Wind, a sensible movement of the air with reference to the earth's surface. The horizontal component of this movement is usually that which is specially referred to by the term wind, but the powerful vertical movements that take place in tornadoes and other severe storms may also be included under the same heading. In general, a wind is the result of an initial, local disturbance of the density of the atmosphere, in consequence of which the air is set in motion; its motion is then modified by the resistance of the earth's surface and by the diurnal rotation of the earth about its axis, and may be propagated into regions far distant from the original seat of disturbance. - In keeping a record of the wind observed at any station, it is important to note both the duration and the force or velocity; directions should be recorded with reference to the true, not the magnetic meridian, and should be given to the nearest half point of the compass card. The force may be estimated and expressed in an arbitrary scale on which zero (0) denotes calm, and 10 the high winds of a destructive hurricane or tornado. Arbitrary scales of 0 to 12, 0 to 6, and 0 to 4 are in use in various countries; but the scale 0 to 10 will, it is hoped, gradually supersede these.

When possible, the force of the wind should be measured by an anemometer, the principal forms of which instrument are: the Robinson anemometer, measuring the velocity of the horizontal movement of the wind by the revolution of a vertical spindle to the top of which are attached four horizontal arms, each bearing at its extremity a hemispherical cup; and Wild's tablet anemometer, consisting simply of a small plane tablet so suspended from a wind vane that its axis is always perpendicular to the direction whence the wind blows; this tablet hangs by a hinge, so that it is deflected from its vertical position by an angle increasing with the force of the wind, and which is measured by a scale of degrees. This latter instrument gives therefore the strength of each gust, while the Robinson anemometer gives the total movement of the air during any prescribed interval. The numerous careful investigations that have been made into the accuracy of the records of these simple instruments have justified their extended adoption within the past few years. Other forms of the anemometer are described under that title.

As the velocity of the wind is independent of the instrument by means of which it is measured, while the pressure of the wind varies according to the shape, etc, of the surface against which it acts, it is now customary to convert the indications of all forms of anemometers into velocities, i. e., miles per hour or metres per second. The most recent investigations bearing on this matter have been published by cavallero (Turin, 1873), Phillips (London, 1874), Hagen (Berlin, 1874), and Thiesen and Dohrandt (St. Petersburg, 1875). - The great importance of correctly understanding the laws of the winds has led to the accumulation of a vast mass of observations thereon. Among the treatises containing large collections are Kämtz, Lehrbuch der Meteorologie; the article on wind by Muncke in Gehler's Wörterbuch; Maury, "Wind and Current Charts;" Buys-Ballot, Windkaartjes tan den noorden Atlantischen ocean; Buchan, "Distribution of Pressure and Winds;" Muhry, Allgemeine Klimatologie; Hann, Untersuchungen über die Winde; Schmid, Meteorologie; the admiralty wind and current charts; and above all, Coffin's "Winds of the Globe "(Washington, 1876). This latter immense work is a new edition of Coffin's "Winds of the Northern Hemisphere" (Washington, 1856), and embraces a collection and summary of the results of all the observations during the past 100 years.

The study of so extensive a series of observations shows that some winds have a local or temporary character, while others are very permanent or prevail over large areas. These latter movements are generally spoken of as currents, as distinguished from the local or temporary movements which retain the designation winds. 1. If we compute the resultant of all the motions to which the air at any place has been subjected during a year or a month, we arrive at the most general view that can be taken of the movement 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 number of observations of the actual wind velocities is insufficient, an attempt has been made, first by Lambert, but principally by Coffin, 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 follows: Between the latitudes 30° N. and 30° S. the lowest stratum of air moves westward; between these parallels and the respective poles the resultant motion of the lowest stratum of air is eastward. 2. The movements of the higher strata of the atmosphere are of equal importance to a proper understanding of the whole subject, but are much more difficult of investigation.

In general, the observations of the motions of the clouds, of the courses of balloons, and of the winds on mountains show that between the tropics the upper current has a resultant motion toward the east, while beyond the tropics the resultant is toward the east only for the highest stratum of cirrus clouds, and is for the lower clouds occasionally toward the west, depending apparently on some special causes. A few observations of the trails left 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. 3. The most important of the general and periodical currents are the trade winds, which prevail between the parallels of 30° N. and 30° 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 irregularities depending on the seasons. The whole system of trade winds is divided by a belt of calms in the neighborhood of the equator, separating the N. E. winds of the northern from the S. E. 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 September and southward in March of its mean position; 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 anti-trades designates the currents that prevail quite uniformly above the trade winds, at a height of 3,000 ft. or more above the earth's surface; this current, known also as the "upper trade "or " return trade," seems to be merely the return to the temperate zones of the air that had flowed at the surface toward the equator. 5. The term monsoons, from a Malaysian 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 deflected 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 northern portion of the Indian ocean is completely reversed during summer, but in winter is greatly reenforced 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 50) from mountain ranges or coast lines, and are much feebler than the monsoons. 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, tramontana; the N. E., greco; the E., levante; the S. E., sirocco'; the S., ostro; the S. W., libeccio; the W., ponente; the N. W., maestro or maestrale. 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 etymology 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 customary 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 Sicilv the sirocco wind is so oppressive by reason of the heat and dampness that both men and animals suffer extremely.

In Spain the same wind is known as the solano, and in Turkey as the samiel. The föhn 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 weather; it blows down the mountain sides and valleys, and, although generally spoken of as a S. wind, is often a N. wind. On the windward side of a mountain range the föhn is moist like the sirocco; on the leeward side it is drier. It is found on a careful examination that winds similar to the föhn exist in every mountainous region; indeed, in the Rocky mountains and the Himalaya its distinctive characters are even more decided. - Of the hot winds, none is more famous than the simoom of northern Africa and Arabia. The many fables and exaggerated accounts of ancient travellers have been materially modified by exact observations of recent investigators, from which it appears' that this is a strong, hot, dry wind drawn from the heated interior of the continent; it is frequently accompanied by sand clouds or sand pillars, and its deadly qualities, if such it has, are simply the result of the oppressive heat and the very fine dust.

Similar hot winds prevail in Egypt in May and June, and are there known as the khamsin. - Of the dry winds that flow out from the interior of continents or down the slopes of mountain ranges, some are cold, others warm. Thus the northers of Texas are due to a thin surface layer of dry air, which as it flows from the Rocky mountains, from Kansas, and from Minnesota, southward or southeastward, continually loses by radiation the heat it receives from the sun, and, underflowing the warmer, moister air of the gulf of Mexico, rushes over the smooth surface of the water with thrice the velocity that is observed in the interior of the continent. On the W. coast of Africa the dry E. and N. E. winds are known as the harmattan; these are cooling on account of their extreme dryness, and in every detail offer a parallel to the northers, except that they have a higher temperature, and are frequently accompanied by sand, which is rarely or never found in Texas. The dry cold wind flowing southward from the Himalaya over India is there known as the tereno.

In South America, from Patagonia to Brazil, there occurs a similar dry wind known as the pampero, which flows almost uninterruptedly from the Andes E. and S. E. to the Atlantic. Similar strong, cold, dry winds flowing from central Europe southward over the Adriatic and Black seas are known there as Bora (with which the euryclydon of St. Paul may be identical). At Malta the N. E. wind is the gregale. In southern Arabia the cold N. N. AV. wind of winter is the Belat. Perhaps the most thoroughly desiccated of any of the winds that have been observed as yet are the S. E. or puna winds of eastern Peru and the N. or Buran of Thibet, on which table lands cold dry gales prevail, which are highly disagreeable to human beings and even to animals. Similar dangerous gales are called purgas in Labrador, guxen in Switzerland, gallegos in Spain, and tourmentes in France. The dry east winds of spring in Great Britain have been from time immemorial proverbial for their injurious effects on delicate constitutions, and the very dry west winds of the United States E. of the Rocky mountains contribute, it is very plausibly urged, to the nervous temperament of the nation.

The prevailing winds being westerly both in Europe and North America, it follows that the former continent enjoys a much moister and more agreeable climate, and one much less provocative of nervous diseases. - Of the names given to certain storm winds as such, we may mention the levante, a strong east wind in the eastern part of the Mediterranean; the hurricane, a term derived from the ouracan of the Carib Indians, and applied by them to the terrific storms of the West Indies; the typhoon, a term of most ancient origin, nominally derived from the Chinese tae-fun as applied to the great storms of the Pacific ocean, but curiously related to the name Typhon applied by the Egyptians and Greeks to a dreaded divinity; finally, the tornado, a term applied in America to destructive winds that rush in narrow paths over long belts of territory, accompanied by whirling clouds and heavy rain or hail. The name tornado (Port, tornar, Sp. tornear, to turn) was originally given by the Peninsular navigators to the violent local storms that occur a short distance off the coast of Africa, and was subsequently applied very properly by them to the similar violent storms of our southern states.

Less perfectly developed but still destructive tornadoes occur in all parts of the United States. Of local American terms we record only the expressive name "blizzard " given in the states W. of the Mississippi river to the blinding storms of sleet or snow and high N. wind that suddenly follow warm spring-like days in winter and early spring. Similar storms of similar origin occur in the steppes of southern Russia, where they are known as viuga. In the Sandwich islands the S. wind preceding a hurricane and interrupting the regular N. E. trades is called the Kona. On the W. side of the Crossfell range of hills in England are formed during easterly winds and previous to rains two peculiar clouds, from one to five miles apart, under which it is calm, while between them is felt a strong east wind locally known as the "helm wind of the Crossfell." 8. Our sketch of the winds would be imperfect without enumerating the remarkable areas of variable light winds and calms that constitute an important feature in terrestrial meteorology.

These calms are sufficiently defined by their names: 1, the calms of the tropic of Capricorn; 2, the equatorial belt of calms, or the doldrums; 3, the calms of the tropic of Cancer, or the horse latitudes. 9. The explanation of the cause of the general system of terrestrial winds has been already given in Meteorology. But besides the winds caused by differences of barometric pressure and of density, there are a few phenomena of occasional and minor importance, due to other causes; for instance, the gusts of wind that precede by a few minutes heavy local showers of rain and hail, result from the mechanical action of the falling drops, which communicate a part of their motion to the air; this, pressing down against the ground, is forced outward, so that on the edge of a rainy region the wind appears to blow from the rain. The existence of this wind complicates considerably the phenomena of local thunder storms, and has even misled some authorities into erroneous explanations of their origin and structure. - Pressure of the Wind, Owing to their great importance in relation to innumerable practical interests (such as the construction of windmills, the art of gunnery, the theory of the pendulum, the driving of railroad trains, and the sailing of vessels), the twin questions of the force of the wind and the resistance of air or water to moving bodies have been studied by very many eminent philosophers and experimenters.

The results thus far obtained are almost entirely empirical. As regards the connection between pressure and velocity, the law announced by Newton, that the resistance should be as the square of the velocity of the moving body, is, for ordinary winds, sufficiently exact. The resistances or pressures vary directly as the density of the medium; they even vary slightly in the air for the ordinary ranges of the temperature and barometer; but they vary in a remarkable manner with every change in the form and the dimensions of the resisting body. The laws of the variation of the resistance as depending on velocities and forms and dimensions can only be satisfactorily given in the shape of an abstract of the numerical results deduced from each experiment. In general, it may be said that a concave surface exposed to the wind offers greater resistance than an equal sectional area of plane surface, and that a convex surface offers less resistance than a plane. The resistance offered by any body depends quite as much on the configuration of its hinder as of its front portions. The resistance offered by a plane surface which is not normal to the wind is less than when it is normal; and it diminishes in proportion to the cosine of the angle of incidence.

For normal incidence the resistance is not to any great extent dependent on the nature of the surface, i. e., whether it be rough or smooth. The determination by experiment of the actual pressure exerted by the wind is a very delicate matter. That which has been most widely adopted is known as the Smeaton or Rouse formula ("Philosophical Transactions," 1759), according to which the pressure in pounds avoirdupois on a surface of one square English foot is equal to 0.00492 multiplied by the square of the velocity expressed in miles per hour; the pressures calculated by this formula are given in the following table. A more trustworthy formula was deduced by Muncke (1842) from the observations of Borda, Hutton, and Woltmann, according to which the above constant coefficient should be 0.00499; but the difference between the two is insignificant in consideration of the extreme variations which depend on the size and shape of the resisting object. In very recent times this important subject has received further elucidation by both theoretical and experimental methods. (See the works of Stokes, Rankine, Thomson, Duchemin, Russell, Robinson, Saint.Venant, Cavallero, Dohrandt, etc.) Maxwell ("Proceedings of the Mathematical Society," 1870) has given theoretical formulas and curves showing the movements of the particles, of an incompressible fluid streaming past a moving obstacle; while Hagen (Berlin, 1872) has experimentally investigated these motions.

Helmholtz (Berlin Monatsbericht, 1873) has shown that for moderate velocities it is very approximately proper to consider the air as an incompressible fluid, free from friction. Finally, Thiesen (Wind.Repertorium, 1875) has made a careful theoretical study of the experiments of Hagen and Dohrandt, and established the rule that the pressure of the wind against an inclined rectangular plate is really very nearly proportional to the square of the velocity and the cosine of the angle of incidence of the wind, while the absolute value of the normal pressure is as given by Hagen's observations. The latter physicist (Berlin, 1874) has embodied the results of very careful observations at moderate velocities in a formula which, converted into English measures, is as follows: P=(0.0028934 + 0.0001403^)Av2; where the velocity v is expresed in miles per hour, the area A of the surface is in square feet, the perimeter p of the surface in linear feet, and the resulting pressure P is in pounds avoirdupois per square foot. By introducing the term p Hagen has expressed the fact that the pressure depends to a considerable extent on the shape as well as the surface of the resisting body.

The formula applies to plane surfaces placed normal to the incident wind, and assumes that the density of the air is that belonging to the barometric pressure, 29.84 in., and the temperature 59° F. For the resistance to shot at high velocities, see Gunnery.

PRESSURE OF THE WIND.

VELOCITY.

PRESSURE PER SQUARE FOOT, POUNDS.

Miles per hour.

Feet per second.

Rouse and

Smeaton, per sq. foot.

Hagen.

Circular

Square

Triangular

plates of one square foot.

1

2 3

1.47 2.98 4.40

0.005 0.020 0044

0.003 0.014 0.030

0.003 0.014 0.031

0.004 .

0.014

0.032

4

5 10

5.87

7.33

14.67

0.079 0.123

0.492

0054 0.085 0.339

0.055 0.086 0 345

0.057

0.088 0.353

15 20 25

22.00 29.34 39.67

1.107 1.968 3.075

0.763 1.356 2.119

0.777 1.382 2.159

0.795 1.413

2.208

80 35 40

44.01 51.34 58.68

4.429

6.027 7.873

3.052 4.154 5.425

3.109

4.232 5.527

3 180

4.328 5.653

45 50 60

66.01 73.35

88.02

9.963 12.300 17.715

6.866

8.476

12.208

6.996

8.637

12.437

7.155

8.a33

12.719

80 100

117.36 146.70

31.490

49.200

21.701 33.908

22.110 34.546

22.613

35.332