88 



ANALYSIS OF THE ENVIRONMENT 



point of maximum intensity of incident 

 radiation. The absorption on a clear humid 

 day rarely amounts to more than 15 per 

 cent of the incident energy. Thus 85 per 

 cent of radiations from the sun that are not 

 stopped by other causes pass the water 

 barrier in such an atmosphere. In contrast, 

 water vapor absorbs almost all the terres- 

 trial radiation. If the atmosphere holds only 

 the equivalent corresponding to 1 cm. pre- 

 cipitation, it absorbs 72 per cent of the 



TOTAL RADIANT ENERGY 



The mean value of the amount of radia- 

 tion received from the sun at the upper 

 level of the earth's atmosphere is 1.94 gm. 

 calories per square centimeter per minute. 

 This is called the solar constant. If this 

 amount of heat could be absorbed and re- 

 tained, it would warm a layer of cool water 

 1 cm. deep at the rate of 1.94° C. per 

 minute. The atmosphere screens out inci- 



en ° 



I- 



2! oo 

 o mi;- 



— fON 



-■y RAYS— » 



■X-RAYS- 



ULTRA 



VIOLET 



♦ ksUISHJ * 



INFRA 



L 



RED 



VISIBLE 



•HERTZIAN 



Fig. 6. The electromagnetic spectrum. (Redrawn with slight changes from Heyroth's revision 



of Ellis andWeUs.) 



earth's radiation. This phenomenon is called 

 the "greenhouse effect" and acts so that 

 solar radiation is transmitted and the earth's 

 radiation is retained. The effect is still 

 strong in a relatively dry atmosphere. 



Scattering and reflection brought about 

 by dust particles in the atmosphere produce 

 an "inverse greenhouse effect." The sun's 

 radiation is screened out by such particles, 

 and the earth's radiation is not affected. The 

 "greenhouse effect" results in a warmed 

 earth, and the "antigreenhouse effect" pro- 

 duces a lowering of the surface temperature 

 (Laurens, 1933). The portion of the sun's 

 ultraviolet radiation that passes through 

 the earth's atmosphere approximately coin- 

 cides with the so-called near ultraviolet. 

 The middle and extreme ultraviolet rays 

 have many biological effects and great 

 theoretical value, but so far as we now 

 know they are not important in outdoor 

 ecology. The parts of the whole radiation 

 spectrum that are ecologically significant 

 will be considered in the following chapters 

 in the order of their decreasing wave- 

 lengths. 



dent energy the more, the greater the 

 distance of air mass that is traversed, the 

 greater the amount of water vapor in the 

 air, and the more dust (Brooks, 1926), The 

 amount of energy that reaches the earth's 

 surface is also affected by the distance of 

 the earth from the sun and by variations 

 in the energy radiated by the sun. Other 

 conditions being equal, the solar radiation 

 received in early January is about 7 per 

 cent greater than that of early July, since 

 the earth is nearer the sun in January (see 

 p. 82). 



The amount of water vapor in the atmos- 

 phere decreases, in general, with latitude 

 and distance from the ocean, and increases 

 with temperature. Radiation intensity is de- 

 creased on the order of 2 per cent by an 

 increase of 1 mm. in water vapor pressure. 



The intensity of solar radiation differs 

 greatly at different points on the earth and, 

 at the same point, at different hours of the 

 day or night. At Washington, D, C, 127 

 meters above sea level, the amount of 

 energy received at noon is on the order 

 of 60 per cent of the mean solar constant. 



