Geography Reference
In-Depth Information
a water resource, unless it is torrential enough to fill stream channels (often dry in the
summer) with flowing water that can be collected in reservoirs. Another interesting ef-
fect on the rate and timing of snowmelt is the possible addition of airborne dust carried
in the winds, often from arid-land sources hundreds or thousands of kilometers away.
Studies conducted in the San Juan Mountains of Colorado indicate that a series of dust
deposition events (usually occurring during the late winter or spring) can decrease the
snow-pack albedo there and thus speed up the melt season by one full month (Painter
et al. 2007).
“Permanent” Snow and the Snowline
Many areas of the globe are covered by snow and ice year round. Latitude plays a dom-
inant role in the distribution of permanent snow and ice. However, high-altitude moun-
tains can completely overpower the effect of latitude and provide a permanent abode
for snow and ice even at the equator. The zone between seasonal snow that melts every
summer and the permanent snow that does not melt is represented by the
snowline.
This zone has fundamental implications for environment and process. The varying dis-
position of the snowline in time and space has resulted in different interpretations of its
significance, and has caused considerable confusion in the literature, with terms such
as
climatic snowline, annual snowline, orographic snowline, temporary snowline, tran-
sient snowline,
and
regional snowline
(Charlesworth 1957; Flint 1971; Østrem 1964a,
1973, 1974; Seltzer 1994). Use of the term “snowline” without an accompanying explicit
definition is fairly meaningless. To appreciate the problem, consider the following con-
ditions: At one extreme is the delineation between a snow-covered and a snow-free area
at any time of the year. Obviously, this snowline varies from day to day and will be low-
est in the winter, reaching sea level in middle latitudes, and highest in summer. There
is also a snowline establishing the lower limits of persistent snow in winter, a matter of
great importance for the location of ski resorts and road maintenance (Elsasser 2001).
Our primary concern, however, is the location of the snowline after maximum melting
in summer, since this is the level that establishes the glacial zone and largely limits the
distribution of most plants and animals. The position of this line is likewise highly vari-
able and difficult to delineate. For example, avalanches may transport large masses of
snow to valley bottoms where, if shaded, they may persist for several years. Similarly,
some mountain glaciers occupy sheltered topographic sites and receive greater accu-
mulations from drifting snow and avalanches than do the surrounding slopes. Glaciers
also can experience less melting because of their “shadow climate” and the natural cool-
ing effect of the larger ice mass. As a result, the snowline is generally lower on glaciers
than in the areas between them. In mountains without glaciers, or on slopes between
glaciers, the snowline is commonly represented by small patches of perennial snow,
where distribution is largely controlled by slope orientation and local topographic sites
(Alford 1980; Brozovic et al. 1997; Mitchell and Montgomery 2005).
The disparity among the various snow limits, and the difficulty of establishing their
exact locations, has led to the use of several indirect methods of approximation. One
is to use the elevation where the average temperatures are 0°C (32°F) or less during
the warmest month of the year. Since this is determined primarily through the use of
radiosondes and weather balloons, a snowline can be established even where there are
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