Heat and its Importance in Meteorology. 373 



radiate from the sun or the earth. The corresponding trans- 

 missions for the same rajs, when they have penetrated only 

 down to the layer at 10,000 meters altitude or down to a pres- 

 sure of 21T4 mii , are 0-939 in the red, 0-864 in the yellow and 

 0-769 in the violet; hence 0-14 and 026 and 0-37 are the 

 respective fractions of the original heat absorbed from the ray 

 in its passage through the lowest 10,000 meters of the atmo- 

 sphere. Now the relative thermal intensities of these radia- 

 tions are respectively 68, 108 and 52 and for the present 

 approximation we may consider that about 20 per cent of the 

 original incident solar heat (3 gram-calories per sq. centimeter 

 per minute) is distributed by absorption along the 10,000 

 meters of this lower path. Therefore during the slow descent 

 of air in the midst of a high area the principal external source 

 of heat is the direct absorption of solar heat and this, at its 

 maximum for zenithal rays, can in the lower atmosphere only 

 amount to an absorption of -20x3-^10,000x1 00 = 0-000,0006 

 calories per minute per cubic centimeter of air; in northern 

 latitudes and on the average of the ordinary 6 to 12 hours of 

 sunshine the absorption becomes much less ; for Central Eu- 

 rope in November the average for the whole duration of sun- 

 shine would be less than 000,0002 calories per minute. 



(11.) As opposed to this gain of heat we have only the loss 

 by direct radiation which is, according to Maurer, expressed by 

 0-000,0007 gram-calories per minute per cubic centimeter or 

 0006 calories per kilogram mass of air when the surround- 

 ing enclosure has an average temperature 1° C. less than that of 

 the radiating mass. 



Without discussing the nature of the diffusion of heat by 

 radiation or conduction within a gas, we remark that Maurer's 

 coefficient of radiation relates strictly speaking to a mass not 

 to a volume ; we may plausibly assume that the unit mass of 

 air has the same coefficient whatever the density of the air, at 

 least within a pretty wide range, therefore it will follow that 

 the rate of linear descent within our atmosphere that produces 

 a rate of compression and dynamic warming sufficient to com- 

 pensate for the cooling by radiation must be greater at high 

 altitudes than at low ones. 



Dusty, hazy or moist air, as also the ammonia and the hydro- 

 carbons or the carbonic acid ordinarily present in the air, have 

 somewhat larger specific heats than those for clear dry air, but 

 they also all have larger coefficients of radiation so that their 

 cooling by radiation is more rapid than that of pure, dry air.* 



* A parallel process must take place in the solar atmosphere if we suppose the 

 gases that ascend from the immediate surface of the sun to be at such a high 

 temperature that all the chemical combinations ordinarily known to us are disso- 

 ciated : when a mixture of such elementary gases in this highly heated condition 

 ascends untd by expansion it is cooled to the temperature at which chemical com- 



Am. Jotju. Sgi.— TriiRD Series, Vol. XLIII, No. 257 —May, 1892. 

 25 



