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THE STUDY OF PLANT COMMUNITIES ■ Chapter V 



Other forms of precipitation include snow, which is formed 

 like rain but at temperatures below freezing and under conditions 

 that permit the crystals to fall before they melt. Sleet is rain that 

 falls through air strata of low temperature and then reaches the 

 earth as clear pellets of ice. If rain falls on a cold surface and 

 freezes, it is called glaze. Hail, which falls almost exclusively in 



FlG. 37. Northern hardwood stand of birch, hard maple, elm, and ash 

 after a glaze storm in New York. Scarcely a tree escaped damage.— U. S. For- 

 est Service. 



summer because of its dependence on convectional storms, starts 

 with a snow or ice nucleus, which falls to a stratum of sufficiently 

 high temperature to be partially melted. When carried upward 

 again, the moisture on the surface freezes, and condensation adds 

 to the size. If the process of falling and being carried up again 

 is repeated several times, the successive thawing, freezing, and con- 

 densation will form a concentrically layered mass of snow and ice 

 of sufficient size to fall to the earth as a hailstone. 



Since hail is primarily a summer phenomenon occurring only 

 under exceptional conditions, it is of little consequence to plants 

 as a source of water. It may, however, do serious phvsical damage, 

 often stripping foliage completely from woody plants and damag- 

 ing herbaceous structures beyond recovery. Sleet and glaze are in 



