6 BULLETIN 1495, U. 8. DEPARTMENT OF AGRICULTURE 
TEMPERATURE, SOIL MOISTURE, AND PRECIPITATION 
An investigation * conducted by the Forest Service, which covered 
the relation of air temperature and soil moisture to the principal 
timber types, except Douglas fir, in the central portion of the Sierra 
region, has brought out the differences between the upper and lower 
portions of the commercial timber belt. (Table 2.) The frostless 
season ranged in length from 97 days in the fir types to 162 days in 
the western yellow pine and length of the growing season from 143 
to 200 days. But neither length of frostless season nor that of the 
growing season can be regarded as directly controlling the length of 
fire season; rather, each is a useful indicator thereof. The difference 
between the two extremes in the frostless season of 65 days and that 
of 57 days in the growing season both exceeded the difference between 
the same types 1n length of fire season—42 days. (Table 8.) 
TaBLE 2.—Climaiological data from stations characteristic of various forest types on 
Plumas National Forest, 1914-1917 
Air-temperature data 
Soil 
Eleva- Mean Hours tem- | Soil 
Cover type tion of | Aspect | ‘tom. | Frost- | Grow. | Guring growing| pera- mois- 
station rene ee ine season— ture, oe 
ee A t £ 
jee : season | season iia eee 
i 85° F. | 70° F. 
d Feet bey ik Days Days |Number|Number| ° F. |Per cent 
Western yellow pine_______ 2,7 Bouthes 74. 4 162 200 498 | 1,396 72 3 
4,000 ;--.do____ 
iVrixed conifers ss 5 = 5, 000 |---do__-__ | 70. 4 158 180 |. 262] 1,070 69 5 
4,000 | North ___|! 
SHmarspine-Hre si ee oe 5000) do 66. 5 108 155 84 841 66 6 
F F 6, 000-| South___ = 
Ua Ge ee ete Se { 6, 000 North — \ 66.3 104 | 150 14 360 63 8 
. 7,060 | South___ = = 
igre. Ay Se { 7 000 North} 51.1 90 | 135 | 1 210 |S nee eee 
' 
{ 1 
The same study gave further information on type differences. The 
mean temperature for August, a peak month of the fire season, ranged 
from 58.7° F. in fir to 74.4° in western yellow pine. The temperature 
differences between types are also expressed by the comparative 
number of hot hours (over 85°) during the growing season, which 
range from 7 in fir to 498 in western yellow pine. The direct effect 
of temperature on spread of fire has not been fully analyzed, although 
undoubtedly it has some effect. In so far as temperature modifies 
relative humidity, it has a direct bearing on the spread of fire. 
Soil moisture is important because of its effect on moisture content 
of the litter and hence on inflammability and rate of spread. Experi- 
ments in California ° and Idaho ° indicate that with a litter moisture 
content of over 8 per cent (on an air-dry basis) fires do not start 
readily from sparks or matches, and with over 25 per cent litter 
moisture even bonfires do not spread. In the fir type the low soil 
moisture content for the season is reached in August at 8 per cent; 
whereas in the western yellow pine the moisture drops to 3 per cent 
4 No comprehensive study of types and climate has been completed for this region. The only published 
studies are: VARNEY, B. M. MONTHLY VARIATION OF THE PRECIPITATION-ALTITUDE RELATION IN THE 
CENTRAL SIERRA NEVADA REGION OF CALIFORNIA. Mo. Wea. Rev., 48: 648-650, 1920. SEASONAL PRECIPI- 
TATION IN CALIFORNIA AND ITS VARIATION. Mo. Wea. Rev., 53: 148-163; 208-218, 1925. 
5 SHow, S. B. CLIMATE AND FOREST FIRES IN NORTHERN CALIFORNIA. Jour. Forestry 17: 965-979. 1919. 
6 GISBORNE, H. T. MEASURING FOREST FIRE DANGER IN NORTHERN IDAHO. U.S. Dept. Agr. Misc. 
Pub. 29, 45 p., illus. 1928. 
