v.] SPRING FROSTS 133 



Effect of Situation and Exposure, 



Other conditions being equal, in the northern hemi- 

 sphere the soil temperatures will always be higher on 

 land sloping toward the southern quadrant than with 

 any other aspect. King found a difference of about 

 3 F. down to the third foot between a stiff red-clay soil 

 with a southern slope of 18 and the same soil on the 

 flat ; Wollny obtained a mean difference of i°-5 between 

 the north and south sides of a hill of sandy soil inclined 

 at 1 5 . The chief cause of these differences is the fact 

 that in this country the sun is never vertical, hence a 

 beam of sunlight represented by xy, Fig. 12, is spread 



FlG. 12. — Distribution of the Sun's Rays on Southerly and 



Northerly Slopes. 



over an area represented by AB when the ground is 

 flat ; if the ground slopes to the south, the same beam is 

 spread over the smaller area represented by AC ; if the 

 ground slopes to the north, it is spread over the larger 

 area represented by AD. During the winter half-year, 

 also, the southern slope will have a longer duration of 

 sunlight than the northern slope. 



Though in a general way the temperature both of 

 the air and the soil decreases with elevation above 



