LIGHT AND VEGETATION 37 



winter is accentuated. At 60° N. (Stockholm, Leningrad) the 

 sun's rays are inclined from the vertical at an angle between 

 84° and 36°, according to the season, and the ratio of the 

 fluxes received is nearly 1 to 50. Within the polar circles, the 

 flux received at the winter solstice is nil. On the other hand, 

 in the equatorial regions, the differences between the seasons 

 are scarcely perceptible. 



It is interesting to note that the maximum illumination 

 at the confines of the atmosphere in summer at the latitude 

 of Bordeaux represents more than nine-tenths of the illumina- 

 tion at the equator. At the latitude of Stockholm it reaches 

 nearly six-tenths and if, in addition, we remember that the 

 days are much longer we can understand why some crops 

 can be grown so successfully and in such a short time at 

 high latitudes in countries which are reputed to be cold. 



Atmospheric Absorption 



We have discussed the nature and intensity of the solar 

 radiation which reaches the upper atmosphere. In eight 

 minutes it has travelled 93,750,000 miles through inter- 

 planetary space without undergoing any modification other 

 than its flux density being diminished as the inverse of the 

 square of the distance. 



But during its passage through a few miles of gas of the 

 terrestrial atmosphere it is weakened — sometimes consider- 

 ably — and its composition is changed because the radiations 

 of diff'erent wave-lengths are differently affected. 



The degree of this weakening depends on the state of the 

 atmosphere and on the thickness of air traversed, i.e., on the 

 height of the sun. This explains why our eyes can bear the 

 light of the setting sun and why it is red in colour. 



The thickness of air traversed is expressed by what is 

 called the "air mass" (abbreviated symbol m)\ when the sun 

 is at the zenith, by convention m=\. 



When the rays of the sun make with the vertical an angle 

 called the zenith distance, they pass through a larger air mass. 

 This mass is doubled for a zenith distance of 60°, that is, 



