SOIL HEAT 



229 



119. The effect of slope on absorption. — The second 

 phase to be considered in the rise of temperature of a given 

 soil is the angle of incident of the sun's rays. The greater 

 the inclination of a soil from a right angle interception, the 

 less rapid will be the rise in temperature. As a consequence, 

 the total insolation received in the tropics to a unit area is 

 greater than that attained by a corresponding area in the 

 temperate zone. Moreover, any condition in a temperature 



Fig. 42. — Curves showing the temperature variations of different colored 

 soils at a four inch depth compared with air temperature. Munich, 

 June 23, 1876. 



region which tends to bring a unit surface more nearly normal 

 to the sun's rays will increase its absorbed energy and raise 

 its average seasonal temperature. In the north temperate 

 zone this is of course a southerly inclination. The diagram 

 (Fig. 43) illustrating conditions on the 42d parallel at noon 

 on June 21 makes clear this relationship. 



It is seen that in this case a southerly slope of 20° received 

 the greatest amount of heat to a unit area with the level soil 



