158 HUMPHREYS: METEOROLOGICAL PARADOXES 



the phenomenon itself, however contrary to experience it may 

 seem, one of great importance and almost continuous occurrence. 



This paradoxical result is easy to explain with a diagram. 

 To this end let AB and A'B', figure 2, be two adiabatic gradients 

 of the free air; that is, let each indicate a temperature change of 

 1° C. for every 100 meters change in elevation — the relation 

 between the temperature and elevation of a rising or falling mass 

 of air that during its travel neither gains heat from, nor loses it 

 to, any outside object, such as the surrounding atmosphere. 

 Let EE be any actual temperature gradient (nearly always less 

 than the adiabatic), in this case i ° C. per 1 20 meters change of 

 elevation. If, then, under these conditions, a mass of air having 

 the temperature and elevation indicated by C, say, of the figure, 

 be heated i ° C, or shifted in the figure to W, it will corresponding- 

 ly expand and consequently be forced up by the surrounding denser 

 air — will ascend, as we say. As it rises, it will cool, by expansion, 

 along the adiabatic gradient A'B', and, therefore, will come into 

 equilibrium with the surrounding atmosphere where this gradient 

 intersects the actual gradient EE, or at the level and temperature 

 indicated by W. Clearly, then, under the assumed conditions, 

 such as are very common in nature, a mass of air heated 1° C. 

 rises 600 meters, and in so doing cools 6° C, or to a temperature 

 5 ° C. lower than it had before it was heated. Of course, the warm 

 air does not rise strictly adiabatically, though probably very 

 nearly so; but in so far as it actually does lose heat it comes to 

 equilibrium at a correspondingly lower level and warmer tem- 

 perature. 



It is precisely this paradoxical process of cooling by heating, 

 the heating being mainly at the surface, however, that leads to 

 the formation of cumulus clouds and generates the familiar 

 "heat" thunderstorm. In fact, it is quite possible to produce 

 a cumulus cloud, and even a local shower, through the action of a 

 large surface fire. It should be noted in this connection that 

 though combustion adds much water vapor to the air, five 

 ninths the weight of the fuel consumed even in the case of ab- 

 solutely dry cellulose, nevertheless, the cumulus cloud over the 

 fire is due essentially to the expansional or dynamical cooling of 

 the ascending air. 



