The Weather 51 



low as shown on the weather maps for December 24, 25, 26, 

 1915, pp. 6-8. Tropical storms, as the West India hurricanes, 

 are attended by exceptionally heavy rainfall. But the rain 

 that follows the passage of a low center is generally very light. 

 Rain sometimes falls with the approach of the high-pressure 

 or anticyclonic area. Why? 



Distribution of rainfall. The inequalities of the land sur- 

 face have much to do with the distribution and mean annual 

 amount of rainfall in the different sections of our country. 

 Make a comparative study of a physical and a rainfall map of 

 the United States. The information thus gained, together with 

 the facts concerning the paths of cyclones and local storms, will 

 help in understanding the distribution of rain. 



In the western section of the country the moisture-laden 

 winds from the Pacific are forced upward by the Coast range 

 to a high altitude, where the low temperature causes much of 

 their vapor to condense and fall as rain. The rainfall amounts 

 to as much as eighty or one hundred inches a year. East of the 

 mountain range is a lowland region which gets only about one 

 fourth as much rain in a year. The Cascade, Klamath, and 

 Sierra Nevada mountains rob the winds of still more moisture, 

 while the land to the east as far as the Rocky Mountains has 

 a mean annual rainfall under ten inches. In the Rocky Moun- 

 tain region the rainfall is again large, amounting in certain 

 localities to about forty inches a year. Why? 



Exercise. Explain the fact that the semi-arid strip of country 

 east of the Rocky Mountains, including the western part of Nebraska, 

 Kansas, Oklahoma, and a narrow strip of Texas, has a very light rain- 

 fall. Why do not the moist winds from the Pacific Ocean yield mois- 

 ture for this region? Explain why the Atlantic coast states have a 

 larger rainfall than the central states. Why do they have less rain- 

 fall than the western coast ? From what source do the Atlantic coast 

 states get their moisture? Show direction of winds bringing moisture 

 from the Gulf and the Atlantic Ocean, and their relation to the low- 

 pressure areas. Why is the precipitation greatest in the central 

 states during the summer season? 



