134 Our Surroundings 



Prevailing Westerlies. The air masses settling to the earth 

 at the horse latitudes are made up of the anti-trade winds and 

 also of air masses drifting at high levels toward the equator 

 from the poles. On approaching the earth the part of the descend- 

 ing air masses which does not flow to the equator as trade winds 

 flows toward the poles and is called the westerlies. The wester- 

 lies flow over a large portion of both temperate and frigid zones. 

 North of the equator they are turned from a northward path to 

 a northeasterly direction, and south of the equator from a south- 

 ward path to a southeasterly direction, due to the rotation of 

 the earth. The climate in the regions over which the prevailing 

 westerlies blow is variable. In these regions are the greatest crop 

 producing countries and the most highly developed civilization. 



In the polar regions the westerlies are drawn by the earth's 

 motion into great whirls of air, forming areas of low pressure. 

 Here the air rises and then turns back toward the horse latitudes. 



Storms and Local Winds. The storm conditions and rapid 

 variations of local weather which we experience are largely due to 

 eddies or whirls in these great air currents of the world, caused by 

 such local conditions as mountain ranges and the uneven heating 

 of sections of land and water. These include cyclones, hurricanes, 

 thunderstorms, tornadoes, waterspouts and land and sea breezes. 



Cyclones or Lows. Winds blow from all directions toward 

 an area of low pressure. The revolution of the earth gives such 

 winds a whirling motion. The direction of this whirl in the north- 

 ern part of the earth is opposite that of the hands of a clock, and 

 in the southern part the same as that of the hands of a clock. 

 These low pressure areas, or lows, are very small as compared to 

 the belt of calms, although they may be a thousand miles in 

 diameter. They are caused by unequal heating of the land sur- 

 face. The whirl of the air around these low pressure areas is 

 called a cyclone. These cyclones move along with the westerly 

 winds and as they pass they change the direction of local winds. 

 That accounts for a north wind one day and a south wind 

 another. We commonly read of cyclones occurring in certain 

 localities and tearing down buildings or rooting up trees. Such 

 storms are not properly called cyclones. They are tornadoes. 



