SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 413 



ofteu rise like columns in the blue sky of summer, and plainly grow out- 

 ward from the interior. The warmer the air, or the higher the tem- 

 perature at which it is saturated with vapor, by so much the greater is 

 the upward impulse it experiences on the occurrence of precipitation. 

 Therefore the precipitations of the ascending currents in summer, and 

 especially in most tropical climates, occur under the most favorable cir- 

 cumstances. When, during calm, sunny weather, the above-described 

 overheating of the lower strata of air, which is always accompanied by 

 abundance of aqueous vapor, has been brought about, and there arises, 

 if only in the highest strata, a disturbance of the equilibrium already 

 stretched to its utmost limit, and thereby a small initial condensation 

 of the vapor, the air immediately receives a strong upward impulse, and 

 draws the lower strata rapidly with it in motion. Hence the numerous 

 thunder-storms in calm, hot summer weather, and in the tropics on the 

 cessation of the regular trade-wind. These thunder-storms do not indi- 

 cate a change in the weather: they only restore the equilibrium of the 

 atmosphere disturbed in a vertical direction. The lower atmospheric 

 strata are cooled by evaporation and by the cooler rain-water; the upper 

 are warmed, and thus the equilibrium becomes again stable. 



The formation of hail is, as Eeye has shown, most easily explained 

 by such ascending moist currents of air. The occurrence of hail in the 

 hottest time of the day and the year and the prevailing calms point 

 directly to the superheating of the lower strata as their immediate 

 cause. But the chief difficulty consists in the question, Whence comes 

 the cold that is necessary for freezing the precipitation, and how can it 

 be due to the high temperature of the lower strata of air? Mohr 

 assumes a descent of higher cold air. We have previously shown how 

 seldom this process can occur, and that it can produce no considerable 

 cooling, because the air is greatly warmed in its descent. Most natural 

 is it to assume with Eeye, inversely, that the lower, moist, hot air is rap- 

 idly carried upward. We have previously explained that on the occa- 

 sion of irruption of cold winds the upper strata rush ahead of the lower, 

 so that the stagnating lower, hot, moist air is equally surrounded by the 

 upper cold air. The former thereby acquires an especially strong up- 

 ward impulse, which is increased by the intense condensation, so that 

 it must soon arrive at altitudes where the precipitation freezes. This 

 explains the fall of hail on the advancing side of a thunder-storm or 

 tornado; its recurrence in bands; its dependence upon local peculiari- 

 ties (broad river valleys are more intensely warmed, and have no equal- 

 izing currents of air, such as in narrow mountain valleys; therefore 

 hail is more frequent in the former, as has, for instance, been demon- 

 strated for Carinthia by Prettner); the descent, by way of compensa- 

 tion, of the cooled air; finally, the maximum of hail in early summer, 

 at which time the upward diminution of temperature is most rapid, and 

 the greatest difference of temperature exists between the earth's sur- 

 face, warmed by the high sun, and the upper strata of air, that still 

 retain their winter cold. 



