408 



SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



quite near the earth's surface. Glaisber's observations, during numer- 

 ous balloon voyages, gave, for the free atmosphere, the following ratios 

 for the diminution of temperature with altitude (Report of the British 

 Association, 1864) : 



Diminution of Temperature i)er 100 meters in Celsius degrees. 



This gives for the summer season, up to au altitude of 5,000 English 

 feet (1,500 meters), an average diminution of temperature of Oo.GS C, 

 very nearly equal to that observed in the Alps. The observations 

 therefore show the mean condition of the atmosphere to be that of stable 

 equilibrium. The diminution of temperature with elevation is slower in 

 winter (in Switzerland it is 0.15) for two reasons : First, because then 

 the earth's surface exerts a cooling influence on the lower strata, and 

 often so intense that the temperature increases upward; to a less 

 extent this is also the case at night. Secondly, because the condensa- 

 tion of aqueous vapor takes place during this season most frequently 

 in the lower strata ; often, indeed, at the surface itself of the earth. But 

 of most importance is the first-named circumstance, for on clear days 

 and in clear winter months the diminution of temperature with the alti- 

 tude is slower than during cloudy weather or precipitation. The differ- 

 ence of temperature (8° C.) between Geneva and St. Bernard (difference 

 of altitude 2,070 meters) is in December less than it would be in a cur- 

 rent of moist air, saturated at 20° C, blowing from Geneva to St. Ber- 

 nard. The slight difference of temperature can therefore not be 

 explained by means of the heat of condensation ; it is in by far the 

 greater part to be ascribed to the greater cooling of the lower strata by 

 the earth's surface and to the underflow of the air cooled upon the 

 mountains. 



In summer, the diminution of temperature is more rai)id, because 

 then the heated surface of the earth acts more powerfully by conduction 

 and radiation upon the lowest strata of air. Since this stratum for the 

 same reason is at this time relatively drier, wherefore the first conden- 

 sation of vapor occurs at higher altitudes, the ascending air can show a 

 diminution of temperature nearly the same as for dry air. 



We ought, however, not to represent to ourselves the warming of the 

 atmosphere from below on calm, clear summer days as taking place by 

 means of a continuous ascending current. Above limited localities, 

 such a process certainly occurs, and the cumulus formations are a proof 

 thereof. But, in general, the warming proceeds gradually from below 

 upward, through the rising of warmer and sinking of cooler air and 

 their intermixture, and the work of one day is continued on the next 



