WIND, a current of air established at cer tain times and places within the body of the atmosphere at large, and flowing during pe riods longer or shorter in certain general direc tions; such currents being occasioned chiefly by differences of temperature at different times or localities, and by variations in the production and condensation of uatery vapor. The por tion of the surface of the globe over which any particular wind, permanent or occasional, may extend, is comparatively small, as is conse quently the tract of the entire aerial ocean resting on that surface that is involved. At all times, also, there are parts of the atmosphere that are sensibly at rest or calm; and such ap parently motionless tracts of air are sometimes of very great extent. (For a statement of the physical properties of the air, see ATMOSPHERE; and in reference to the mechanical principles of equilibrium, mobility and disturbance of a fluid mass circumstanced as is the air, see Pstzu MAT1CS). The atmosphere is held to the earth only by gravity and the action of this force does not interfere with its fluidity or elasticity, nor with the effect of any pressures acting at points within it; so that its parts have entire freedom of motion about or among each other, and it is in every part sensitive to the slightest disturbing forces. Since, however, the globe with its aerial envelope is to be re garded as moving in unresisting space, and since the friction of the earth's surface upon the low est stratum of air, and of the strata successively one upon another, has sufficed to communicate to the entire body the earth's own velocity, it follows that the atmosphere, if it were left at rest within itself, must partake of the earth's movements as perfectly as if it were a solid part of that body. The simplest of the dis turbances affecting the atmosphere are the movements of 'atmospheric waves' of greater or less magnitude and duration, but of two sorts, the daily and the occasional or irregular, the occurrence of which is shown by certain periodical or rare, but gradual variations of barometric pressure. From the nature of the medium, these waves are, as compared with those of water, on a vast scale. The indica tions of the barometers at stations scattered over a large area of country show that these waves move singly, and indicate their breadth, and the direction and rate of advance; a gener ally increased or maximum pressure showing at a given time the presence of the crest, while at distances on either side of this a minimum press ure shows the margins or accompanying troughs of the wave. Of daily atmospheric waves, or tides, there are two: (I) that due to attraction of the sun and moon, and which in periods and character is, therefore, similar to the oceanic titles, but which, its maximum ef fect on the mercury column not exceeding ifs of an inch, cannot be a cause adequate to pro duce winds; (2) the heat tide, or elevation of a crest of air along a mendional line following the sun at no great distance, while the cooling on the opposite side of the globe occasions the ad vance of a corresponding line of depression, this tide having, therefore, for its period a solar day, and within that period but a single crest, instead of two opposite ones. Beside these pe riodic fluctuations, there are occasional vast at mospheric waves, due perhaps to previous winds, to great local disturbances of temperature, or to combinations of causes not yet understood. The disturbances by heat that give rise to or dinary periodical or irregular winds are such as occur along certain latitudes, or as are local and irregular altogether. An increase of tem perature equal to 50° F. dilates the air receiv ing it by only about one-tenth of its volume. From the direct rays of the sun air absorbs heat chiefly near the surface of the earth, and yet slowly even here, the warming of the air being more largely due to secondary radiation from the heated surface of the land or water. The heat acquired within a given time is usually by a very gradual increase and limited in amount. If the warming of the air is quite uniform over a large surface, the equilibrium between the af fected and the surrounding bodies may be steadily adjusted and preserved, so that no wind shall result; and it is a common experience of the hot season that, though the air at a place may be intensely heated, or through many de grees within a few hours, yet no wind may occur. During subsequent cooling of the same
body of air a wind is more likely to arise, and especially so if clouds form at no great dis tance. Very generally, however, the effect of heating a tract of air in excess over that around it is to occasion expansion and diminution of density; the column of air so affected moves or flows upward, and while the effect of its mo mentum further relieves its lowermost portions of pressure and diminishes the resistance they can oppose to the surrounding air, the ascending body, losing at considerable height its excess of heat, acquires the density of air at such eleva tion and flows over or outward, increasing the weight and pressure of some or all the sur rounding portions. The lateral equilibrium be low is thus destroyed and a double movement of the air established, the air flowing in from one or more directions below the heated space, and flowing out above. But the momentum acquired in some given direction by the air rushing into the affected space may predom inate, and, the conditions of neighboring por tions of air favoring, a wind may thus be estab lished that shall blow far beyond the point of first disturbance, as well as successively affect portions of atmoshphere further hack of it, and also extend widely, continuing for a long time before equilibrium and calm are restored. As a well-known fact, however, high or widely ex tending winds are more likely to arise just be fore or during storms in which a considerable body of watery vapor is condensed and precip itated from the air, and yet more likely to be felt chiefly after such storms. Winds are also known to be produced in consequence of rapid and great evaporation, and even during the rapid formation of belts or masses of cloud without rain. In all the great oceans, however, there are certain winds, called trade-winds, which always blow in the same direction, though with seasonal variations in the area over which they tilos. These are cold currents of air constantly flowing in from the polar regions to replace the warmer and lighter air which is constantly ascending from the tropical belt and which finds its way hack, at first entirely through the upper strata of the atmosphere, to the re gions in which the cold currents take their rise The direction of these winds, which is nearly due west, but slightly south or north, according as it is a northeast or southeast trade-wind. results from the axial rotation of the earth from west to east. The general character of the air-movement is twofold. There is a move ment from the poles toward the equator, and a return movement from the equator to the poles In low latitudes the latter takes place exclu sively in the higher strata of the atmosphere, but in higher latitudes its effect is often felt on the surface of the earth. In these latitudes. then, the winds may be divided into equatorial and polar, the former being as a rule more or less westerly, the latter more or less easterly. The equatorial winds arc distinguished in gen eral by the highest temperature, the greatest degree of saturation, the most cloudy weather. the most frequent rainfall and the lowest at mospheric pressure; and the polar by the low est temperature, the least degree of saturation. the clearest weather, the least rainfall, and the highest atmospheric pressure. This explains why the southwest wind is that which brings the most rain, and why. a falling barometer is as a rule a sign of approaching rain, and also why the barometer, as is well known, usually shows an upward tendency with an east wind Certain winds have a seasonal character, being either confined to certain seasons of the year, as the harmattan of the Guinea coast and the cue sian winds that blow from the north in summer in the eastern part of the Mediterranean, or changing their direction at certain seasons, such as the monsoons of the Indian Ocean. (See