74 VEGETATION OF A DESERT MOUNTAIN RANGE. 
In order to compare the altitudinal shortening of the frostless season 
on the Santa Catalinas with the same datum for a series of valley 
stations located at progressive altitudes, figures were collected which 
are shown in table 12. These figures are based on the last vernal and 
first autumnal occurrence of a temperature of 32°, for 1903 to 1912 
inclusive, without regard to the reports of frost made by the voluntary 
observers at these stations. In figure 15 the length of the frostless 
season at the several stations is graphically shown by horizontal lines, 
and the limits of the frostless season for the Santa Catalinas (for 1909 
to 1914) are shown by oblique lines. 
Spring opens at an earlier date at 3,000 and 4,000 feet on the Santa 
Catalinas than it does at Tucson and Benson, but at 4,000 and 5,000 
feet it does not open at so early a date as it does at Cochise and Fort 
Huachuca. At all four of the elevations mentioned the close of the 
Taste 12.—The altitudinal shortening of the frostless season in southeastern Arizona, as 
shown by the dates of the last spring occurrence and the first autumn occurrence of a tempera- 
ture of 32° at eight stations at graduated altitudes, in the decade of 1908 to 1912. 
6 : 6 Length of 
Station. Last 32 * Le sd frostless 
in spring. | in autumn. | ..oson (days). 
Yuma, 141 feet................ Jan. 28 Dec. 18 325 
Phoenix, 1,108 feet............ Feb. 1 Dec. 18 321 
Tucson, 2,390 feet............. Mar. 15 Nov. 19 248 
Benson, 3,523 feet............. Mar. 26 Nov. 7 226 
Cochise, 4,219 feet............ Mar. 10 Nov. 6 241 
Fort Huachuca, 5,100 feet...... Mar. 26 Nov. 8 227 
Flagstaff, 6,907 feet........... June 11 Sept. 24 105 
Chlarson’s Mill, 7,200 feet *.... May 137 Oct. 19T 159 
* The elevation of Chlarson’s Mill is reputed by the proprietor to be 8,000 feet, and it is so 
stated in the publications of the Weather Bureau. Several aneroid determinations by the writer 
indicate that it is approximately 7,200 feet. 
+ These dates are based on an incomplete record. 
growing season comes sooner at the valley stations than it does on the 
mountains. The length of the frostless season at Flagstaff is notably 
shorter than it is at the same elevation in the mountains. 
The advent of spring is retarded at Tucson and Benson by the cold- 
air drainage of the Santa Cruz and San Pedro rivers respectively. 
Cochise is situated in the middle of the eastern bajada of the Dragoon 
Mountains and Fort Huachuca at the top of the northern bajada of 
the Huachuca Mountains. Each of these stations is therefore removed 
from the operation of cold-air drainage, as is manifested by the failure 
of their greater altitude to impose upon them shorter frostless seasons 
than those of Tucson and Benson. 
The length of the frostless season at Marshall Gulch and at the 
similarly situated mountain station at Chlarson’s Mill (Pinaleno Moun- 
tains) is greater than at Flagstaff, which is at a slightly lower altitude. 
This is to be attributed partly to the higher latitude of Flagstaff, but 
