The United States can be divided into many climatic zones; but four climate 

 classes were enough to provide the broadscale plant moisture responses needed for rat- 

 ing fire danger. These four climate classes were adapted from Thornthwaite' s earliest 

 climate classification system (Thornthwaite 1931) . His arid and semiarid provinces 

 were grouped into Climate Class 1 because the true desert is of little real concern to 

 fire management. Also, in terms of fire behavior, the subhumid province groups better 

 with the humid province than with the dry, subhumid province. The climate class des- 

 criptions, the general geographic areas to which they apply, and the vegetation charac- 

 teristic to each, are provided in table 1. Figure 2 shows the general locations to 

 which the climate classes apply. 



Table I . --Climate class selection guide 



NFDRS 

 climate 

 class 



: Thornthwaite'* 

 : humidity 

 : province 



: Characteristic : 

 : vegetation : 



Regions 



1 



Arid 



Desert (sparse 

 grass and scat- 

 tered shrubs) 



Sonoran deserts of west Texas, New 

 Mexico, southwest Arizona, southern 

 Nevada, and western Utah; and the 

 Mojave Desert of California 





Semiarid 



Steppe (short 

 grass and shrubs) 



The short grass prairies of the Great 

 Plains; the sagebrush steppes and 



pinyon/ juniper woodlands of Wyoming, 

 Montana, Idaho, Colorado, Utah, 

 Arizona, Washington, and Oregon; and 

 the grass steppes of the central 

 valley of California 



The Alaskan interior; the chaparral 

 of Colorado, Arizona, New Mexico, the 

 Sierra Nevada foothills, and southern 

 California; oak woodlands of Cali- 

 fornia; ponderosa pine woodlands of 

 the West; and mountain valleys (or 

 parks) of the Northern and Central 

 Rockies 



Blue stem prairies and blue stem- 

 oak-hickory savanna of Iowa, 

 Missouri, and Illinois 



Almost the entire Eastern United 

 States; and those higher elevations 

 in the West that support dense 

 forests 



4 Wet Rain forests Coast of northern California, Oregon, 



(redwoods, and Washington, and southeast Alaska 

 spruce-cedar- 

 hemlock) 



^Deeming and others 1977. 

 '^Thornthwaite 1931. 



Subhumid (rain- Savanna (grass - 

 fall deficient lands, dense 

 in summer) brush and open 



conifer forests) 



Subhumid (rain- 

 fall adequate in 

 all seasons) 



3 



Humid 



Savanna (grass- 

 lands and open 

 hardwood 

 forests) 



Forests 



4 



