66 



BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE. 



Vapor pressures at the control station. — In Table 14 are presented the 

 records of vapor pressures for the control station, by decades, up to 

 September, 1918. These vapor pressures have been computed from 

 a table prepared in 1911 by B. C. Kadel, of the Weather Bureau, for 

 a barometric pressure of 21.42 inches, which is the mean for Wagon 

 Wheel Gap, but slightly below the mean for Fremont. 



It will be noted that up to June, 1914, these records pertain to 

 conditions prevailing at 7 a. m., while thereafter the psychrometer 

 readings were taken at 8 a. m. This change has probably had a very 

 slight effect on the values, increasing the later ones by about 0.0050 

 inch. 



As these data are only of indirect interest in the study, their de- 

 tailed consideration is hardly necessary. However, a few interesting 

 facts from the meteorological standpoint may be noted. 



(1) The amount of vapor in the air varies greatly at times from 

 one decade to the next. This is not surprising in view of variations 

 in cloudiness, wind direction and velocity, and other conditions which 

 are related. However, it is surprising to find that different years 

 may vary by a margin as great as 25 per cent. In general, these 

 variations show little relation to mean temperatures; or, if they show 

 any such relation, they indicate that high vapor pressures tend to 

 lower temperatures, rather than that high temperatures permit high 

 humidity. This is more plainly true of growing seasons than of 

 whole years. Some of this variation may be ascribed to changes in 

 observers and to consequent changes in the standard of psychrometer 

 readings. 



(2) The seasonal variation, as indicated by the averages for eight 

 years, or indeed by most of the individual years, keeps pace with the 

 rise and fall of temperature, but is to some extent modified by pre- 

 vailing wind directions and velocities. The highest humidities are 

 recorded in the latter half of July, with the highest temperatures; 

 but the middle of August is likewise a moist period, with low winds 

 and prevailing cloudiness, as previously mentioned. 



As humidity readings at other stations in the same locality as the 

 control station have always followed those at the control station by 

 an interval varying from a few minutes to two hours, and as it is 



Eroposed to use the 8 a. m. humidity as a measure of the mean 

 umidity for whole days, it is necessary to determine to what extent 

 the humidity varies during the day. In Table 15 the data are given 

 for representative months from March, 1910, to February, 1912, 

 during which time observations were made daily at 7 a. m., 1 p. m., 

 and 7 p. m. 



Table 15. — Vapor pressures at the control station at various hours, 



to January, 1912. 

 [Average vapor pressures in inches] 



April, 1910, 





Hour. 



January. 



April. 



July. 



October. 



Mean 



annual. 



increase over pres- 

 sure at 7 a. m. 





Amount. 



Percent. 



7 a.m 



a. 0678 

 .0800 



.0670 



0.1344 

 . 1694 



• 



0.2844 

 .8128 



0. 1352 

 . 1 726 

 . 1 167 



0.1628 

 . 1812 







1 j.. in 



o. 0284 

 .0108 



19 



7 



7p.m 





