Geography Reference
In-Depth Information
The extensive variety of surface characteristics of mountains—snow, ice, water, pas-
tures, extensive forests, shrubs, soils, bare bedrock—affects the absorption of incoming
solar radiation (Pomeroy et al. 2006). The effects of two factors—groundcover and to-
pographic setting—illustrate this. Dark-colored features, including vegetation, absorb
rather than reflect radiation, receiving increased amounts of energy. Snowfields, glaci-
ers, and light-colored rocks have a high reflectivity (albedo), so that much of the incom-
ing short-wave energy is reflected back into the atmosphere. If the snow is in a valley
or on a concave slope, reflected energy may bounce from slope to slope, increasing the
energy budget of the upper slopes. The opposite occurs on a mountain ridge or con-
vex slope, where the energy is reflected back into space. Consequently, valleys and de-
pressions are areas of heat build-up and generally experience greater temperature ex-
tremes than do ridges and convex slopes. Reflected energy can be an important source
of heat for trees in the high mountains (Pomeroy et al. 2006). Snow typically melts faster
around trees because the increased heat is transferred, as long-wave thermal energy,
to the adjacent surface. On a larger scale, the presence of forests adds significantly to
the heat budget of snow-covered areas. The shortwave energy from the sun can pass
through a coniferous forest canopy, but very little of it escapes again. The absorbed en-
ergy heats the tree foliage and produces higher temperatures than in open areas. This
results in rapid melting rates in the snowpack (Pomeroy et al. 2006).
Variation in the components of the surface energy budget provides the main driving
force for regional differences in climate. In particular, the relative magnitude of sensible
and latent heat fluxes reflects the influence of prevailing weather systems, and plays
an important role in determining atmospheric temperature and moisture content
(McCutchan and Fox 1986; Kelliher et al. 1996). These factors in turn influence the
development of local wind systems. Surface energy budgets can vary significantly in
mountains due to the effects of both complex topography and surface characteristics.
When snow or ice is present, energy must first be partitioned to ablation before temper-
atures rise and, once the snow melts, there are large changes in albedo (Cline 1997).
These variations affect the distribution of incoming and outgoing radiation, influencing
net radiation, soil heat flux, and sensible and latent heat, producing a range of topo- and
microclimates (Germino and Smith 2000).
Temperature
The decrease of temperature with elevation is one of the most striking and fundamental
features of mountain climate. Those of us who are fortunate enough to live near moun-
tains are constantly reminded of this, either by spending time in the mountains or by
viewing the snowcapped peaks from a distance. Nevertheless, many characteristics of
the nature of temperature in mountains are subtle and poorly understood. Alexander
von Humboldt was so struck by the effect of temperature on the elevational zonation of
climate and vegetation in the tropics that he proposed the terms tierra caliente, tierra
templada, and tierra fria for the hot, temperate, and cold zones, respectively. These
terms, commonplace in the tropics today, are still valid there. Their extension to higher
latitudes by others, however, under the mistaken assumption that the same basic kinds
of temperature conditions occur in belts from the equator to the poles, has been unfor-
tunate, though some textbooks still use this simplistic approach.
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