972 JOURNAL OF FORlCSTRY 



SEASONAI, MARCH OF EVAPORATION 



Chart 4 shows the average evaporation rate for each month of the 

 fire season on the north and south slopes (elevation, 3,500 feet), meas- 

 ured by the U. S. Weather Bureau evaporation pan and corrected for 

 precipitation. The rate on the north slope is uniformly about 25 per 

 cent lower than that on the south slope or flat (which are nearly equal) ; 

 the peak is reached in July; the rate holds up fairly well in August, 

 and then drops rapidly. The evaporating power of the air in July is 

 2% times as great as in April, 1^ times as great as in May, 1%. times 

 as great as in June, and 1% times as great as in September. Evapora- 

 tion as a factor is chiefly important in reducing the moisture content 

 of the litter to the danger point after a rain. 



SEASONAL VARIATIONS IN WIND VELOCITY 



Chart 5 shows the daily wind movement for each month at the ex- 

 periment station, based on four years' average. The months, April to 

 September, inclusive, or roughly, the fire season, show values above the 

 average, while the others are below. Being based on one station only, 

 the figures, of course, cannot be applied generally, but serve, perhaps, 

 as a fair indication of the general trend of wind movement in the 

 Sierras. Values vary for individual years and months, but the general 

 tendency of wind velocity to follow the same annual course as air 

 temperatures is well marked. 



DAILY VARIATIONS OF WIND VELOCITY 



During the period, June to November, 1916, an automatic-recording 

 wind register was maintained on Mt. Hough at the lookout station. 

 Chart 6 shows the average wind movement for different hours of the 

 day for this station, based on the entire period of observation. The 

 fluctuations are very striking ; the maximum velocity is reached about 2 

 p. m., or the hottest part of the day, and the minimum early in the 

 morning. The general tendency already noted, for wind velocity to 

 follow temperature, is here very marked. It is perhaps unnecessary to 

 say that individual days show an entirely different set of values, espe- 

 cially when wind direction is changing, but the data are sufficient to 

 warrant the statement that wind velocity may be expected to be highest 

 during the hottest part of the day and lowest at night. 



This record was made at an elevation of something over 7,000 feet, 

 and presumably represents the so-called "master-winds." or the main 



