LIGHT AND VEGETATION 45 



Inside the Arctic Circle the sun remains visible for several 

 days in succession in the summer and is hidden for several 

 days in succession in the winter. 



The following table gives the length of this polar "day" 

 and polar "night" for dififerent latitudes: 



Energy Received per Day 



Let us assume, for the sake of simplicity, that the atmo- 

 sphere is perfectly transparent. The daily quantity of solar 

 energy received on a given horizontal surface, for example, 

 1 sq. cm., depends both on the zenith height of the sun, 

 which varies in the course of the day, and on the length of 

 the day. 



At the equator, the days are always twelve hours long 

 and the daily energy received varies little in the course of the 

 year. It is a httle greater at the equinoxes, when the sun is at 

 the zenith, than at the solstices, when the zenith distance at 

 midday is 23J°. 



At higher latitudes, the solar illumination is generally 

 weaker, but the days are longer in summer and shorter in 

 winter. As a result, the differences between summer and 

 winter become more and more accentuated. In summer the 

 greater length of the days compensates for the diminution of 

 illumination and the total energy received in a day may be 

 more than that received at the equator. 



A striking fact is the large quantity of solar energy 

 which reaches regions of high latitude in the summer; this 

 explains the surprisingly rapid growth of northern vegetation, 

 caused by a daily quantity of solar energy almost equal to 

 that at the equator and spread over very long hours of 

 dayhght. 



