154 C.F. TOEMAN 
are given and doubtless later we will be told the average of the century. 
A knowledge of the average precipitation tells us little. "The observa- 
tions should also be arranged to determine the density of the preci- 
pitation, the unit taken being a rate of fall measured in centimeters 
per hour, then the figures for the rate of fall of each shower, or the 
most violent portion of the same would furnish some numerical 
foundation for further deduction. The assumption that increasing 
aridity always involves increasing torrentiality is not wholly justified, 
for conditions can be set up deductively which will cause an increasing 
precipitation of increasing torrential character. 
In this preliminary analysis, therefore, torrential concentration is 
chosen as the first of the important factors governing desert climates. 
Livingston reports' that summer rains in Tucson often reach the 
amount represented by 1 cm in ten to fifteen minutes, and I estimate 
that, under the conditions that there obtain, a density of 3 cm per hour 
can probably be considered the lower limit of torrential precipitation. 
The climate of the Salton Sink and San Felipe desert in Lower 
California is a good example of Type 3. MacDougal states? that the 
rainfall is distributed through the year so that only a small precipita- 
tion is received within any one month, and that at the Raza Islands 
no precipitation occurred for more than an entire year. At Fort 
Yuma the average for twenty-six years is 2.84 inches per yéar, with 
a fairly even monthly distribution, excepting a notable decrease for 
April, May, and June. This region shows to a marked degree the 
increased importance of wind action, and the extensive slopes are 
of gentler gradient than those developed under more torrential 
conditions. Without carrying analysis farther it is safe to conclude 
that under a non-torrential and distributed precipitation, the main 
quantitative factor is the increase in the relative importance of the 
wind action. 
If now rainfall increases moderately without decided torrential 
concentration, it is probable that vegetation, especially the grasses 
and the forests, will increase in importance, and the material of rock 
disintegration is held back, chemical weathering starts, talus slopes 
t Loc. cit. 2 Op. cits. Pp. 43. 
3See Douglass, The Crescent Dunes of Southern Peru (in press); (Tolman, 
“The Crescentic Dunes of the Saltan Sea,” Jour. of Geography (in press). 
