CLIMATIC FACTORS 



63 



zon, the altitude decreases, until at the horizon the light 

 must penetrate 12 to 35 times as much atmosphere, and 

 its total effect is weakened to about one-fiith the full 

 effect at 90 degrees. The effect at different altitudes is 

 expressed in the following selected altitudes : '■ — 



TABLE XI 



From the equator to 40 degrees latitude, the total sun- 

 shine received at a given place from March to September 

 is about one-third of the total possible sunshine at that 

 point if the sun stood at zenith during the hours of day- 

 light. In the northern latitudes the longer days of mid- 

 summer compensate for the lower altitudes of the sun, so 

 that during the months of June and July, as much heat 

 is received at the north pole as at any lower altitude. 

 In fact, for a period of about 90 days, more heat units 

 are received at the north pole than the equator, but due 

 to the great amount of ice is not sufficient to raise the 

 temperature above freezing. The relative quantities of 

 heat received at different latitudes in the Northern Hemi- 

 sphere are shown by the following table, as calculated by 

 Aymonnet : ^ — 



' Abbe, loc. cit., p. 85. 



2 Ibid., p. 92. 



