48 



PRINCIPLES OF STRATIGRAPHY 



is a calm on their southern side, with little if any increase in tem- 

 perature, and with high humidity. In most cases also, a few hours 

 after the foehn has begun to blow, rain and snow begin to fall on 

 the southern slopes and the summits of the Alps, the precipitation 

 being often extraordinarily heavy ; exceptional precipitation on the 

 south often coinciding with extreme violence of the foehn on the 

 north. The following table, copied from Hann, shows the average 

 weather conditions which prevailed simultaneously at J\Iilan, on the 

 southern side, at Bludenz, Austria, in a tributary alpine valley of the 

 Rhine, and at Stuttgart in the northern plains, during twenty winter 

 days on which there were foehn winds. (40:550.) 



The high temperature of the wind and its dryness do not exist 

 on the alpine summits, but are acquired. during the descent on the 

 northern side. They are not imported from the region to the south 

 of the Alps. This is shown by the observations on the weather 

 along the St. Gotthard Pass during the foehn of January 31-Feb- 

 ruary i, 1869, as shown in the following table. ( H ann-40 :j50.) 



