410 SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



The rapid rate of diminution of temperature during the rainy period, 

 which was also accompanied by strong winds, is a consequence of the 

 ascent of the air on the flanks of the mountains. In the free atmosphere, 

 the air at the same altitudes is certainly somewhat warmer and the tem- 

 perature diminution slower. I do not, however, think that in the sub- 

 sequent slower rate of diminution the radiation of heat from the surface 

 of the earth exerted any disturbing influence. Such radiation acts 

 upon the thermometer more at the lower than at the upper station, and 

 must therefore tend to increase the differences of temperature. It can, 

 however, not be denied that the true temperature of the air in the neigh- 

 borhood of a mountain is somewhat higher than at the same level in the 

 free atmosphere. But the observations of Glaisher up to 5,000 feet ele- 

 vation have given during fine weather an average rate of diminution of 

 only 0'^.67 C. per 100 meters. Even during the afternoons, the diminu- 

 tion of temperature with altitude does not reach the degree that cor- 

 responds to that of an ascending current of air. 



Bate of Diminution of Temperature at 1^ p. m. 



Thus it seems that the warming of the lower strata of the atmosphere 

 by the heated surface of the earth takes place mostly by the gradual 

 mixing of the lowest strata with the higher, whereby the ascending 

 columns of air with a slower movement gradually equalize their temper- 

 ature with that of the surrounding air without reaching those altitudes 

 to which a continually ascending current of air of equal temperature 

 could attain j the heating of the air by the radiation from the earth's 

 surface must also come into consideration. Thus there is formed a stra- 

 tum pretty uniformly heated up to a certain altitude above the earth's 

 surface within which the diminution of temperature takes place slowly, 

 but above which it probably takes place rapidly. So long as the dry 

 winds which bring clear weather blow strongly, the upward diminution 

 of temperature is rapid, because the particles of air warmed below must 

 attain their appropriate altitude; but as soon as the dry winds diminish, 

 and calms finally ensue, the above described overheated stratum of air 

 develops itself continually more and more, and with it first an indif- 

 ferent, then, finally, perhaps, an unstable equilibrium between this 

 and the higher strata. From the 20th to the 28th of July, 1872, the 

 warming from below had gradually extended above the level of the 

 highest station in Switzerland, and, therefore, before the outbreak of 

 the thunder storm no unstable equilibrium is demonstrable. But on the 

 2Sth, when the stronger west wind first prevailed high above Switzer- 



