CLIMATE OF THE SANTA CATALINA MOUNTAINS. 57 



and 8. These figures compare the summer rainfall curves of the Santa 

 Catalinas and those of selected stations for the same summers. In 

 figure 7 the rainfall of July, August, and September 1911 has been used, 

 for 13 stations located in southeastern Arizona, east of Phoenix and 

 south of Fort Apache. The rain has been averaged for each group of 

 stations lying within the same thousand-foot interval of altitude. 

 Figure 8 shows the curve for the Santa Catalinas for 1912 and the curve 

 for 21 stations in the same area. A single record above 5,000 feet has 

 been available for this curve, that at Chlarson's Mill, in the Pinaleno 

 Mountains. 



The significance of the comparison of these rainfall records for a 

 single season is entirely different from that of averages for long series 

 of years. Such a comparison as this makes possible the contrasting of 

 records which are strictly contemporaneous and serves to show the 

 way in which the same complex of meteorological conditions affected 

 the precipitation at various altitudes, and how these conditions affected 

 the rainfall of a single mountain range in comparison with that of an 

 extended adjacent area lying at different levels. The extremely small 

 number of rainfall records secured at localities above 5,500 feet in 

 southern Arizona does much to vitiate such a comparison. In 1911 the 

 gradient of rise was greater between 3,000 feet and 5,000 feet in the 

 Santa Catalinas than it was in the Weather Bureau stations. In 1912 

 the rise was sharper in the mountains from 3,000 to 4,000 feet than 

 it was in the valleys, but the fall from 4,000 to 5,000 feet was paralleled 

 by a rise in the curve of the valley stations. The fall at 7,200 feet at 

 Chlarson's Mill was far below that at 8,000 feet in the Santa Catalinas 

 for the same period. 



The shape of the averaged curve of rainfall in the Santa Catalinas 

 for the three summers is correlated with the nature and movement 

 of the convectional storms to which the summer precipitation is due. 

 It would appear that certain rains are derived from low-lying clouds 

 which form over the desert and are then driven against the mountain 

 wall by the prevailing southwest winds of summer. These rains in- 

 crease in intensity as they pass up the mountain slopes and yield their 

 maximum downpour at about 4,000 or 5,000 feet, according to the 

 conditions. The rainfall at the Xero-Montane Garden was greater 

 than that at the 6,000-foot station (700 feet above it and only half a 

 mile distant) for four of the six summers in which records have been 

 kept in the two localities (see table 4). The Garden is located at the 

 head of Soldier Canon, and just above it there is a sharp increase in 

 the gradient of the mountain slopes. It is probable that the head of 

 the caiion is the terminating point in the course of many of the desert 

 rain storms. The rapid increase of rainfall between 6,000 and 7,000 

 feet may be due to a similar topographic cause, as mentioned in a 

 preceding paragraph, or it may give indication that the rains of the 



