254 THE MECHANICS OF THE EARTH'S ATMOSPHERE. 



ragged clouds surrounding the border of the continuous cloud cover, 

 an excellent picture of the mixture just described. 



Of course it is understood that all these considerations relate only to 

 the conditions that ordinarily occur in the interchange of air between 

 cyclone and anti-cyclone. 



Processes in which we have to do with unstable equilibrium (such as 

 occur, for instance, in the great thunderstorms in front of an advanc- 

 ing current of air, where a whirl *with a long horizontal axis rolls 

 rapidly forward and brings simultaneously on the side of the descend- 

 ing current heavy rain-fall and great cooling with higher barometric 

 pressure, while on the front or ascending side the cloudiness is just 

 beginning) — such processes demand a very special investigation that 

 may be postponed to some future occasion. At present only one more 

 consequence will be drawn from the propositions relative to potential 

 temperature which seems to me calculated to throw a new light on the 

 interchange of heat in the atmosphere, and that especially demands 

 consideration from a climatological point of view. 



III. ON COMPLEX CONVECTION. 



It has been shown above that in the adiabatic transfer of air out of 

 the cyclone into the anti-cyclone, the potential temperature in the de- 

 scending branch is higher than in the ascending. Hence it follows 

 that in the descending branch a higher temperature prevails after 

 attaining the initial pressure than prevails at the initial point, and a 

 still higher temperature prevails at the end of the descending branch, 

 that is to say on the ground in the anti-cyclone where, according to 

 experience as well as for mechanical reasons, the pressure is always 

 higher. Therefore in this transfer of air we are concerned not only 

 with a simple transfer of the quantity of heat belonging to the air at 

 the base of the cyclone, which we can here temporarily call the original 

 quantity of contained heat, but this quantity of heat is increased by 

 that heat of condensation which in the condensation stage did a part of 

 the work of expansion and thereby diminished the cooling to a smaller 

 quantity than it otherwise would be. 



Even when in consequence of the stronger abstraction of heat at the 

 base of the anticyclone the air is fiually colder than it would have 

 been in purely adiabatic interchange ; and even when temperature 

 inversion has occurred, still the temperature at the end of the process 

 is still always higher than if the transportation of the air had taken 

 place at the level of the earth's surface and the cooling influences had 

 remained the same. 



The heat of condensation or negative heat of evaporation, or as it teas 

 formerly called the liberated latent heat, accrues to the advantage of that 

 region in which the descending current has reached the carWs surface. 



We can therefore compare the whole process with that of a steam 

 heater. 



