46 ELEMENTS OF GENERAL SCIENCE 



greater, until finally, at some distance above the earth, satura- 

 tion may be reached and moisture may be condensed in the 

 form of clouds. If condensation continues, rain will fall. 

 As pointed out before, the warm southern winds are blowing 

 northward in the front part of the storm (the eastern half), 

 and it is here that clouds and rain are found. 



In the western half of the storm the winds are coming 

 from northern and cooler places and are being warmed. 

 Capacity for moisture is increasing, and there is no conden- 

 sation in this part of the storm area. The air in the western 

 part of a cyclone is usually clear, dry, and relatively cool. 



Large bodies of water and great irregularities of the earth's 

 surface, such as mountain ranges, may cause marked local 

 variations in the character of the weather which accompanies 

 the passage of cyclones. 



45. Passage of a cyclone. On the average, several cyclones 

 pass eastward over the United States each we.ek. Cyclones 

 and anticyclones commonly follow so closely upon one an- 

 other that we are practically always in one or the other. 

 Most of the winds which we experience are cyclonic winds, 

 and most of the rainfall in the Mississippi Valley is produced 

 by these winds. The events that occur in connection with 

 the passage of a cyclone have been described as follows in a 

 pamphlet issued by the United States Weather Bureau : 



When the wind sets in from points between south and south- 

 east, and the barometer falls steadily, a storm is approaching 

 from the west or northwest, and its center will pass near or to 

 the north of the observer within twelve to twenty-four hours, 

 with winds shifting to northwest by way of southwest and west. 

 When the wind sets in from points between east and northeast, 

 and the barometer falls steadily, a storm is approaching from 

 the south or southwest, and its center will pass near or to the 

 south or east of the observer within twelve to twenty-four hours, 

 with winds shifting to northwest by way of north. The rapidity 

 of the storm's approach, and its intensity, will be indicated by 

 the rate and amount of the fall in the barometer. 



