October to April the N. W. and from April to Oc tober the S. W. winds are blowing with great steadiness. Their reversal in May and October is often accompanied by violent hurricanes and deluges of rain. Monsoons are the direct consequence of the unequal effect of the sun's heating power over large tracts of land, in con trast with its effects over a large ex panse of ocean. On a smaller scale sea and land breezes have a similar origin; on S. shores the land becomes powerfully heated in the day time, while at night its temperature sinks below that of the adjacent ocean. This circumstance gives rise in the morning to the welcome sea breeze, setting in toward land, at first gently, but reaching a stiff breeze at the time of the greatest heat of the day, or about 2 P. M. After this hour it sinks gradually to a calm in the evening, to be followed by the contrary or land breeze, blowing strongly from the land seaward during the night, again dying away in the early morning.
Tornadoes also have their seasons of frequency, occurring in the United States most frequently in April, May, June, and July, in the order named; while the hours of greatest frequency are between half past three and five o'clock in the after noon—i. e., soon after the warmest part of the day, when the warm ascending currents are most likely to meet the cooler descending current, to the meeting of which currents the violence of the tornado is, in a great measure, due. We must also notice a remarkable law, known as Dove's or Buys Ballot's, from its importance to the navigator, as its knowledge will enable him to avoid the most destructive part of revolving storms. It has been noticed that in the Northern Hemisphere the wind has a de cided tendency to veer round the com pass according to the sun's motion, i. e., to pass in the direction (say) from N. through N. E., E., S. E., etc., round to N., and that a revolution in the opposite direction is very rare. Ballot's law may be stated as follows: (1) Stand with your back to the wind, and the center of depression or of lowest barometer will be to your left in the Northern Hemi sphere and to your right in the Southern Hemisphere; (2) stand with the high barometric pressure to your right and the low pressure to your left, and the wind will blow on your back, the posi tion to be reversed for the Southern Hemisphere. In general, the resultant wind can be worked out from a knowl edge of the isobaric curves, or graduated lines of equal pressure corresponding to the time and place. In cyclones, then, the rotation of the air in the Northern Hemisphere is in a direction contrary to the hands of a watch laid face upper most, and in the same direction as the hands for the opposite hemisphere. At the equator violent cyclones do not occur. It is well known that at the time the center of a cyclonic depression passes over a place there is a calm or lull in the wind, which lull is preceded and followed by winds of equal violence, but in op posite directions. Anemometers, or in struments which measure the force and direction of the wind, are of two kinds: those which record at stated intervals, and those which give an automatic and continuous registration. Some are self registering instruments for the direction, others for the pressure of the wind. Of velocity anemometers that of Robinson has a high reputation. It registers the velocity by the revolutions of a vertical shaft connected with arms and hemi spherical cups, on which the wind im pinges. It thus registers the number of miles passed over by the wind in a given time. See ANEMOMETER; CYCLONE; TOR NADO; WEATHER BUREAU.