90 CARNEGIE INSTITUTION OF WASHINGTON. 



seasonal growth. It appears possible that these opposing influences 

 operate in such a manner as to give the end-result of nearly equal 

 growth at different altitudes. Such a relative uniformity of growth- 

 rate would not contradict the probability, for which there is some 

 strong evidence, that the lower limit of the yellow pine is determined 

 by deficiency of soil-moisture, together with high evaporation, and its 

 upper limit by the length of the frostless season. 



Rainfall Conditions of Desert Mountains, hy Forrest Shreve. 



The measurement of summer rainfall on the Santa Catalina Moun- 

 tains has been carried on at three stations since 1907 and at seven 

 stations, at 1,000-foot intervals, since 1911. The published results of 

 the earlier readings, together with those more recently secured, have 

 brought out four features of general interest: 



1. The gradient of increase of rainfall on a relatively abrupt moun- 

 tain side is much steeper than it is on a gently rising stretch of country, 

 such as that occupied by stations of the Weather Bureau in Arizona, 

 with which comparisons have been made. 



2. There is a very sharp rise in the rainfall between the base of the 

 Santa Catalina Mountains, at 3,000 feet, and the station at 4,000 feet, 

 a feature believed to be true of the lowest interval of 1,000 feet on all 

 mountains in the southwestern deserts, regardless of their basal altitude. 



3. There is another sharp rise in the gradient of increase at the 

 elevation at which heavy forest is first encountered. 



4. In certain years the rainfall has been less at 9,000 feet than at 

 8,000 feet. 



The emergence of these facts has made it seem desirable to continue 

 the readings at the same stations and to estabUsh others for com- 

 parison. For the summer of 1917 a new series of accumulative rain 

 gages was installed, embodying the same features as the gages pre- 

 viously used, in which kerosene is employed to prevent evaporation, 

 and the readings are made by volume, with reduction by the area of 

 the funnel. A new series of stations was located on the south face of 

 the Pinaleno Mountains, in Graham County, Arizona. This range was 

 selected because it has a greater basal altitude than the Santa Cata- 

 linas and a greater maximum altitude, and because the lower limit of 

 forest is nearer the base and farther from the summit — ^in terms of 

 vertical distance — than in the Santa Catalinas. It will be possible, 

 therefore, to test out all of the above conclusions by possessing parallel 

 series of data for the two mountains. It will be of particular interest 

 to see if the Pinaleno Mountains exhibit a sharp rise of rainfall with the 

 first occurrence of forest and whether the increase of rainfall with 

 altitude is reversed at the 10,000-foot station, as is suggested by the 

 occasional reversal at 9,000 feet on the Santa Catalinas. 



