54 VEGETATION OF A DESERT MOUNTAIN RANGE. 
5,000, and 8,000 feet, and between June 20 and July 18 at 6,000 and 
7, 000 feet. In similar manner the less frequent readings of 1912 and 
1913 have been divided into the early summer and late summer falls, 
by the latest July reading, and the averaged curves for early summer 
and late summer rain are of nearly the same shape, but the late summer 
curve is not so steep. This short record does not seem, therefore, to 
corroborate the suggestion of Smith. 
TaBLy 6.—Intraseasonal distribution of summer rainfall at the Desert Laboratory and at 6 
elevations in the Santa Catalina Mountains for 1911. 
Rainfall of the maximum period in heavy type. 
Apr. 25-27 | June 20-21 | July 18-19 | Aug. 22-24 | Sept. 22-25 
Station. to to to to to Totals. 
June 20-21. | July 18-19. | Aug. 22-24. | Sept. 22-25. | Oct. 12-14. 
Des. Lab... 0.01 1.37 4.08 2.90 1.61 9.97 
‘3,000 feet... -00 1.08 3.39 1.80 1.46 7.73 | 
4,000 feet... .00 2.15 4.65 2.65 1.69 11.14 
5,000 feet... .00 3.62 5.40 2.95 2.75 14.72 
6,000 feet... -00 5.71 3.45 1.91 1.92 12.99 
7,000 feet... .00 7.36 3.68 4.82 2.16 18.02 
7,600 feet... .69 8.30 8.83 3.48 2.56 23.86 
The increase of rainfall which accompanies increase of altitude is a 
phenomenon of general occurrence throughout the southwestern 
United States. The curves by which such increase may be expressed 
differ from each other most strikingly, according to the horizontal dis- 
tance of the successive stations from each other, according to the 
coastal or continental position of the series of stations, or according 
to the size of the mountain range on which the successive elevations 
are secured. Although it is possible to deduce mathematical formule 
for the vertical increase of rainfall, it is necessary to introduce into all 
such formule a constant for the particular region or mountain involved, 
and the figures thus secured are merely in the nature of hypothetical 
means near which the normal conditions may fall. It would be of very 
great interest in the extension of plant geography to possess data on 
the actual amounts of rainfall at successive elevations in a large number 
of mountains and shelving plains throughout the southwest. The mean 
rainfall conditions which are expressed in a gradient based on a long 
climatological record are of great importance in connection with vege- 
tation, but only when consideration is also given to the extremes of 
rainfall, and particularly to the lower extremes, if a semi-arid country 
is under consideration. The securing of typical normal gradients of 
altitudinal increase of rainfall is not of so much importance in plant 
geography, therefore, as a knowledge of the actual oscillations of the 
rainfall conditions from year to year throughout the series of stations 
or localities involved. 
