194 



ELEMENTARY SCIENCE 



weather commonly accompanies areas of high pressure. In 

 view of what has just been stated you should be able to ex- 

 plain this fact. Remember that rising air is cooled and so 

 tends to give up moisture, while as to descending air, the 

 reverse is true. 



Now, to go back to our case of the rains of the Mississippi 

 Valley, it is a fact that areas of low pressure frequently 

 originate in the region of the Gulf of Mexico, and, passing 

 across it, move up the Mississippi Valley, and then are 



FIG. 70. Side view of a cyclone as described in the text. After SALISBURY. 



carried by the prevailing winds off to the east and north- 

 east. By this means much air, carrying moisture from the 

 Gulf, is drawn northward, and, as it goes, it cools and 

 delivers this moisture as rain. Can you not see from this 

 why it is that an east wind, throughout the Mississippi 

 Valley, is considered a ''sign of rain"? The cyclonic 

 winds, which have just been described as the principal rain 

 bringers in this region, first make their presence felt by an 

 easterly or southeasterly breeze, which gradually veers 

 around to the north and northwest as the "storm center" 

 moves to the east. 



Study of Weather-Maps. Now you should be in a 

 position to study and interpret such a weather-map as 

 that given on page 181, on which isotherms, isobars, direc- 



