72 VEGETATION OF A DESERT MOUNTAIN RANGE. 
temperature of 32° or less was registered at a station during an interval 
of two or three weeks, the date of such minimum could be determined 
by finding the exact date of the lowest temperature for the same period 
at the Desert Laboratory, and such determinations undoubtedly have 
a very slight possibility of error. Taking into account the number of 
direct observations and the larger number of estimations, the limiting 
dates of the frostless season, given in tables 10 and 11, may contain 
errors of as much as 7 to 10 days. The swamping of these errors by 
averaging the dates for the 6 years of observation reduces the probable 
error to about 5 days. 
TaBLe 11.—The altitudinal shortening of the frostless season in the Santa Catalina Mountains, 
as shown by the dates of the last spring occurrence and the first autumn occurrence of a 
temperature of 32° at & altitudes in 1911, 1912, and 1918. 
(Data for 9,000 feet are partially interpolated.] 
Last 32° First 32° 
Station. Year. : * 
In spring. in autumn. 
feet; length of frostless season, 282 1912 Feb. 26 Dec. 9 
Desert Laboratory; elevation, 2,663 1911 Feb. 20 Dec. 25 
days. 1913 Mar. 31 Dec. 8 
Averawe dates): <s30 3s awneseus sees g's '93| settee Mar. 7 Dec. 14 
feet; length of frostless season, 248 1912 Apr. 15 Dec. 9 
Climatological Station; elevation, 5,000 | (1911 Feb. 27 Dee. 4 
days. 1913 Apr. 14 Nov. 21 
Average dates'scneisc.acéosscciaaioestnoabis tees 64 Mar. 30 Dec. 1 
feet; length of frostless season, 187 1912 May 13 Nov. 18 
Climatological Station; elevation, 7,000 | (1911 Apr. 3 Oct. 30 
days. 1913 May 5 Oct. 13 
Average dates... 0... cece eee cece fer en neces Apr. 27 Oct. 31 
Marshall Gulch; elevation, 7,600 feet; 
length of frostless season, 148 days. 1912 May 18 Oct 2 
1911 May 16 Oct. 30 
1913 May 9 Sept. 26 
Average:dates : cascitieae Vora wondlaacawd es May 14 Oct. 9 
Mount Lemmon; elevation, 9,000 feet; 
length of frostless season, 122 days. 1912 May 20 ed 
1911 May 29 Oct. 16 
1913 June 16 Sept. 8 
Average dates: icici cuidewaces vs ean | lv dene ee June 1 Sept. 30 
In figure 2 are shown separately the curves for altitudinal shortening 
of the frostless season for the years 1908, 1909, and 1910, and the years 
1911, 1912, and 1913. In the latter group of years the advent of spring 
was nearly a month earlier at the middle altitudes than it was during 
the former years, and the advent of winter was correspondingly later, 
in spite of the fact that the frostless season was of approximately the 
same length at the Desert Laboratory during the two periods. A con- 
