GENERAL CIRCULATION OF THE ATMOSPHERE 281 



land cooler than the ocean in winter, and warmer in summer. In 

 winter they yield moisture, even at low levels. This gives the 

 low lands of California their wet season. As the winds blow on 

 across the mountains back from the coast, they yield more moist- 

 ure, so that all the area west of the top of the first high ranges, 

 the Sierras at the south and the Cascades at the north, is supplied 

 with rain and snow in the winter season. As the winds blow 

 beyond the Sierra and the Cascade mountains, the air descends 

 and becomes warmer, and therefore dry. East of these mountains 

 lie the semi-arid lands of the Great Basin with its Great Salt 

 Lake, and of eastern Oregon and Washington. 



When these winds reach the higher parts of the Rocky Moun- 

 tains, which are higher in many places than the mountains farther 

 west, they again yield some moisture. But farther east, all the 

 way to the Atlantic, these winds are dry, for they cross no more 

 high mountains, and they do not generally go far enough north to 

 reach a temperature as low as that of the mountains they have 

 passed. For some distance east of the mountains the rainfall is 

 slight; but east of central Kansas and Nebraska the lands are well 

 supplied with moisture. It is therefore clear that something be- 

 sides the westerly winds brings rainfall to this region. This agent 

 is the aperiodic cyclonic wind, to be studied in the next chapter. 



The winds which blow from the Pacific to the continent in 

 summer have a different effect upon rainfall. At this time of year, 

 the winds find a temperature on the low lands higher than their 

 own. They are therefore "dry" in this region, and give to much 

 of California its dry season. Blowing inland, these winds reach 

 mountains so high that the temperature is low enough to cause 

 condensation and precipitation, even while the low lands to the 

 west are dry. 



Farther north the case is somewhat different. In Washington, 

 for example, the mountains near the coast are high enough to occa- 

 sion precipitation even in summer. In Alaska, where some of the 

 mountains are always covered with snow, precipitation is heavy in 

 the summer, and at high altitudes it often falls as snow instead of 

 rain. 



Monsoon winds may yield much moisture. In general, they 



