68 PERIODIC PHENOMENA OF VEGETABLE LIFE. 



come on in the beginning of October; any strikingly early 

 appearances of them cause a considerable deficiency in the 

 harvest. * 



We have elsewhere noticed that at the height of 4500 feet, 

 taking the mean of several years, no month is without snow, and 

 that from this height upwards, the number of days of snow rapidly 

 increases, without entirely excluding falls of water even at great 

 heights. In extreme years there are still considerable tracts of 

 snow at between 4000 and 5000 feet in the beginning of May, 

 which, however, then rapidly disappear. In September, and 

 especially in October, falls of snow become frequent at the 

 further limits of Cereals ; falls of snow are not unfrequeut in all 

 months. In the table of heights, the 15th June is given as the 

 mean of ordinary years at an elevation of 5000 to 6000 feet. 

 The flowering of Cereals often suffers considerably by such falls, 

 and it not unfrequently happens that they are entirely covered 

 with snow for a short time not long before their ripening. 



The commencement of winter is usually accompanied by the 

 heavier falls of snow ; sometimes, however, the latter take place 

 somewhat after the sinking of the temperature. 



The number of days of snow in comparison with that of rainy 

 days iuci*eases with the altitude, so also the length of time during 

 which the ground is covered with snow. The latter, however, 

 inci'eases with the altitude more than the former ; an increase 

 of one day in the fall of snow corresponds with an increase of 

 several days in its duration on the ground. Thurmannf found 

 that, in the Jura, an increase of three days in the fall of snow 

 entailed an increase of ten days in its duration on the ground. 

 The figures given on this subject show some interesting results 

 as to their relations in elevated regions. They indicate also 

 numerous variations according to the quantity of snow fallen on 

 each snowy day, \ according to the degree of heat in summer, or 

 to the daily maxima, according to the radiation on isolated 

 summits, &c. 



The duration of vegetation, that is, the period intervening 

 between the reappearance of vegetation in spring, and the com- 



* As an instance of an early frost, very injurious to Maize at an elevation 

 of 1700 feet, we may state on the authority of private letters, that a very 

 severe one took place at Innspruck in the nights of the 14-15th September, 

 1850, after a continued east wind. 



t Essai de Phijtostatique, v. i., p. 67. 



j The quantity of snow which falls in a day diminishes at considerable 

 elevations. 



