372 C. Abbe — Atmospheric Radiation of 



the time during which radiation has been progressing and thi& 

 duration will enter as an exponent in the analytic formula. 

 Again the enclosure to which this heat is radiated, as imagined 

 in connection with Maurer's coefficient, is on the average 1° 0. 

 cooler than the radiating mass, therefore its dimensions may 

 be calculated and it appears that, near the earth's surface, the 

 lower portion of such a sphere is cut off and is replaced by the 

 cold ground which is equivalent to diminishing the size of the 

 sphere or increasing the rate of radiation ; a similar considera- 

 tion obtains at the outer limit of the atmosphere ; in general 

 the radiation of a descending mass causes it to cool least rapidly 

 when at a medium altitude above the ground or in the midst 

 of the atmosphere. 



As for conduction between the earth and the air it is easily 

 recognized, as Maurer has shown, that this has an inappreciable 

 influence during twelve hours at three meters above the soil 

 or during 48 hours at six meters ; it can only become mode- 

 rately important for calm air during long arctic nights. As- 

 concerns convection of heat by currents rising from the earth 

 this will directly affect the dry, cold, clear, calm air in the 

 midst of high pressures only up to a moderate altitude, say 

 1000 meters, and that only in the warmer part of the day 

 because the rising currents are rapidly cooled by mixture and 

 cease rising at an altitude whose limit in such weather is 

 usually defined by a thin layer of dust and haze. Up to this 

 altitude therefore atmospheric radiation of heat, during 24 

 hours of day and night, may be partially compensated by con- 

 vection during sunshine but this limit is much lower for still 

 air lying over ice, snow or water. As concerns the cooling due 

 to evaporation this can be computed from the known amount 

 of evaporation and is a very small quantity for the cold air 

 over the land north of latitude 50° where our high areas form. 



Therefore the factors that principally affect temperature in 

 extensive areas of high pressure are : 1. The direct absorp- 

 tion by the air of the solar radiation. 2. The direct radiation 

 by the air of its own heat. 3. The thermal changes due to 

 changes of barometric pressure in the radiating mass which are 

 themselves due principally to its descent in the atmosphere. 



(10.) With regard to the direct absorption of solar radiation, 

 the bolometric researches of Langley show that the general 

 coefficient of transmission of solar radiation through the atmo- 

 sphere varies from about 8 in the red and 0*6 in the yellow 

 to 0*4 in the violet and averages approximately 0*6 for the 

 whole spectrum. These figures relate to the general absorp- 

 tion of a zenithal ray passing through the whole atmosphere 

 down to the layer whose elastic pressure is 760 mm and give no 

 idea of the special absorption of specific wave-lengths that may 



