298 On the Physical Geography of the Alps. 



10. The depression of temperature with elevation is greater 

 in summer than in winter, amounting, for example, to 1° C. 

 for 440 feet in July, and for 710 feet in January. The cause 

 of this is, that in the lower parts of the mountains the dif- 

 ferences between single months are greater than in the 

 higher parts. 



11. The elevation of the point, near which, on a vertical 

 line, the depression of temperature is the greatest, is a dif- 

 ferent one in every month. It is the highest, when the Alps 

 are covered with snow, in December and January ; from 

 March to September this point is generally found near the 

 limit of snow ; in October and November it lies below the 

 snow line. 



12. The height of the snow line in the months does not 

 coincide always with the same isothermal. In January the 

 snow line and the isothermal of 0° are both nearly on the 

 base of the Alps ; but from this time to July and August, 

 the isothermal of zero moves quicker upwards than the 

 snow line, and from August to January quicker downwards. 

 The snow line, therefore, in the first period coincides with 

 isothermal s warmer than 0° C. ; in July it is even at x 5° 0. 

 The snow line, in the usual sense, that is to say, its highest 

 limits in summer, is, at the mean temperature of the year, 

 - 4° C. 



13. Over large masses of snow and glaciers there is re- 

 marked, particularly on fine days, a descending current of 

 air (glacier wind), which has a great influence on the general 

 depression of temperature near the limits of snow. 



14. The absolute extremes of cold on single days are at 

 the lower stations sometimes so great, that they are com- 

 paratively but little surpassed by those on the higher points. 

 But the differences between the higher and lower part are 

 much greater if we consider the maxima of heat. The abso- 

 lute maxima seem scarcely ever to exceed 5° or 6° 0. on the 

 highest summits of the Alps. On all days the decrease of 

 temperature is greater at the time of the maximum than at 

 the minimum. 



15. Compared therefore to the temperature of high lati- 

 tudes, the summits of the Alps correspond nearly to 70° N. 

 lat. But the climate of the highest elevations on the Alps 



