Distribution of Temperature in the Alps. 297 



the central parts at nearly 7000 ; for the group of Mont 

 Blanc it seems to be at a still greater elevation. 



5. The height corresponding to a depression of tempera- 

 ture of 1° C. is in the mean 540 F. feet = 90 toises = 166 

 metres, if we compare the lowest stations of continued obser- 

 vation with the highest stations ; but if we consider the tem- 

 perature of the highest summits, the depression becomes a 

 little greater (510 feet in the central Alps). 



6. The mean temperature of the air seems to be, for the 

 highest summits from —. 13° to — 15° C. 



7. At the greater elevations the temperature of single 

 months is generally altered in this manner : — -the tempera- 

 tures of February and January, of August and July, differ less 

 from each other than they do at lower stations. 



8. The influence of the general form of the surface on the 

 temperature is particularly evident when we consider the 

 mean temperature of the months. The valleys during the 

 winter are in general colder than the mountains, the cold air 

 sinking down, and being accumulated in them ; during the 

 summer they are comparatively warmer, the heat being re- 

 flected and radiated by the insulated masses near them, and 

 circulation produced, especially in the horizontal direction ; 

 their climate is therefore subject to greater extremes, though 

 in the annual mean it scarcely difi^ers from the Alps in ge- 

 neral. The declivities during the winter are comparatively 

 warmer^ since the air near the surface, after sinking down in 

 the valleys, is replaced readily by less cold strata. During 

 the summer, particularly in southern exposures, and if the 

 relative height above the bottom of the valley is not great, 

 they are also a little warmer, since then they can be partially 

 reached by the ascending current of air. But this increase 

 of temperature being smaller than that of winter, these situ- 

 ations have a more constant climate than the valleys. The 

 mean temperature of the year on declivities, particularly 

 with southern exposures, is therefore a little higher than the 

 mean of the Alps in general. 



9. Summits and declivities, with exposure to north and to 

 north-east, show also the character of a constant climate ; but 

 the temperature of summer is much lower, and consequently 

 the annual mean is also sensibly depressed. 



