CLIMATIC FACTORS : RADIANT ENERGY 



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FlG. 65. A generalized profile of altitudinal zones of vegetation in the 

 mountains of Utah, which illustrates the effects of northern and southern ex- 

 posures.— Adapted jrom Woodbury . 



276 



Cover and Temperature.— Anything that absorbs or reflects in- 

 solation before it reaches the earth will reduce both soil and atmos- 

 pheric temperatures. Thus it is cooler in cloudy or foggy areas 

 than in similar areas without clouds or fog, and any given area 

 tends to be warmest on clear days. But, because heat radiated from 

 the earth and clouds is held below a cloud blanket, the lowest tem- 

 peratures also occur on clear days, and extremely low temperatures 

 are not to be expected on cloudy days. Temperatures in and above 

 bare soil, particularly dark soil, are higher than if that soil has some 

 form of cover. Any type of vegetation must absorb some radiant 

 energy and, consequently, reduce temperatures between itself and 

 the soil, the reduction being proportionate to the closeness of the 

 stand and how many strata compose it. Temperatures in forest 

 stands in midsummer are usually ten degrees lower by day than in 



