IIEAT 



91 



again without making any change either on 

 the earth or in its atmosphere. Roughly 

 another third is absorbed by the atmos- 

 phere, and the final third is absorbed by the 

 earth itself. These are average figures for 

 the earth as a unit when all seasons are 

 considered. 



On a clear day, when the sun stands 

 overhead at the zenith, approximately 92 

 per cent of the radiation at sea level comes 

 from the sun directly; the other 8 per cent 

 comes from the sky. The relative differ- 

 ences decrease until they are equal, though 

 both are much less, when the sun is some 8 

 degrees above the horizon. The intensity of 

 direct radiation from the sun increases with 

 an increase of height above sea level; con- 

 versely, the intensity of sky radiation 

 decreases with altitude. When the sun is 

 overhead in an overcast sky, if the cloud 

 layer is uniform, the brightness is surpris- 

 ingly uniform; brightness decreases about 

 10 per cent 45 degrees from the zenith and 

 about half of that at a point almost at the 

 horizon (Humphreys, 1942). 



ECOLOGICAL RADIATION UNIT 



Under many conditions, the amount of 

 radiation received in a given biotic com- 

 munity, or a fraction thereof, can be 

 summarized by the ecological radiation unit 

 that may be stated in terms of energy or of 

 light intensity (O. Park, 1931). This unit 



represents a summation of (1) the inten- 

 sity (a) under the open sky, (b) under 

 different degrees of shade, and (c) in sun- 

 flecks under a canopy of vegetation; (2) the 

 area in the community which receives ra- 

 diation of each of the recognized inten- 

 sities. In a representative case, the ecologi 

 cal radiation unit of the forest floor can be 

 calculated as follows: 



Let A =: unit area 



P = portion of unobstructed radiation 

 Shi = shaded portion of density 1 

 Sh2 = shaded portion of density 2 

 S = portion covered by sunflecks 

 Q = intensity of unobstructed radiation 

 Qi = mean intensity in sunflecks 

 qi = mean intensity in Shi 

 qz = mean intensity in Sh2 



When P + S -^ Shi -t- Sh= = A, the fol- 

 lowing simple formulation can be stated: 



PQ + SQ, + Shiq, + Shsqj 



AQi 



= Ecological radiation unit. 



The ecological radiation unit may sum- 

 marize all radiation, or it may be broken 

 into different fractions, as, for example, the 

 originally proposed ecological light unit (O. 

 Park, 1931; Strohecker, 1938). The latter 

 has distinctly different values in the several 

 stages of the dune-forest succession. 



6. HEAT 



EFFECTS OF HEAT ON THE PHYSICAL 

 ENVIRONMENT 



Heat is a form of energy, of which two 

 important ecological factors may be recog- 

 nized. There is (1) the intensity factor, 

 temperature, and (2) a capacity factor, 

 heat capacity. Temperature is measured in 

 degrees on some temperature scale; in this 

 book the centigrade scale will be used un- 

 less otherwise stated. The capacity for heat 

 is defined as the quantity of heat taken to 

 raise the temperature of the given substance 

 through 1° C. The standard unit, the 

 calorie, is the quantity of heat required to 

 raise 1 gm. of water from 15° to 16° C; 

 this is a gram-calorie and represents a rela- 

 tive!" <unall amount of energy. When large 



quantities are involved, the kilogram-calorie 

 is often used as the basic unit, especially in 

 human nutrition; this is 1000 times larger 

 than the gram-calorie. 



Ecologists, and biologists in general, fre- 

 quently use the words "heat" and "tempera- 

 ture" as though they were synonyms; often 

 they are. A familiar phenomenon will illus- 

 trate one basic difference. Much heat 

 energy must be spent to melt ice, yet, until 

 it is melted, the temperature remains con- 

 stant. It requires 3500 gram-calories per 

 square centimeter of surface to change the 

 ice on Lake Mendota at Madison, Wiscon- 

 sin, to water; an amount equal to the heat 

 from some 195,000 tons of anthracite coal 

 is necessary to melt the ice on the whole 

 lake, yet the temperature of the water is 



