TEMPERATURE 



305 



vertical rays pass through a thinner stratum of air than the other, 

 and more heat will reach the surface from a vertical sun. The 

 effect of greater inclination is compensated for in summer to some 

 extent by the longer sunshine period in twenty-four hours for high 

 latitudes. 



7. Slope. The slope of land has somewhat the same effect as 

 latitude on the concentration and distribution of heat. The effect 

 is to cause the rays from the sun to strike the south slope at a less 

 and the north at a greater angle from the perpendicular (Fig. 13(5). 

 With the sun 45 degrees above the horizon and the hill having the 

 two slopes of 20 degrees of equal length, the south one would receive 

 twice as much heat from the sun as the north one. 



Wollny found that the average temperature of the south slope of 

 a 15-degree hill was 1.5 degrees F. higher than the north slope. 



Fia. 13(>. Effect of slope on the area covered by the sun's rays. Angle of sun's rays 

 30 degrees from vertical. KK i.s KM) per cent greater than !)!',. BC is 40 per cent greater 

 than AB. 



King found that on July 31 a south slope of IS degrees had a tem- 

 perature 3.1 degrees F. higher than the level at a depth of one foot ; 

 2.7 degrees at two feet and 'J.S degrees at three feet. For early 

 crops a south slope is desirable. Plants that are liable to injury 

 from spring frosts should be placed on north slopes so that growth 

 will be retarded as much as possible. 

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