390 SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



sclirift der natnrforschenden Gesellschaft in Zurich, 14. Jahrg., 1869). 

 " The tbunder-storm begins with a precipitation of vapor in con- 

 sequence of the cooling of the air ; this precipitation is, however, ac- 

 companied by a development of heat, and this heat developed by the 

 condensation of vapor constitutes the chief difficulty in the explanation 

 of thunder-storms." Wettstein now computes for a few cases the quan- 

 tity of heat set free by the condensation of vapor, and investigates 

 whether the cooling processes that occur in nature suffice to remove 

 this quantity of heat and to render possible the condensation of vapor. 

 We will delay somewhat over this chapter of the interesting work of 

 Wettstein, because the citation of his computations and the proof of 

 the errors that "have slipi^ed in will best lead to the recognition of the 

 important part that the ascending movement of the air must play in all 

 heavier processes of precipitation. 



Wettstein first considers the cooling of ascending moist air. He 

 computes the diminution of temperature according to a well-known 

 formula of the mechanical theory of heat for the case where no conden- 

 sation takes ijlace. For an ascent of 1,000 meters, there results a cool- 

 ing of 10° C. ; for 2,000 meters, 20° ; for 3,000 meters 29° ; for 4,000 

 meters, 38° ; for 5,000 meters, 47°. If, now, the air was originally 25° 

 C. warm, and saturated with vapor, then, by its ascent, vapor will be 

 condensed ; as, for instance, for 1,000 meters ascent, 22.83 — 12.74 = 

 10.09 grams per cubic meter j the latent heat thereby liberated is 10.09 x 

 0.590 = 5.96 units of heat, which suffices to raise the temperature of 

 the air and the condensed water by more than IS^.S C. Since the cool- 

 iug due to expansion amounts to only 10° C, therefore Wettstein con- 

 cludes that precipitation is impossible. In a similar manner, he finds 

 for an ascent of 2,000 meters an excess of nearly 10° C. of warmth, for 

 3,000 meters nearly C, and for 4,000 meters, 0o.7 C. 



For a current of air rising up to 5,000 meters, the cooling is 47°.2, and 

 the warming 41°.4; therefore the resultant cooliug5°.8; from these figures, 

 it follows, says Wettstein, that the lowering of the temperature of tha 

 air, in consequence of its expansion, will, up to heights of more than 

 4,000 meters, be neutralized by the heating due to the condensation of 

 vapor, and that the ascent of warm moist air can be accompanied by 

 preci|)itation, first at very great altitudes, between 4,000 meters and 5,000 

 meters. 



The vicious reasoning here presented consists in this, that an amount 

 of precipitation is introduced in the computation that cannot possibly 

 occur in nature. The air, by an ascent of 1,000 meters, will, indeed, in 

 consequence of vapor, cool by less than 10° C, but cool it will somewhat, 

 and consequently some precipitation must occur. We shall subsequently 

 see that in tbe supposed case, air saturated at 25° C, the cooling amounts 

 to 0°.4 for each hundred meters of ascent, and therefore for a thousand 

 meters amounts to 4°. This would give for each cubic meter a precipi- 

 tation of 4.66 grams. If the ascending current has the very moderate 



