188 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 19 3 5 



the atmosphere and an equal amount of energy passes outward from 

 the earth in the form of dark heat radiation. Only certain gases 

 in the atmosphere take part in this absorption and emission of radia- 

 tion; nitrogen, for example, is nearly transparent to all the types 

 of radiation passing through the atmosphere, but oxygen, ozone, 

 and water vapor strongly absorb and radiate certain particular 

 types, being transparent to others. Thus oxygen strongly absorbs 

 the radiation of the very shortest wave lengths received from the 

 sun, namely the extreme ultraviolet radiation. Ozone strongly ab- 

 sorbs radiation of rather longer wave lengths though still in the in- 

 visible ultraviolet region of the spectrum, and also absorbs a little 

 in the yellow-green, and again a little in the long-wave infrared 

 region. Oxygen and ozone together absorb about 6 percent of the 

 total energy of the sun coming to the earth, and since the absorp- 

 tion is very strong this energy is absorbed by the air at a very great 

 height in the atmosphere. Water vapor is nearly transparent to 

 all the visible and ultraviolet radiation received from the sun, but 

 strongly absorbs most of the long-wave infrared radiation emitted 

 by the earth. 



The radiation from all bodies at temperatures below red heat takes 

 the form chiefly of infrared radiation. Oxygen and ozone can emit 

 little radiation of this kind, so that the loss of heat by emission of 

 radiation is chiefly due to the water vapor present. If only water 

 vapor were present, it would absorb little energy from the sunlight 

 but would absorb the infrared radiation emitted by the earth and 

 would bring the air to a temperature of about 50° C. below zero, at 

 which temperature it would be absorbing and emitting equal 

 amounts of radiation so that its temperature would remain constant. 

 It is this process which chiefly governs the temperature of the 

 stratosphere, as oxygen and ozone do not play any appreciable part 

 here, because all the solar radiation which they could have absorbed 

 has already been absorbed much higher in the atmosphere. 



Passing now to the extreme upper limits of the atmosphere we 

 come to the region where oxygen and ozone absorb the 6 percent of 

 the sunlight mentioned before. This amount of energy is very large 

 and at these heights there is not much water vapor, so that the 

 atmosphere cannot easily lose heat by radiation. The result natu- 

 rally is that the temperature of the air rises very much until the 

 small amount of water vapor present is able to emit as much energy 

 as is absorbed. Thus it will be seen that the actual temperature at 

 any level depends on the amount and character of the radiation 

 absorbed and radiated away and therefore on the relative amounts 

 of water vapor and oxygen and ozone which are present. 



If we knew the amount of oxygen, ozone, and water vapor present 

 at every height in the atmosphere it would be possible to calculate 



