6 INTRODUCTION 



out — namely, that the condensation of water vapor in the atmosphere 

 accounts for nearly half the amount of energy which the atmosphere receives. 



The Atmosphere as a Thermodynamic Machine 

 So far, we have considered only the average heat budget of the atmos- 

 phere and have not examined the geographic and seasonal distribution 

 of the regions in which heating and cooling take place. Such an examina- 

 tion cannot be made in detail because our knowledge of the processes 

 of radiation in the atmosphere and of the regions where condensation of 

 water vapor takes place is inadequate. It appears certain, however, 

 that the greater heating takes place in the Tropics and the greater cooling 

 in the high latitudes, so that on an average heat must be transported from 

 the Tropics to the polar regions in order to maintain an approximately 

 stationary distribution of temperature. It is also probable that this 

 transport of heat takes place mainly by air currents, which implies that 

 the atmosphere is a thermodynamic machine that transforms heat into 

 kinetic energy. This kinetic energy is dissipated by friction and trans- 

 formed back again into heat that is lost by radiation. If the machine 

 runs at a constant speed, the amount of heat which in unit time is trans- 

 formed into kinetic energy must equal the amount of heat which in unit 

 time is generated by dissipation of kinetic energy. 



In a thermodynamic machine that produces kinetic energy the heating 

 must take place, according to a theorem formulated by V. Bjerknes 

 (Bjerknes et al, 1933), under a higher pressure and the cooling under a 

 lower pressure. Applied to the atmosphere the theorem states that heat- 

 ing must take place at a lower altitude and cooling at a higher altitude. 

 This condition is fulfilled in the atmosphere because condensation of 

 water vapor does take place at a relatively low altitude; absorption of 

 long-wave radiation from the earth also takes place at relatively low 

 altitudes, whereas radiation to space from the water vapor and the upper 

 surfaces of clouds takes place from high altitudes. Thus, the general 

 features of the thermodynamic machine represented by the atmosphere 

 can be recognized, but the details are not understood. A discussion of 

 the interaction between the oceans and the atmosphere will supply some 

 of the details that have to be considered and is therefore necessary for 

 the better comprehension of the atmospheric circulation. 



Several attempts have been made at an examination of the general 

 character of the atmosphere as a thermodynamic machine, the latest and 

 most widely quoted being that by Simpson (Bjerknes et al, 1933; Brunt, 

 1939). Simpson considers only the distributions with latitude of the 

 incoming short-wave radiation and the total outgoing long-wave radia- 

 tion, making certain simplifying assumptions as to the absorption spec- 

 trum of the water vapor. He finds that the earth receives a surplus of 

 radiation up to latitude 35° and concludes that this surplus must be 



