22 



PROTECTION OF WOODLANDS. 



CHAPTEE II. 



DAMAGE CAUSED BY ATMOSPHERIC PRECIPITATIONS. 



14. Different kinds of Precipitations. 



The precipitation of the aqueous vapour contained in the atm< 

 phere can take place in many ways, as is well known. It may 

 deposited in a watery form as dew, always beneficial to vegetation, 

 or, when condensed in the upper aerial layers, as rain, which fall 

 with more or less violence. When the temperature of the plant 

 sinks by radiation of warmth below the freezing point, rime is 

 formed ; and when the cooling down of the upper aerial layers 

 equally great, snow falls in place of rain, the flakes being smal 

 and dry if the cold is great, but large and moist if the teni] 

 ature be only slightly below the freezing point. When the sno> 

 hanging to the trees freezes after having previously thawed, 01 

 after having fallen as moist snow, an ice-accumidation is forme 

 as also occurs when rain suddenly falls after previous severe cold 

 whilst rain-drops that have become greatly cooled in falling 

 through the lower-lying cold layers of air crystallise into ice 

 soon as they come in contact with cold bodies, and cover branch( 

 twigs, dry foliage, &c., with sometimes quite a thick layer 01 

 smooth sheet-ice or frozen sleet. Hoar-frost is formed when the 

 aqueous vapour (mist) of the atmosphere is deposited in the shape 

 of ice-crystals on bodies cooled below the freezing point, as often 

 takes place to a large extent on branches and foliage ; and, finally, 

 the well-known injurious phenomenon of hail is caused by sudden 

 depressions of temperature in the atmosphere, combined with 

 electrical disturbances. 



The influence of these phenomena on woodland growth is 

 manifold, and under certain circumstances very injurious. 



