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 11^ 30°" a. m. was 10 per cent, and it was the same at 

 4^ 30°" p. m. At O** 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 Canon, 

 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 7 p. 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 4^ 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 conamunicated 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 days in 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. 



