68 VEGETATION OF A DESERT MOUNTAIN RANGE. 
On May 22, 1911, in the east fork of Sabino Basin, at 3,400 feet 
elevation, the humidity at 3" 30™ p.m. was 6 per cent, at 6 p. m. it 
was 8 per cent, and at 8° 30™ p. m. it was 12 per cent. At 4° 30™a.m., 
on the following day, the humidity had risen to 24 per cent. These 
figures show the prevalence of desert humidities at a locality which is 
low in elevation but is well in the heart of the mountain mass as a whole. 
At Marshall Gulch, at 7,600 feet in the Forest region, on May 20, 1911, 
the humidity at 112 30™ a. m. was 10 per cent, and it was the same at 
45 30" p.m. At 9°15™ a.m. on the following day the humidity was 
16 per cent, and at 3 p. m. it was 11 per cent. Although these figures 
are roughly twice those of the readings in Sabino Basin, they are never- 
theless indicative of a low humidity for a forested locality and show 
that in the arid fore-summer there are days on which the humidity is 
nearly as low as it is on the Desert. 
A number of humidity readings were taken in June 1911, but none 
of them showed as low values as those just mentioned. In Bear Cajion, 
at 6,100 feet elevation, on June 21 (a dull and intermittently cloudy 
day), the humidity was 46 per cent at 1 p. m. and 42 per cent at 3 p. m., 
falling to 41 per cent at 7p.m. In Marshall Gulch on June 23 (a nearly 
cloudless day), the humidity at 5 a.m. was 33 per cent and it fell 
steadily to 22 per cent at 12 noon, with a temporary rise during an 
interval of cloudiness at 10 a.m. In the afternoon the percentages 
rose from 25 per cent at 3 p. m. to 29 per cent at 6 p. m., but fell again 
to 26 per cent at 8 p.m. The highest humidity observed at Marshall 
Gulch was 48 per cent at 45 30™ p. m. on June 8, 1911, after the summit 
of the mountain had been covered several hours with cumulus clouds. 
Continuous records of relative humidity have been secured in yellow 
pine forest at the Fort Valley Experiment Station at 7,200 feet eleva- 
tion, near Flagstaff, Arizona, by Pearson.* The monthly mean values 
for May and June (1909-1912) are 38 and 34.9 per cent respectively. 
Some of the lowest extremes involved in the composition of these means 
have been kindly communicated to the writer by Pearson. The number 
of days in June on which the humidity fell to 16 per cent or less was 
as follows: 8 days in 1909; 11 days in 1910; 6 daysin 1911. The lowest 
humidities were a single occurrence of 10 per cent and two occurrences 
of 11 per cent. Humidities as low as 11 per cent also occur in July, 
and values as low as 16 per cent occur between May and October. 
These figures for the Coconino Plateau are in agreement with the 
lowest figures secured at Marshall Gulch in May 1911, and show that 
desert humidities are of frequent occurrence in the arid fore-summer, 
both on isolated mountains, such as the Santa Catalinas, and on ex- 
tended plateaus in the midst of the arid region. On the days that 
exhibit such low humidities at higher elevations there is practically a 
* Pearson, G. A. A Meteorological Study of Parks and Timbered Areas in the Western 
Yellow Pine Forests of Arizona and New Mexico. Mo. Weather Rev., 41: 1615-1629, 1914. 
