] 9 the what and the why of desert country 



stance, it is about four inches at Yuma, nearly eleven near 

 Tucson. Four inches means sand dunes which look like 

 those pictures of the Sahara which the word "desert" calls 

 to most people's minds. Eleven means that where the soil 

 is suitable, well-separated individuals of such desert plants 

 as the cacti and the paloverde trees will flourish. 



But if scanty rainfall makes for deserts, what makes for 

 scanty rainfall? To that there are two important answers. 

 One is simply that most regions other than the mountain- 

 ous ones tend to be dry if they lie in that belt of perma- 

 nently high atmospheric pressure which extends some 

 thirty or thirty-five degrees on each side of the equator 

 where calms are frequent and winds erratic. Old sailors 

 used to call this region "the horse latitudes" though no- 

 body knows why and you can take your choice of three 

 equally unconvincing explanations. One is that it was be- 

 cause horses tended to die when the ships lay long in the 

 hot calms. Another, because the boisterous changeable- 

 ness of the winds when they do come suggests unruly 

 horses. A third is that they were originally called after the 

 English explorer, Ross, which was mistaken by the Ger- 

 mans for their old word for "horse." In any event, the 

 latitude of Tucson puts it just within the "horse latitudes." 

 Most of the important deserts of the world, including the 

 Sahara and the Gobi, lie within this same belt. 



The other important answer to the question, "What 

 makes for scanty rainfall?" is, "Mountains lying across the 

 path of such moist winds as do blow." In our case the 

 Coast Ranges of California lie between us and the Pacific. 

 From my front porch, which looks directly across the 

 desert to some nearer mountains of the southernmost 

 Rockies, I can see, on a small scale, what happens. Many, 



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