THE GENERAL STORM 253 



the pressure areas that cause the various winds. There are 

 still other results from the wind movements. 



Exercise 56. Completing the Weather Map 



Chart, next, the state of weather at each station, whether clear, 

 partly cloudy, cloudy, raining, or snowing ; also the amount of precipi- 

 tation that had occurred in the last 24 hours. Note that the cloud 

 sheet attended the low-pressure area, while the high-pressure area 

 was mostly clear. Use the symbols given in the "Explanatory 

 Notes" on a Daily Weather Map. 



EXPLANATION. The inflowing ground winds about a LOW 

 are continually pushed in from all sides by the surrounding 

 higher pressure; having no other escape, they are crowded up- 

 ward over the region in and around the center of the LOW. 

 In rising, they become cooled and their moisture is condensed 

 into clouds and often into rain or snow. The amount of clouds 

 and of rain or snow about a LOW varies much at different times 

 and in different LOWS. Sometimes no rain or snow is pro- 

 duced, and in a few instances, only scattered cloudiness has re- 

 sulted. In high-pressure areas, fair weather usually prevails, 

 because the air in a HIGH is settling downward and warms by 

 compression as it descends. This rising temperature increases 

 the capacity of the air for water vapor (see Table XII, Art. 

 250) and enables it to evaporate clouds and clear the sky. 



278. In Review. The pressure differences cause the winds 

 and give them their direction and velocity. The winds carry 

 temperature conditions and help to make some regions warmer, 

 others colder. The winds crowded upward in the low-pres- 

 sure area furnish the moisture to form its cloud sheet and its 

 rain or snow. The winds settling downward through the 

 high-pressure area evaporate clouds and clear the sky and 

 bring down the fresh invigorating air usually noticed in the 

 clearing weather following a storm. Thus the pressure areas, 

 i.e., the HIGHS and LOWS, by controlling the movements of the 

 air become the chief cause of most of the general weather 

 features of the temperate zones. 



