Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/153

Rh PEEVAILING WINDS.] METEOROLOGY 143 the sums of the influences directly and indirectly at work throughout the year in increasing or diminishing the pressure of the atmosphere. There are two regions of high pressure, the one north and the other south of the equator, which pass completely round the globe as broad belts of high pressure. The belt of high pressure in the southern hemisphere lies nearly parallel to the equator, and is of nearly uniform breadth throughout ; but the belt north of the equator has a very irregular outline, and shows great differences in its breadth and its inclination to the equator. These irregularities wholly depend on the peculiar distribution of land and water which obtains in the northern hemisphere. These two zones of high pressure enclose between them the com paratively low pressure of the tropics, through the centre of which runs a narrower belt of still lower pressure, towards which the trade- winds on either hand blow. Considered in a broad sense, there are only three regions of low pressure, the equatorial one just referred to, and one round each pole bounded by or contained within the zones of high pressure just described. The most remarkable of these, so far as it is known, is the region of low pressure about the south pole, which remains low throughout the year, playing the principal role in the wind systems of the Antarctic zone, in its heavy snowfall and rainfall, and in the enormous ice bergs which form so striking a feature of the water of the Southern Ocean. The depression around the north pole contains within its area two distinct centres of still lower pressure, the one filling the northern part of the Atlantic and the other that of the Pacific. Of these two the low-pressure area round Iceland is the deeper, and is probably occasioned by the steeper barometric gradients and stronger winds which prevail over the North Atlantic. The broad equatorial zone of low pressure also contains two distinct regions characterized by still lower pressures. The larger of the two stretches across southern Asia from Assam to the head of the Persian Gulf, and is entirely due to the very low pressures which form so marked a feature in the summer meteorology of that part of Asia. The regions of the middle Indus and upper Ganges occupy the centre of tliis low- pressure area, where normal pressure falls short of 29 - 80 inches. The second area of lowest equatorial pressure is in the centre of Africa. It may be here pointed out that the whole of these areas of low mean annual pressure possess the common characteristic of an excessive amount of moisture in the atmosphere. The Arctic and Antarctic zones of low pressure, and the equatorial low-pressure zone generally, maybe regarded as all but wholly occasioned by the com paratively large amount of vapour in their atmosphere. As regards the region of low pressure of southern Asia in summer, it is remarkable that, while the eastern half which overspreads the valley of the Ganges is characterized by a moist atmosphere and large rainfall, the western half of it is singularly dry and practically rain less, and that the central portion of this remarkable depression occupies a region where at the time the climate is one of the driest and hottest anywhere to be found on the globe. Hence, while the vapour is the more important of the disturbing influences at work in the atmosphere, the temperature also plays no inconspicuous part directly in destroying atmospheric equilibrium, from which result winds, storms, and many other atmospheric changes. The Prevailing Winds of the Globe. If atmospheric pressure were equal in all parts of the earth we should have the physical conditions of a stagnant atmosphere. Such, however, is not the case. Let there be produced a concentration of aqueous vapour over a particular region, or let one region show a higher temperature than what prevails around it, then from the different densities, and consequently different pressures thereby produced, the equilibrium of the atmosphere is destroyed, and, as might be expected from the laws of aerial fluids, move ments of the air, or winds, set in to restore the equili brium. Now every one of the isobaric maps we have given, as well as every isobaric map which has been made from recorded observations, indicates very considerable disturbance of the equilibrium at the surface of the earth. All observation shows that the prevailing winds of any region at any season of the year are simply the expression of the atmospheric movements which result from the dis turbance of the equilibrium of the atmosphere indicated by the isobaric maps for that season and region. All winds maybe regarded as caused directly by differences of atmospheric pressure, just as the flow of rivers is caused by differences of level, the motion of the air and the motion of the water being both referable to gravitation. The wind blows from a region of higher towards a region of lower pressure, in other words from where there is a surplus to where there is a deficiency of air ; and this takes place whether the differences of pressure be measurable by the barometer, as is generally the case, or not readily measur able, as in the case of sea breezes, squalls, and sudden gusts of wind which are of short duration. So far as is known, differences of atmospheric pressure, and consequently all winds, originate in changes occurring either in the temperature or the humidity of the air over restricted regions. Thus, if two regions contiguous to each other come to be of unequal temperature, the air of the warmer region, being specifically lighter, will ascend, and the heavier air of the colder region will flow in below to- take its place. Of this class of winds the sea and land breezes are the best examples. Again, if the air of one region comes to be more highly charged with aqueous vapour than the air of surrounding regions, the air of the more humid region being lighter will ascend, while the heavier air of the drier regions will flow in below and take its place. Since part of the vapour will be condensed into cloud or rain as it ascends, heat is thereby disengaged, and the equilibrium still further disturbed. In this way originate gales, storms, tempests, hurricanes, and all the more violent commotions of the atmosphere, except some of the forms of the whirlwind, such as dust storms, in the production of which very great differences of temperature are more immediately and exclusively concerned. The Trade-Winds. From fig. 14, giving the isobarics for January, it is seen that atmospheric pressure in the Atlantic is lower near the equator than it is to north and south of it ; and the arrows indicate that to the north of the tract of lowest pressure N.E. winds prevail and to the south of it S.E. winds. These are the well- known N.E. and S.E. trade- winds, which thus blow from regions of high pressure towards the tract of lower pressure situated midway between them. The trade- winds do not blow directly to where the lowest pressure is, but in a slant ing direction at an angle of about half a degree. The devia tion from the direct course is due to the influence of the rotation of the earth on its axis from west to east, an influence to which all winds and all currents of the ocean are subject. In virtue of this rotation, objects on the earth s surface at the equator are carried round towards the east at the rate of about 17 miles a minute. On receding from the equator, however, this rate of velocity is being continually diminished, so that at 60 N. lat. it is only about 8| miles a minute, and at the poles nothing. From this it follows that a wind blowing along the earth s surface in the direc tion of the equator is constantly arriving at places which have a greater eastward velocity than itself. As the wind thus lags behind, these places come up, as it were, against it, the result being an east wind. Since, therefore, the wind north of the equator is under the influence of two forces one, the low pressure near the equator, drawing it southwards, and the other, the rotation of the earth, deflecting it eastwards it will, by the law of the composi tion of forces, take an intermediate direction, and blow from north-east. For the same reason, south of the equator the south is deflected into a south-east wind. In the Atlantic the north trades prevail between latitudes 7 and 30 N., and the south trades between lati tudes 3 N. and 25 S. These limits are not stationary, but follow the sun, being farthest to the south in February and to the north in August. The tract of low pressure between these wind systems is named the region of calms, owing to the calm weather which often prevails there, and it is also characterized by the frequent occurrence of heavy rains. This region of calms varies its position with that of the sun, reaching its most northern limit, lat. 11 N.,