CLIMATE AND FOREST I-IRF,S OTl 



deduction that during the summer the effect of any rain may be entirely 

 lost in from one to two days, when the Htter is exposed to direct sun- 

 hght, and there is even a sHght circulation of air. The rate of moisture 

 loss is lower and the period for drying out is longer when the litter is in 

 the shade, with air, temperature, humidity, and wMnd movement the 

 same as in the sun. 



It is probably true that the rate of drying out of small samples is 

 somewhat more rapid than in nature, but the available evidence cer- 

 tainly indicates that even a soaking rain, which completely saturates 

 the litter, may lose its effect in a very short time, and that an attitude 

 of complacency and a let-down in protection alertness are not justified 

 during the fire season. A study of the moisture relations on Mt. 

 Hough also indicates that the end of the fire season comes when the 

 content of the litter is above the ignition point, and not when a good 

 rain early in October apparently ends the danger for the year. Cer- 

 tainly the records show that fires can and do occur after the fall rains, 

 and after the protective organizations have been disbanded. Such 

 fires frequently are larger than those during the recognized fire sea- 

 son, as, for example, on the Shasta National Forest in 1911 and 1916. 



An interesting example of rapid dessication of litter occurred in 

 September, 1915. On September 13, a rain of 0.14 inch fell between 

 2 p. m. and 6 p. m. During the rain, brush piles were burned on the 

 grounds of the Feather River Experiment Station, and in no case did 

 the fire spread. The morning of the 14th was cloudy until 10 a. m., 

 after which the sun shone, and more brush was burned between 8 and 

 10 a. m. without the fire spreading. On the afternoon of September 14 

 a series of ten fires was set on pine needles, each fire being allowed 

 to burn 15 minutes. In every case the fire spread freeh' and it is 

 apparent that the effects of the rain were lost in a very few hours. 



PROPERTIES OF AIR-DRY LITTER 



Air-dry litter is markedly deliquescent, that is, it takes up moisture 

 from the air, independently of precipitation. Chart 3 shows this, and 

 it may be said that even dunng the hot summer season (June to 

 August) the litter takes up from 5 to 6 per cent of its air-dry weight 

 every night. This explains to a very large extent the well-known fact 

 that fires burn more slowly by night than by day, though other factors 

 — wind movement and temperature — also play a part. 



In June, the litter begins to take up moisture at about 5 p. m., and 

 starts to lose it at about 6 a. m. Later, or earlier in the season, this 

 period is of course correspondingly longer. 



