CUMATOI.OGY 



505 



movements, especially at changes of season in spring and fall. 

 These winds often carry considerable dust and are unpleasant but 

 not destructive. Table II records the average hourly, wind move- 

 ment as observed for a number of years at various representative 

 localities of the State. 



In general there are two precipitation maxima, one in midwin- 

 ter and one in midsummer. Winter precipitation, which is of great- 

 er importance in northeastern Arizona, occurs either as snow or a 

 gentle downfall of rain and penetrates quite completely into the 

 ground. Individual showers usually cover a considerable territory. 

 Summer rains, of primary importance in southern Arizona, occur 

 as local torrential showers often lasting but a few minutes and 

 rarely continuing more than a couple of hours. It is difficult to 

 have these rains penetrate well, since one-half inch of precipitation, 

 occurring in a few minutes, is apt to start a considerable surface 

 runofif, and occasionally more than 75 per cent of the heavier 

 showers is lost in this way. The mean annual precipitation varies 

 with the altitude ; but, owing to local topographic features, this 

 variation is usually not in direct proportion. For example, the mean 

 annual precipitation at Tucson, altitude 2425 feet, is slightly greater 

 than at Snowflake, altitude 5644 feet. This is due to the somewhat 

 abrupt southwestern border of the Colorado Plateaus Region. The 

 sudden increase of altitudes causes precipitation of a great deal of 

 moisture from southwestern winds, and it is not until these winds 

 reach a considerably higher altitude that much further precipitation 

 takes place. A second example is that of the Prescott Dry-farm 

 which receives fully two inches less moisture each year than Pres- 

 cott, about seven miles south, while Jerome Junction, about ten 

 miles north, receives still less. The difference in altitude between 



