CLIMATIC FACTORS : RADIANT ENERGY 131 



more situations or habitats. For this purpose, the photoelectric 

 method is quite usable. 



The method has, however, other limitations, and its use requires 

 certain precautions. Preferably two or more instruments should 

 be available and the readings should be made simultaneously. Even 

 so, readings should be made only on a clear day and, when pe- 

 riodic observations are made, at the same time of day. Results 

 should be expressed as percentages of full sunlight at the time 

 when each observation is made. At sea level this would be approxi- 

 mately ten thousand foot candles on a clear day at noon, but values 

 as high as twelve thousand foot candles have been obtained in the 

 clear air of high mountains. If for any reason the readings in the 

 open are low on a given day, no further observations should be 

 made. 



Because of its concave sensitive surface, the instrument can be 

 operated in only one plane at a time. If readings are made simul- 

 taneously at noon with the instrument in a horizontal plane, many 

 complicating factors are automatically eliminated. The instru- 

 ments are extremely sensitive to slight variations in light, and this 

 necessitates numerous readings to arrive at average conditions. 

 The slightest air movement shifts the position of leaves and per- 

 mits bright sun flecks to come through a forest canopy. These 

 flecks come and go, first at one point and then at another, and 

 cannot be ignored in evaluating light in a stand. Their inclusion is 

 best accomplished by making observations at a rather large num- 

 ber of uniformly or randomly distributed predetermined points 

 and averaging the results. In all instances, the instrument should 

 be in the same position relative to the observer and the ground. 



A sensitive surface of spherical form is usually more desirable 

 than a flat one. Where reflected light is appreciable, a sphere will 

 record from all directions. If a continuous record is to be obtained, 

 the sphere records accurately because one-half its surface always 

 faces the sun regardless of its position. Several radiometers, which 

 measure heat effects and are nonselective of wave lengths, are 

 spherical in form and are advantageous in other respects. If a 

 photoelectric cell is given more than a short exposure to strong 

 light, the current it generates falls off because of solarization, but 

 the radiometer can be exposed indefinitely without such effects. 



