SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 411 



land, the stagnating overheated stratum of air could no longer remain 

 in equilibrium, and a stormy mixture of the lower and upper masses of 

 air, which of course was accompanied with precipitation and thunder 

 and lightning, precipitated itself mostly from the upper strata in special 

 localities downward to the earth's surface. 



The occurrence of unstable equilibrium in strata of air lying vertically 

 above each other after a period of sunny and calm weather is facilitated 

 by the fact that the inflow of a strong wind almost always first occurs 

 in the upper strata, whose temperature thereby rapidly sinks, while in 

 the lower strata, it perhaps still rises. Since the flow of the air near the 

 surface of the earth is much obstructed and delayed by friction and other 

 obstacles, the cooling is completed more rapidly in the upper strata and 

 the inflowing colder air surrounds from above the warmer lower strata^ 

 which then must break through with violence.^ 



In a sinking and warming current of air, no condensation of aqueous 

 vapor can take place, therefore in it the increase of temperature of 1° 0. 

 per 100 meters, as required by theory, must clearly stand forth. In fact, 

 the Fohn wind shows this rapid increase of temperature between the 

 summit and base of the mountains over which it descends. In general, 

 descending masses of air can have only a warming eflFect upon the earth's 

 surface, since their increase of tem| erature is more rapid than the, average 

 upward diminution of temperature. If, for instance, in summer the 

 strata in which float the ice crystals of the cirrus clouds at altitudes of 

 G,000 to 7,000 meters had a temperature of even — 30° C, still would they 

 by sinking to the earth's surface have a temperature of +30° or +40° C. 

 Only in the case of a diminution of temperature of more than 1° 0. per 

 100 meters, which can, however, only be local and occur in summer, but 

 never in winter can it happen that masses of air descending from the 

 upper regions to the earth's surface arrive there cooler than the air 

 that they displace.^ 



One of the effects of descending currents of air must consist in clear- 

 ing up the sky, and dissipating the precipitations, instead of causing 

 them. Now, however, we still observe precipitation on the advent of an 

 upper current of air that slowly descends to the earth's surface and then 

 brings warmth. When, after clear weatlier, the increasing cirrus clouds 

 announce the speedy advent of foul weather, one can often enough clearly 

 observe how the layer of cirro-stratus sinks, and is thereby thickened, 

 and gradually mer^^ies into a rain cloud. Often after a hot summer day 

 I have seen thunder-storms form in this manner j the formation of the 

 lower layer of cumulo-stratus unmistakably proceeded from a layer o( 

 sinking cirro-stratus. In this case, however, the upper current of air 

 could be relatively so cold that even after sinking it could cool the lower 

 strata. In winter, such an assumption would not be allowable; we must 

 assume that the upper warm current increases in volume, continually 

 extends into deeper strata (but does not form a descending air-current), 

 and by mixing with the lower strata produces a precipitation continually 



