WIND 



0310 



WIND 



earth's surface. The atmosphere over this belt 

 josequently warmer and lighter than that 

 on either side, and is forced upward by the 

 higher pressure of the heavier air. Over the 

 .itorial regions then, there is constantly an 

 :iding current, and since this current is not 

 perceptible, sailors call this region the belt oj 

 equatorial calms. As the air rises there is a 

 constant surface current from the tropical lati- 

 tudes toward* the equator. Were it not for the 

 rotation of the earth these currents would be 

 due north and south currents; but in flowing 

 from the polar towards the equatorial regions 

 the atmosphere is constantly coming in con- 

 with portions of the earth whose velocity 

 of rotation is greater than that of the region 

 from which they come. Since the air is unable 

 to acquire this velocity as rapidly as the solid 

 ii. these currents lag behind, as it were. In 

 the northern hemisphere they blow from the 

 northeast and in the southern from the south- 

 east. 



The ascending current over the equator soon 

 reaches an altitude where the surrounding at- 

 mosphere is of equal density, and it becomes a 

 horizontal current, flowing towards the poles. 

 The farther this current flows the cooler it be- 

 comes, until, in the temperate latitudes, it 

 reaches the same temperature and density as 

 the surface layers of atmosphere, and gradu- 

 ally mingles wifch them. Since this mingling of 

 currents of equal density equalizes the atmos- 

 pheric pressure over these regions, there may 

 be large areas on the ocean where for days to- 

 gether there is no wind. These regions are 

 known as the calms of Cancer and the calms of 

 Capricorn. Outside these belts of calms the 

 currents flowing from the equator become sur- 

 face currents, forming westerly winds. Here 

 again the deflection from a north and south 

 course is caused by the rotation of the earth. 

 Currents flowing towards the polar regions are 

 flowing from regions of higher to those of lower 

 velocity of rotation, and they run ahead or 

 blow towards the east. 



In the upper regions of the atmosphere winds 

 often blow contrary to those on the surface, a 

 fact easily verified by observing the clouds. 

 Lofty mountains, the shape of the continents, 

 and minor local causes affect the surface cur- 

 rents, so that a detailed study of winds is de- 

 cidedly complex. 



Classification of Winds. A steady wind is 

 one that blows constantly in the same direc- 

 tion, as the trade winds. A periodic wind is 

 one that blows at stated intervals, such as land 



and sea breezes and monsoons. Variable winds 

 are irregular, and are due to local causes, such 

 as the sudden rise in temperature over a small 

 area on a hot summer day. 



Effects of Wind. A cold wind blowing into 

 warm regions has its capacity for moisture in- 

 creased. Therefore, such winds usually assure 

 clear skies and bright days. On the contrary, a 

 warm wind blowing into a cooler region has it< 

 capacity for moisture decreased. A portion of 

 its water vapor is condensed, forming clouds. 

 and if the condensation is carried to the point 

 of saturation, rain falls. The rainfall of a lo- 

 cality is, therefore, largely determined by the 

 prevailing winds. A gentle breeze equalizes 

 temperature, and keeps the air in circulation, 

 thereby preventing such an accumulation of 

 impurities in any locality as would vitiate the 

 atmosphere. After a heavy fall of rain the 

 wind hastens the evaporation of water and dry- 

 ing of the land. 



Wind is a destructive agent when its velocity 

 becomes so great as to endanger life and dam- 

 age property, as in a^ severe gale or a tornado. 

 Wind is an active agent in the formation of 

 soil, and in wearing away of rocks in desert 

 regions and in the formation of sand dunes. 



The Anemometer. Wind blows from areas of 

 high towards areas of low pressure, and the ve- 

 locity depends upon the difference in pressure. 



THE ANEMOMETER 

 (a) The Robinson 

 device; (&) for meas- 

 uring the velocity of 

 air currents in build- 

 ings. 



When the difference is great there results a 

 high wind, and when it is nearly equal, a gentle 

 breeze. The velocity of wind is measured by 

 the anemometer, which consists of four hemi- 

 spherical cups attached to the ends of horizon- 



