Geography Reference
In-Depth Information
CHAPTER FOUR
Snow, Ice, Avalanches, and Glaciers
LELAND R. DEXTER, KARL W. BIRKELAND, and
LARRY W. PRICE
The presence of frozen water in several forms is fundamental at high altitudes and
provides the essential ingredient for the development of avalanches and glaciers. These
interrelated phenomena, which contribute much to the distinctiveness of high mountain
landscapes, offer a considerable challenge to the inhabitants, both plant and animal, of
these regions.
Snow and Ice
Snowfall and New Snow
Snow is precipitation in the solid form that originates from the freezing of water in the
atmosphere. This leads to one of the great mysteries of nature: Why should snow fall in
the form of delicate and varying lacy crystals rather than as frozen raindrops? The com-
monly held assumption that water must freeze at 0°C (32°F) is incorrect. The freezing
temperature can range as low as −40°C (−40°F), which, coincidentally, is the crossov-
er point of the two temperature scales. Water that remains liquid when cooled below
0°C is referred to as supercooled water. The actual freezing point of water in the atmo-
sphere depends not only on ambient temperature but also on water droplet size, droplet
purity, and mechanical agitation. Smaller droplets are more resistant to freezing. Very
small droplets may resist freezing to the −40°C value mentioned above. Dissolved salts
will retard freezing, but certain particulates will enhance freezing (i.e., promote freezing
at temperatures closer to 0°C) (Knight 1967; Hobbs 1974; Pruppacher and Klett 1987;
Tabazedeh et al. 2002).
Clouds form most readily when certain contaminants are present in the atmosphere.
These contaminants can be divided into two classes, depending on their ability to pro-
mote either condensation or freezing. Condensation nuclei are hygroscopic materials
that attract water, such as salt and smoke. Freezing (more properly called deposition )
nuclei generally are particles that mimic the hexagonal crystal structure of ice, although
dry ice (frozen CO 2 ) is also an effective freezing nucleator based on its low temperature.
Effective freezing nuclei include clays, certain bacteria, and silver iodide. In nature,
most clouds contain a mixture of water droplets formed around condensation nuclei and
small ice crystals formed around freezing nuclei. At typical cloud temperatures of −10°C
(14°F), the freezing nuclei are effective in overcoming the activation energy and hence
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