THERMODYNAMICS OF ATMOSPHERE VON BEZOLD 269 



If now the conditions above described are fulfilled; if there be no 

 decided ascending or descending movement of the air, while the 

 sun heats the ground very hot, then unstable equilibrium will be 

 produced at different portions of the earth's surface and especially 

 where this heating is favored by the character of the ground. 



Thus, during the summer months at least, at any given moment 

 the heating effect may be about the same all along a line inclined 

 to the meridian and trending NNW. and SSE. since points on 

 such a line will have experienced equal durations of insolation. 



Consequently, and independently of the influence of the general 

 distribution of pressure, an approximately equal and simultaneous 

 superheating of the lower air and hence unstable equilibrium will 

 occur on any given day along such a nearly meridional line. 



Thus, then, at first there will be a series of centers arranged along 

 this line, where favored by local peculiarities the lower air will rise; 

 on account of the increase of buoyancy due to the condensation, 

 this ascending current will rise higher and higher until it is no longer 

 able to raise and carry up the condensed mass of water, when it 

 falls and we say the thunderstorm has broken. 



As will be shown later on, this fall generally begins at altitudes at 

 which the temperature is below freezing and in most cases therefore 

 the precipitation at the higher levels consists of hail or sleet, which, 

 however, seldom reaches to the ground but melts during the fall and 

 therefore reduces the temperature of the lowest stratum in the 

 well-known manner. 



Hence, after the thunderstorm begins there is a sudden fall of 

 temperature, the surfaces of equal barometric pressure press closer 

 together within the region of precipitation, while the air rising at the 

 front or eastern edge of the thunderstorm and adjoining the still 

 warmer parts of the atmosphere overflows toward the cooled side 

 and causes a rise of pressure there. 



On the other hand, in the lower stratum, the air flows with great 

 force eastward out of the region of precipitation, the air resting in 

 front of it is disturbed and, if not already so, is now forced to a 

 rapid ascent without necessarily being itself in a state of unstable 

 equilibrium. 



Thus the thunderstorm renews itself continually on its front edge 

 and if the original superheating was great enough, and the atmosphere 

 in general sufficiently quiet, to allow the individual thunderstorms 

 developed along the above-mentioned meridional line to unite into 

 one large band, then the thunderstorm front thus originating rolls 

 farther eastward as a great whirl with horizontal axis until the 



