Page:Popular Science Monthly Volume 90.djvu/43

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Popular Science Monthly

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����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.

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