CLIMATIC FACTORS 



87 



hundreds of thousands of short twigs, so crowded together that one can 

 hardly push an alpenstock into the tangle. The temperature of 

 these Sechss is much below that of suri'ounding areas. In June, and 

 again in August, there are many frosty days with minima from — 1° to 

 ~6°C. Pillichody (1921) published temperatures for three years 

 taken in the frost pocket of La Joux near Les Fonts (1,100 m.). The 

 mean minimum of the week Feb. 20 to 27, 1901, was — 29°C. in the 

 frost pocket; in the high forest just south, only — 14°C. Even in 

 the second half of May a mean minimum of — 5.1°C. was found; and 

 from June 19 to 25, one of —2°. In the same week of June the mean 

 minimum in the bordering high forest was 4.6°C. 



Fig. 46. — The January isotherm of 4.5°C. and the distribution of Ruhia peregrina. 



{After Salisbury.) 



Temperature Inversions. — Another phenomenon due to stagnation 

 of cold air in valleys is the temperature inversions on mountains in 

 midwinter. Very striking is the difference between the truly Siberian 

 winter temperatures of the forested valleys of the Oberengadine and 

 the surrounding naked passes and mountain ridges. In the winter of 

 1905 Saint Moritzbad (1,780 m.) had minima of -36° and -38°C. 

 More than 500 m. higher, at Bernina (2,310 m.), the extreme minimum 

 was — 24°C. Hence it follows that the upper limit of the forests of 

 Picea, Larix, and Pinus cembra in the Alps is certainly not due to low 

 winter minima. Besides, in northern Siberia larch forest {Larix 

 sibirica) withstands temperatures of — 70°C. 



Temperature inversions in the Santa Catalina mountains of Arizona 

 have been shown by Shreve (1912) to be due to cold-air drainage. 



