68 



BULLETIN 12:33, U. S. DEPARTMENT OF AGRICULTURE. 



the growing season are due simply to the fact that these sites do not 

 feel the effect of the diurnal valley breeze quite so early as the control 

 station, these breezes carrying somewhat more moisture than the 

 local air. 



Certain data are given in Table 17 for somewhat more distant 

 stations covering a later period of two years, when observations at 

 the control station were taken at 8 a. m., and those at the outlying 

 stations from one-half to one and one-half hours later, the inter 

 being longer in winter than in summer. At all these outlying station: 

 the psychrometer was swung as close to the ground as possible. 



Table 17. — Departure of vapor pressures near the ground, in north-slope Douglas 

 fir type, from those at the control station, in inches of mercury. 



Station 



No. 



Cover condition. 



January. 



Febru- 

 ary. 



March. 



April. 



May. 



June. 



July. 



F-7-8.... 





-0.0036 

 -.0013 

 -.0027 

 +.0001 



+0. 0050 

 -.0014 

 +.0005 

 +.0036 



+0.0046 

 -.0036 

 +.0026 

 +.0013 



+0.0038 

 -.0032 

 -.0028 

 -.0053 



+0.010S 

 +.0055 

 +.0021 

 +.0087 



+0.0019 

 +.0112 

 -.0007 

 + .0041 



+0. 0061 



F-14 



Half cut 



+ .0305 



F-15 



F-9 



do 



Uncut 



+ .0243 

 + .0146 





rage 





Ave 



-.0019 



+.0019 



+.0012 



-.0019 



+ .006S 



+ .0041 



+ .0190 



Station 

 No. 



Cover condition. 



August. 



Septem- 

 ber. 



Octo- 

 ber. 



Novem- 

 ber. 



Decem- 

 ber. 



Year. 



Growing 

 season. 1 



F-7-s .... 



Clear cut 



+0.0309 

 +.0321 

 +.0065 

 + .0210 



+0.0214 

 +.0060 

 +.0083 

 +.0126 



+0.0067 

 -.0006 

 -.0091 

 + .0026 



+0. 00S9 

 +.0031 

 + .00S2 

 +.0078 



-0. 0088 

 -.0081 

 -.0028 

 -.0014 



+0.0073 

 +.005S 

 + .0029 

 +.0058 



+0.0139 



F-14 



Half cut 



+ .0227 



F-15 



F-9 



do 



Uncut 



+.0099 

 +.0132 



Ave 







+.0226 



+.0121 



-.0001 



+ .0070 



-.0053 



+ .0054 



+ .0149 







1 nrowinir-season means computed by adding three times the June, July, and August differences to the 

 September di (Terence and dividing the total by 10. 



The great irregularity of the relations indicated by the above data 

 is difficult to account for. It should be understood that humidity at 

 each of the first three stations was measured during different decade- 

 of each month; that is, (luring one decade a month for each station. 

 The record for Station F-9 is complete, and therefore might be com- 

 pared with the average of the first three stations. However, even 

 this comparison does not help to clarify the results. Every effort was 

 made to eliminate discrepancies by using the same psychrometer at 

 all stations, and by daily reversal 01 the order of observations. Hence 

 the average time is essentially the same for each of the four stations. 



All of the annual differences show higher humidity at these stations 

 than at the control station. The increase in humidity during the 

 morning hours is (\\\o to the rise in temperature, or to the setting up 

 of the mountain breeze, or to both. In either case the greatest change 

 is hkclv to occur in the early part of the day, and especially in summer 

 between 8 and !• a. m. Hence, the generally higher humidity at these 

 si at ions, as well as t he great excess during the growing season, may be 

 w holly accounted for by the difference in time between these stations 

 and ilic control station. On the other hand, the variations between 

 i he four stations are unaccountable, except on the ground of variations 



in the amounts of sunlight at the points of observation and of the im- 



ibility of entirely shielding the psychrometer from this influence. 

 Sunlight on the psychrometer baa the effect of lessening the wet-bulb 

 depression and thereby making the humidity appear higher than it is. 



