Popular Science Monthly 



27 



^^^^fi^m 



Rime deposit on a wire fence. The moistvire 



was deposited from a fog driven by the 

 wind against the wires. Rime is a common 

 phenomenon in mountainous districts 



A tree laden with hoarfrost. The deposit is 

 so heavy that it resembles snow. Hoarfrost 

 crystals form on cold surfaces in still air in 

 an almost endless variety of beautiful shapes 



not only snowflakes, but also various forms 

 of frost deposit upon terrestrial objects. 

 Hoarfrost is often described as "frozen 

 dew," but it consists less often of frozen 

 dewdrops than of ice crystals deposited 

 directly from the air. It tends to form on 

 horizontal surfaces, cooling rapidly at night 

 by radiation. It may even form on a sheet 

 of ordinary ice, such as the frozen surface 

 of a pond or lake. Beautiful flower-like 

 deposits are produced in this manner. 



A cold fog will sometimes leave copious 

 deposits of feathery ice crystals on the 

 edges and angles of objects against which 

 the fog is driven by the wind. These often 

 grow to several inches in length, and are 

 best obsersed on mountains and in the 

 polar regions. Such deposits are now 

 technically called "rime." 



The frostwork on windowpanes is some- 

 times true frost, produced on dry glass, 

 from water vapor in the air; but the beauti- 

 ful fern-like and feather-like forms are 

 produced in a thin film of water. Many 



varieties of window-frost and window-ice 

 have been photographed. When the indoor 

 air is very moist, a dew-like deposit of 

 moisture, consisting of minute drops, forms 

 on the glass, and this produces a layer of 

 granular ice, differing from frost cr>'stals. 



In spite of their immense variety in 

 detail, all perfect ice crystals have six 

 sides or principal rays. When secondary 

 rays form, they are parallel with the adja- 

 cent primary rays. There are two principal 

 forms of ice cr>'stal; viz., the tabular and 

 the columnar. Sometimes the two forms 

 are combined ; a column or rod, of hexagonal 

 section, will have at one or both of its ends 

 an hexagonal plate. 



Both the size and the shape of snow 

 crystals depend to some extent upon the 

 temperature of the air. The smallest 

 crystals form in the coldest weather. Star- 

 shaped cr>'stals are most abundant when 

 the temperature is not far below the freez- 

 ing point, while at lower temperatures there 

 is a preponderance of hexagonal plates. 



