Wind is air in motion. The force of the wind is measured by Anemometers (q.v.), of which some measure the velocity, and others the pressure. The following are a few velocities of wind, translated into popular language: 7 miles an hour is a gentle air; 14 miles, a light breeze; 21 miles, a good steady breeze; 40 miles, a gale; 60 miles, a heavy storm; and 80 to 150 miles, a hurricane sweeping everything before it. A few of the accepted comparisons of velocity and pressure may be added: 5 miles an hour represents a pressure of 2 oz. on the square foot; 10 miles, lb.; 20 miles, 2 lb.; 30 miles, 4 lb.; 40 miles, 8 lb.; 51 miles, 13 lb.; 60 miles, 18 lb.; 70 miles, 24 lb.; 80 miles, 32 lb.; and 100 miles, 50 lb. During a great storm in 1867 the pressure was 35 lb., the velocity 83 miles. In the Tay Bridge storm (December 1879), which travelled at rates varying from 40 to 70 miles an hour, the velocity of some gusts reached 96 or even 150 miles. The British Association Committee on Wind-pressure have reported cases of 80 and 90 lb. to the square foot; but the whole question of the relation of velocity to pressure cannot yet be regarded as ascertained.
Seamen more than landsmen require to pay attention to variations in the strength of the wind, as well as in its direction, and to adopt such phrases as will render that strength generally intelligible. Anemometers, used on land for this purpose, are unsuited to the requirements of seamen, who have found it convenient to divide winds into twelve kinds, in relation to strength, designated thus: Faint air, light air, light breeze, gentle breeze, fresh breeze, gentle gale, moderate gale, brisk gale, fresh gale, strong gale, hard gale, and storm. This classification was determined in 1806 by Beaufort according to the amount and kind of sail which one of the ships of the royal navy could safely carry at the moment. These estimates of the wind's force by the scale 0 to 12 mean that 0 represents a calm and 12 a hurricane.
All wind may be regarded as due directly to differences of atmospheric pressure at the same level above the sea as observed at different places; these in their turn being ultimately referable to differences of temperature, and, in a less degree, to differences of humidity. Thus, if the temperature of two adjacent regions become, from any cause, unequal, the air of the warmer, being lighter, will ascend and flow over on the other, whilst the heavier air of the colder region will flow in below to supply its place. Hence a difference in the temperature of the two regions gives rise to two currents of air—one blowing from the colder to the warmer along the surface of the earth, and the other from the warmer to the colder, in a higher stratum of the atmosphere; and these currents will continue to blow till the equilibrium be restored. Winds are classed as Constant, Periodical, and Variable Winds.
Constant Winds.—The Trade-winds.—When the surface heated is, roughly speaking, a whole zone, as occurs in the tropics, a surface-wind will set in towards the heated tropical zone from both sides, and uniting ascend, and then separating flows as an upper current in opposite directions. Hence a surface-current will flow from the higher latitudes towards the equator, and an upper current towards the poles. If, then, the earth were at rest, a north wind would prevail in the northern half of the globe, and a south wind in the southern half. But these directions are modified by the rotation of the earth on its axis from west to east. In virtue of this rotation, objects on the earth's surface at the equator are carried round toward the east, at the rate of 17 miles a minute. But as we recede from the equator this velocity is continually diminished; at lat. it is only miles a minute, or half of the velocity at the equator; and at the poles it is nothing. A wind, therefore, blowing along the earth's surface towards the equator is constantly arriving at places which have a greater velocity than itself. Hence it will lag behind—i.e. will come up against places towards which it blows, or become an east wind. Since, then, the wind north of the equator is under the influence of two forces—one drawing it south, the other drawing it west—it will, by the law of the composition of forces, flow in an intermediate direction—i.e. from north-east to south-west. Similarly in the southern tropic the wind will blow from south-east to north-west. All observation confirms this reasoning. From the great service these winds render to navigation they have been called the Trade-winds. It is only in the Pacific and Atlantic oceans that the trade-winds have their fullest development. In other parts of the trades' zones, such as southern Asia and intertropical Africa and America and adjoining oceanic regions, they are more or less diverted from their course by the unequal distribution of land and sea (on which see MONSOON and METEOROLOGY). In the Atlantic the North Trades prevail between and lat., and in the Pacific between and lat.; and the South Trades in the Atlantic between N. and S. lat., and in the Pacific between N. and S. lat. These limits, however, are not stationary, but follow the sun, advancing northward from January to June, and southward from July to December.
The Region of Calms is a belt, or lat. broad, stretching across the Atlantic and Pacific, approximately parallel to the equator. It marks the meeting-region of the north and south trades.
Here also occur heavy rains, and thunderstorms almost daily. This belt varies its position with the trades, reaching its most northern limit in July, and its most southern in January. The trade-winds are more strongly marked in the Atlantic than in the Pacific Ocean; indeed, in the western part of the Pacific, to the east of Japan and the Philippine Islands, they are but obscurely marked at all seasons. When the belt of calms nears the African coast, in the Gulf of Guinea, the copious rainfall gives rise to the strong steady-blowing gales of that coast, called Tornadoes and Hurricanes. In nautical parlance the doldrums may be either the region of calms or the calms characteristic of it.
Periodical Winds.—Land- and sea-breezes are the most general, as well as most readily explained, of the periodical winds. On the coast, within the tropics, a breeze sets in from the sea in the morning, at first a mere breathing on the land, but gradually it increases to a stiff breeze in the heat of the day, after which it sinks to a calm towards the evening. Soon after a contrary breeze springs up from the land, blows strongly seaward during the night, and dies away in the morning, giving place to the sea-breeze as before. These winds are caused during the day by the land becoming more heated than the sea, consequently the air over it ascends, and the cool air from the sea flows over on the land to supply its place; and during night, by the temperature of the land falling below that of the sea, when the air, becoming thereby heavier and denser, flows over the sea as a land-breeze. It is within the tropics that sea-breezes are most marked and constant, because there the sun's heat is strongest, and atmospheric pressure is practically uniform, except in those rare instances where it is disturbed by the passage of cyclones. But in countries such as Great Britain, where atmospheric pressure is most commonly greater or less than that of surrounding regions, the strength of the wind blowing from the high to the low barometer is far stronger than that which would result from the disturbance caused by the unequal heating of land and water; and consequently the sea-breeze is not then felt. In the warm months, however, at those times when barometers are nearly uniform over northern and western Europe, there is a gentle sea-breeze all round Great Britain during the heat of the day, and a land-breeze during night. Thus on the coast of Berwickshire during fine settled summer weather, when the temperature of the land is much warmer than that of the sea during the day, in the morning the wind is north-west till about 10 A.M., when it veers to north, falling all the time till finally it sinks to a calm. A little before noon it springs up from north-east or east, veers to south-east from 2 to 3 P.M., where it continues till 7 P.M., when it veers to south and south-west, and gradually sinks to a calm. About sunset it springs up from west and veers to north-west during the night, where it continues till next morning. On the other hand, on the west coast of Scotland north-west winds diminish in force toward sunset, giving rise to the weather say, 'The west wind is a gentleman and goes to bed.' Quite analogous to the land- and sea-breezes are the monsoons, which, as regards southern Asia, are only the north trades drawn out of their course in summer by the heated land-regions—the south-west monsoon being really only a vast sea-breeze blowing on southern Asia, and lasting several months of the year.
Variable Winds depend on purely local or temporary causes, such as the nature of the ground, covered with vegetation or bare; the physical configuration of the surface, level or mountainous; the vicinity of the sea or lakes; and the passage of storms. Within the tropics all except the last of these is borne down, or all but borne down, by the great atmospheric currents, which prevail there in all their force. But in higher latitudes this is not the case; these, therefore, are the regions where variable winds prevail. The most noted of these winds are the Simoom, Sirocco, Solano, and Harmattan. The Bora (q.v.) is a cold tempestuous wind, blowing from the Alps down on the Adriatic; and the Gregale is a peculiarly cold, parching, and unhealthy wind which in spring and early summer descends on Malta from Greece. The Puna Winds prevail for four months in the year in a high barren tableland in Peru called the Puna; as they are probably part of the south-east trade-wind, after having crossed the Andes they are drained of their moisture, and are consequently among the most dry and parching winds that occur anywhere on the globe. In travelling over such regions it is necessary to protect the face with a mask from the glare and heat of the day, and from the intense cold of the night. The East Winds which prevail in the British Islands in spring are part of the great northern current which at that season frequently descends from northern over southern Europe. Their origin explains their dryness and unhealthiness. It is a prevalent notion that the east winds in Great Britain are damp. It is quite true that many easterly winds are peculiarly damp; all that prevail in the front part of Storms (q.v.) are damp and rainy, these being simply an indraft of the air towards the low barometer which is advancing from the west at the time; and it is owing to this circumstance that in the east of Scotland the greater part of the annual rainfall falls with easterly winds. All of these damp easterly winds, however, soon shift round to some westerly point. But the genuine east wind, which is the dread of the nervous and of invalids, does not shift to the west, and is specially and intolerably dry. In the third week of May 1866 this character was strongly marked, when at many places in Scotland the humidity was only 40, and on some occasions as low as 29; the degree of this dryness will be appreciated when it is stated that the mean driest month during many years showed a humidity only of 73, saturation being 100. While this wind lasted, the daily range of temperature was double the usual amount, the soil was parched, and the leaves of trees and plants were blackened and destroyed. Deaths from brain-diseases and consumption reach the maximum in Great Britain during the prevalence of east winds. The Etesian Winds are northerly winds which prevail in summer on the Mediterranean. They are caused by the great heat of North Africa at this season, and consist in a general flow of the air of the cooler Mediterranean to the south, to take the place of the heated air which rises from the sandy deserts. The Mistral (q.v.) is a steady, violent north-west wind, felt particularly at Marseilles and the south-east of France, blowing down on the Gulf of Lyons. The Pampero blows in the summer season from the Andes across the pampas of Buenos Ayres to the seacoast. It is thus a north-west wind, and so far analogous to the stormy winds which sweep over Europe from the south-west. But since it comes from the Andes over the South American continent, it is a dry wind, frequently darkening the sky with clouds of dust, and withering all vegetation.
Lord Bacon remarked that the wind most frequently veers with the sun's motion, or passes round the compass in the direction of north, north-east, east, south-east, south, south-west, west, and north-west, to north. This is due to the fact that by far the greater proportion of the storms of north-western Europe follow their course to eastward along paths lying to the north of the British Islands. Dove of Berlin first propounded the Law of the Rotation of the Winds, and showed that the whole system of atmospheric currents—constant, periodical, and variable winds—obey the influence of the earth's rotation as they blow from regions of high pressure, where there is a surplus, towards regions of low pressure, where there is a deficiency of air.
Boreas was the north wind of the ancient Greeks, called by the Romans Aquilo or Septentrio; Notus or Auster, the south wind; Eurus, the east or south-east; Zephyrus or Favonius, the west wind. Africus (Gr. Lips) was south-west.
See METEOROLOGY and STORMS, with works cited; EUROCLYDON, HARMATTAN, ROARING FORTIES, SIMOOM, &c.