72 BULLETIN 1233, T. S. DEPARTMENT OF AGRICULTURE. 



Another method, which was practiced in compiling the records at 

 Fremont for a number of years, is to compute the saturation deficit 

 directly from each psychrometer reading, using vapor pressure and 

 saturation pressure tables, or a special table in which the saturation 

 deficits are shown directly. This method, however, is probably less 



Erecise than the one described above, and gives very different results, 

 ecause the saturation deficit varies much more widely with changes 

 in temperature than does the vapor pressure. Hence this method is 

 not to oe recommended where dependence must be placed on single 

 daily observations at a fixed hour. 



- The saturation deficit data for the control station up to September, 

 1918, are given in Table 19. 



It will, perhaps, be of interest to note that during the winter months 

 the saturation deficits are of about the same magnitude as vapor pres- 

 sures for the control station (Table 14) and indicate a relative humid- 

 ity of about 50 per cent. From April onward, through the growing 

 season, vapor deficits are scarcely more than two-thirds of the magni- 

 tude of vapor pressures and indicate relative humidities nearer to 60 

 per cent. This general change results from the cessation of the strong 

 mountain and anticyclonic winds. 



At Wagon Wheel Gap (W-Al) the winter deficits are always of less 

 magnitude than vapor pressures, probably because of the lack of any 

 direct mountain-breeze effects. They are also of less magnitude at all 

 other times, except in May and June, when the relative humidity is 40 

 or 45 per cent. Thus the dependence of vapor deficits on tempera- 

 tures as well as on vapor pressures is clearly Drought out. 



EVAPORATION. 



Evaporation observations were begun at the control station and at 

 several other local stations as early as July, 1914. The instrument 

 then in use was the Piche, as modified by the United States Weather 

 Bureau 5 fitted with 10-centimeter glass plates and with 9-centimeter 

 Glter papers as a standard. Great difficulty is always experienced 

 i:: operating these, insl ruments on a basis of daily observations, because 

 of the necessity for frequent adjustments, according to temperature 

 and dryness, to prevent overflowing or drying of the papers. As the 

 capacity of these instruments — 40 cubic centimeters — was found to be 

 insufficient Tor a. day's evaporation at midsummer, extra fillings were 

 immediately necessary, and smaller papers were soon adopted! The 

 results were corrected to the standard papers on the basis of the 

 exposed areas of the two kinds. 



These observations, unsatisfactory as ih^y seemed at the time, were 

 continued through 19] I and again taken up in April, 1915. During 

 the colder part of this period, to prevent injury to the glass vessels 



es of alcol 



from freezing, vrarious mixtures of alcohol and water were used. It 

 ridenl thai the alcohol would accelerate Losses, and it is equally 

 • •\ idenl thai evaporation of the alcohol would continue freely at tem- 

 peratures below 32°, Hence this expedient was in uowise satisfac- 

 tory, even though the mixtures were compared with distilled water 

 to determine relative evaporation rates at temperatures above 

 freezii 



WhUi I Plchi i rapori employed by the Weather Bureau tor a tufce la comparisons 



other instriimenl ,il Indication: have always been regarded as most untrustworthy. 



