DAMAGE CAUSED BY ATMOSPHERIC PRECIPITATIONS. 



27 



17. Factors determining the Occurrence and Extent of the Danger. 



The nature and extent of the damage done by snow, and the 

 frequency or infrequency of its repetition, are .due to many 

 circumstances and influences. 



Soil and Situation are perhaps of first importance in this respect, 

 as damage is most frequent in high ranges of hills and among 

 the outlying hills of high mountain chains. On plains and in low- 

 lying tracts snow seldom falls in vast quantities ; whilst at higher 

 elevations in mountainous localities, the flakes are mostly dry; 

 small, and therefore comparatively uninjurious. Timber crops that 

 have shot up quickly with clean stems, are naturally much more 

 exposed to danger from snow than the shorter-boled crops on less 

 productive soil. 



But species of tree, age of crop, and density of canopy are also of 

 influence in the matter, as snow-break occurs when the burden of 

 snow on the crowns exceeds the carrying power of the stem, 

 whilst the pressure is determined by the quantity of falling snow 

 which settles on the crown, and by the weight of the snow, which 

 is more than twice as great in the wet state than when dry. The 

 formation of the branches and the nature of the foliage determine 

 the quantity of snow accumulating on the individual tree, whilst 

 the density of canopy determines it for the whole crop. It is 

 therefore clear that the evergreen conifers, Silver Fir, Spruce, and 

 Pine, must be more exposed to danger from snow than broad- 

 leaved deciduous species and Larches that are defoliated in winter, 

 and hence offer a much smaller resting-place for the falling snow, 1 

 although broad-leaved species are by no means exempt from 



1 Interesting measurements made by Biihlers in Switzerland, after heavy snowfall, 

 I gave the following results : 



