58 VEGETATION OF A DESERT MOUNTAIN RANGE. 
higher elevations are derived from a higher cloud level, probably from 
convectional clouds which form at times when the atmospheric con- 
ditions cause condensation at a greater distance from the earth. When 
a long series of records shall have been secured from the 9,000-foot 
station it will probably show that its average rainfall is greater than 
that at 8,000 feet, but the 9,000-foot. record for 1913 indicates that 
there will be occasional years, at least, in which the maximum for the 
mountain is recorded at 8,000 feet. This probably means that at 
10,000 feet on adjacent mountains there is a constantly lower rainfall 
than at 8,000 or 9,000 feet. 
The check in the vertical increase of rainfall which has been described 
as occurring between 4,000 and 6,000 feet appears to be absent from 
all curves derived from widely separated valley stations. The writer 
has seen no such plateau in ANY jus 
curves derived from southwest- ~ " ; oy 
ern data, but there is always the 
possibility that a plateau has 
been smoothed out of the curves 
or that the data have been sub- 
jected to the influence of a 
straight-line equation. The 
character of the increase of pre- 
cipitation with altitude in a sin- 
gle small range of mountains is 
no moreaspecial case than is the 
increase in a widely separated 
series of stations in any locations Dine eae n etre ee 
IG. J. raph showing lack of relation between 
whatsoever. In so far as con- summer rainfall at Marshall Gulch (7,600 feet) 
cernsthestudy of meteorological and at Desert Laboratory (2,663 feet) from 1907 
dynamics, such a mountain ‘19!* 
rangeas the Santa Catalinas offers exceptional opportunities for investi- 
gation, and much more might be learned in a single summer of intensive 
meteorological study on itsslopes than could be ascertained by an exami- 
nation of records of rainfall covering a period of a thousand years. 
As regards vegetation, the most important feature of the study of 
rainfall conditions is the determination of the extremes of variation in 
the amount and seasonal distribution of rain, and the ascertaining of 
the effect of these extremes upon the conditions of soil moisture. Years 
of heavy precipitation are important for the maintenance of the forest 
which clothes the higher mountain slopes and for the general restora- 
tion of the supplies of soil moisture and ground water. The years of 
low precipitation, and especially the series of consecutive years with 
deficient rainfall, are of first importance to the vegetation which occu- 
pies the Encinal region of the mountains. During such years, and 
particularly during the arid fore-summer of such years, the lowest 
nl L 1 1 1 1 
1907 1908 1909 1910 ait 1912 1913 1914 
