Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 164

The regional snowline is lowest in the polar regions, where it may occur at sea
level, and highest in the subtropics, where it occurs between 5,000 and 6,500 m
(16,500-21,500 ft). This is not a straight-line relationship, of course, owing to the inter-
play of temperature and precipitation. The highest snowlines are found between 6,000
and 6,500 m (19,800-21,500 ft) in the arid Puna de Atacama of the Andes (25°S latitude)
and the Tibetan Highlands (32°N latitude). The greater precipitation and cloudiness ex-
perienced in the tropics depress the snowline, while areas under the influence of the
subtropical high at 20-30°N and S latitude receive less precipitation and fewer clouds,
resulting in a higher snowline even though temperatures are lower (Fig. 4.12). At any
given latitude, the snowline is generally lowest in areas of heavy precipitation (e.g.,
coastal mountains) and highest in areas of low precipitation (e.g., continental moun-
tains). Accordingly, there is a tendency for snowlines to rise in elevation toward the west
in the tropics and toward the east in middle latitudes, in accordance with the prevailing
winds. The middle-latitude situation is illustrated by the snowline in the western United
States, which rises from 1,800 m (6,000 ft) in the Olympic Mountains, Washington, at
48°N latitude, to 3,000 m (10,000 ft) in Glacier National Park, Montana, in the Rockies,
800 km (480 mi) to the east (Flint 1971: 66). A similar tendency for the snowline to rise
from west to east is found in the mountains of Scandinavia, the Andes of southern Chile,
and the Southern Alps of New Zealand (Østrem 1964a; Porter 1975).
Other Occurrences of Frozen Water in Mountains
RIME ICE OR HOARFROST
Rime ice, sometimes called hoarfrost, forms by contact freezing of supercooled water
droplets and direct deposition of water vapor onto various nucleating objects in the sur-
rounding environment (Graham 2009). These rime icing events are most often accom-
panied by high-velocity winds. Nucleating objects can be natural (trees, rocks, falling
snowflakes, an old snow surface, or even entire mountain peaks) or human-made (air-
craft wings, buildings, ski lift towers, communications towers, fence posts) (Fig. 4.13).
Rime loading on human structures can become so great that a structure may collapse.
Rime ice often takes on a blade-like form ( rime feathers ) that builds outward from
the collecting object into the oncoming wind. Surprisingly, rime may provide the major-
ity of winter water accumulation in some areas. Polar mountains, for example, receive
so little direct precipitation that the contribution of rime and hoarfrost is often greater
than that of snow. In some very rare instances, rime accumulations have been shown
to release abruptly from their anchorage and “avalanche” in curious rime flow events
(Kokenakais and Dexter 1998).