WIND (AS. /rind, Guth. triads,. 011G. wint, Ger. Wind, wind ; connected with La I- 'yentas, Skt. auto, wind, from rd, to Idim ). _Moving air. The direction of movement, if horizontal, is expressed by stating the point of the compass from which the wind comes. The force was for merly expressed on sonic arbitrary scale, such as zero for calm, and four for destructive to large trees. In 1806 Admiral Beaufort introduced into the British Navy a scale of zero to twelve, arranged according to the amount and kind of sail to lie carried by the standard ship of the Royal Navy. (See BEAUFOET SCALE.) Recently the rnited States Weather Bureau has adopted the rule that in official forecasts the fine subdi visions once used shall be replaced by the follow ing simpler seal( of terms: The above figures are usually understood to refer to velocities as recorded by the Robinson ane mometer during a interval such as one minute or five minutes, but without correction for the errors that are known to he peculiar to that instrument and by reason of which its reeords are from live to twenty-fivi- per cent. too large. lhstead of using the wind velocity• which is the datum specially appropriate to meteor ology, many have attempted to measure the pres sure against a normal unit of surface and thus obtain a scale of wind force. But as the pres sure varies with the shape of the obstacle and the (Tensity of the air. it is generally conceded that velocities should be observed and pressures must he calculated therefrom as hest we can.
From a general meteorological point of view, winds are classified as steady, periodical, and variable. The steady winds are best illustrated by the trade winds at the earth's surface, and the anti-trades above them, by the easterly wind that apparently prevails high above the equato rial region, and by the westerly wind that pre vail, above the north temperate zone. The periodical winds are represented by the diurnal land and sea breezes and the summer and winter monsoons, which are in fact sea breezes and land breezes on a large scale. The variable winds usu ally occur in connection with the areas of high and low pressure, or the storms that move over the earth. They represent ascending and descend
ing movements due to the interaction of upper and lower strata or of cold arctic and warm tropical air, when seeking to obtain more stable equilibrium. The variable winds generally en dure only a few clays, or at the most a week, by which time the disturbance has passed by or per haps altogether subsided, and a change of wind takes place. Sometimes the winds appear to blow with some regularity for a few clays from the north and then for a few days from the south alternately, but this alternation is not often maintained for any length of time, as it is evi dently due to the regularity with which areas of high and low pressure pass over the station. These irregular or variable winds give to ordi nary local weather its characteristic variability, and they have therefore always been a prominent subject of observation and discussion. In the north temperate zone the westerly wind may be considered. as the normal and most frequent; all other winds soon shift to the west.
The local names that have been given to various winds must not be considered as im plying that the peculiarities of the winds are local. Thus the simouin, sirocco, and snlano, which are the warm southerly winds of the northern Mediterranean coast. have their parallels in very similar southerly winds in Siam, India, and the Atlantic coast of the United States. The Nora of the Adriatic and the gregale of Malta have their counterparts in the blizzards of the United States and the purga of Si beria. The dry pnna winds of Peru have a very close parallel in the so-called lint winds of the region from Missouri to Nebraska and Iowa. The east winds of the British Islands prevail in New England as welt as from Scotland eastward into Russia. where, however, they become drier than they are in Oreat Britain. The mistral, or northwest wind of Stmtliern Frailty, is essen tially the same as the northwest wind that fol Imvs, a storm centre passing over the lake region east ward into New York and New England.