WIND. 556 WINDBREAK. 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- vails 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 equililnium. The variable winds generally en- dure only a few days, 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 days 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 irregvilar 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 simoom, sirocco, and solano, which are the warm soutlierly 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 bora 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 puna winds of Peru have a very close parallel in the so-called hot winds of the region from Missouri to Nebraska and Iowa. The east winds of the British Islands prevail in New England as well as from Scotland eastward into Russia, where, however, they become drier than they are in 'Great Britain. The mistral, or northwest wind of Southern France, is essen- tially the same as the northwest wind that fol- lows a storm centre passing over the lake region eastward into New York and New England. Several laws bearing on the winds liave been enunciated. Dove maintained ilie existence of alternat(^ currents, polar and equatorial, exist- ing side by side and moving as a. whole eastward. lie also showed empirically that there was a cerlain law of sequence according to which the winds followed each other day after day, and which we now know is sim|)!y due to the fact that the majority of the storms pass on one side or the other of the station. Redfield, Reid, and other meteorologists showed that the winds cireulale around a storm centre, a fact which, however, was also known to Dampier and other early voyagers. Hence all large storms have the character of whirlwinds. Redfield knew, but did not lay stress upon tlie fact, which Meldrum brought out most forciblj', that the winds are inclined at a considerable angle to the radius vector from the .storm centre, or to the isobars surrounding the storm centre. J. Allan Broun and, many years later, Clement Ley and Cleve- land Abbe independently announced that the upper winds are inclined to the lower winds in a regular sequence, so that as we ascend in the atmosphere the winds are deflected more and more to the right. Ferrel showed that the in- clination of the wind to the isobar depends upon the distance from the storm centre, on the lati- tude of the place, and on the coelfieient of re- sistance of the earth to wind, and he gave the formula connecting these together. Espy and, many years later, Koeppen, independently, showed that the interchange of air between the upper strata and those near the ground causes the decided diurnal increase in the velocity of the wind in the early morning hours up to a maximum at the hour when the interchange is most rapid; also that there must be a cor- responding diminution of the wind in the upper strata, wiiich diminution has been established by actual observations on mountain tops and other elevated points. The general circulation of the atmosphere is not a simple system of steady upper and lower winds. It has been .shown that the dynamics of the air do not allow of the existence of any steady currents maintain- ing the dynamic equilibrium. Therefore the system of upper and lower winds devised by Ferrel as the simplest imaginable must be re- placed by systems of whirls or eddies, so that the great whirhvinds or cyclones and anticyclones that oljservation shows on every daily weather map nnist be considered as an integral and essen- tial part of the general circulation, and would be so even if the globe were a uniform frictionless sphere. Bibliography. On the general subject of the winds, consult: Ferrel. Popular Treatise on the ^Vincls (New York. 1889). On wind pressure, consult JIarvin, Aneinometry (United States Weather Bureau. Washington. 1000). On the general circulation, consult Oberbeck, translated in Abbe. Mcrluniics of the Earth's Atmosphere (Washington, 1891). On recent results of ob- servations on the movements of the upper strata, consult Bigelow. Report on International Cloud Observations (Washington, 1901). For charts of the winds, see Bartholomew's Atlas of Meteor- olof!}/ (London. 1900). For a most comprelien- sive review of all recent results, see Hann, Lehr- huch der Mcteorologic (Leipzig, IflOl). See .Vnkmo.metek; Climate; Meteorolooy. WINDAGE. See Balllstics ; Gunnery. WINDBREAK, or Shelter Belt. Any plant- ing on the wiiiilward side of buildings, orchards, gardens, etc., with the object of overcoming the force of the wind. Windl)reaks are of great ad- vantage in prairie regions in protecting from the cold, lessening evaporation from soils and iilants, retaining snow and leaves as protection, lessen- ing liability to mechanical injury from winds, protecting trees at time of blossoming, and so on. Some disadvantages are that they cause snow to drift to the leeward; prevent the circulation of warm winds in spring, thus retarding the melting of snow and the drying of the soil and roads; and furnish harlxirs for plant pests, etc. The frees are usually jjlanted upon the north and
