40 



WEATHER AND CLIMATE 



i \ \ v* -s^=s- 

 i \ i \ ' \ \ Upper winds blow 

 \ \ ' ' towards cooler area 



\ 



FIG. 67. WHY WINDS BLOW 



unequally healed, and that which is warmer is pushed 

 up by the heavy, cooler air which rushes in, and thus 

 huge convection currents are set up. Near the earth the 

 movement is towards the point where the air is heated, 

 but at higher altitudes the direction is away from the 

 rising warm air column and toward the falling cold air. 

 A study of Figure 67 will make this clear to you. 



What are cyclones and anticyclones, and from what 

 direction do weather changes come in this country? 

 If a barometer were read over an area where the air 

 is warm it would be found to read less than where 

 it is cooler. The place where the reading is lower 

 is called a low-pressure area or low and where it is 

 higher, a high-pressure area or high. Your experiment 

 has taught you that winds blow from a "high" toward 

 a "low," and if you remember from a previous study 

 that warm air holds more moisture than cold air, you 

 can see why bad weather is usually associated wi'th 

 low-pressure areas. The warm, moist air is pushed up 

 by the colder, dry air to higher levels where it ex- 

 pands, cools, and loses some of its moisture. 



The air over a "low" does not rise straight up but 

 has a spiral motion in a direction opposite to the mo- 

 tion of the hands of a clock. This direction is common- 

 ly spoken of as counter-clockwise. These winds blow in 

 toward the center of the low-pressure area. Also the 

 winds about a "high" are spiral in a dockzvise direction, 



blowing away from its center, as your study of the 

 weather maps and the diagram you made have taught 

 you. These spiral winds are caused by the rotation of 

 the earth combined with the rising and falling air cur- 

 rents. The name cyclone is given to low-pressure areas 

 and the name anticyclone to high-pressure regions. 



Low- and high-pressure areas, varying from 400 to 

 1000 miles in diameter, move across North America 

 generally from west to east, bringing our changing 

 weather conditions. The movement is from the west 

 toward the east because this continent lies in the wind 

 belt called the prevailing westerlies. Figure 68 shows 

 how trees are sometimes affected by the prevailing 

 winds of a locality. 



F 



Courtesy Taylor Instrument Company 



FIG. 68. THE WORK OF PREVAILING WINDS 



