18 THE CLIMATOLOGY OP THE UNITED STATES. 



in summer, appearing then like trade winds and are so 

 called. These characteristics are more marked between 

 the Sierra Nevada range and the Pacific coast. The ele- 

 vated plateau between the Rocky mountains and the Si- 

 erra Nevadas, partakes somewhat of the character of the 

 regions on either side ; its rainfall is however less than either 

 of these. The third region consists of New Mexico and Ar- 

 izona ; this like the last has a semi-tropical climate with a 

 wet and a dry season, but these are reversed ; the wet sea- 

 son occurring chiefly in July and August, the total rain- 

 fall, however, being very small. The winters are warm 

 and dry and the summers hot except in the mountains. 



Before proceeding to consider the Pacific climate we 

 will note some of the chief features of our own variable 

 climate. In the first place we shall notice that through- 

 out the year, at intervals of every two or three days, es- 

 pecially in winter, we are visited by storms of large area 

 occupying from twelve hours to two days in passing ; 

 these storms travel in about the same direction and act in 

 about the same manner. A person who is an observer of 

 nature and interested in the phenomena occurring about 

 us from day to day, would naturally put the following 

 questions ? What causes these storms ? Where are they 

 developed? Where do they go? and what becomes of 

 them? A few words and a few simple illustrations may 

 make the subject plain in a general way. 



It is often noticed on a summer day at the seashore, that 

 the air will be quiet and warm, and in the afternoon the 

 wind will start up from the eastward and refresh us with 

 cool ocean breezes ; this is owing to the air over the land 

 becoming heated and rising, causing a current of cool air 

 to flow in from the ocean to take its place ; in this case 

 we may have merely an afternoon breeze created which 

 will go down with the suu. Again, we may take the case 



