Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 101

side of mountains because of spillover effects (Sinclair et al. 1997). Many precipitation
maps of mountainous regions do not indicate this internal variability because of the lack
of weather stations and interpolation between existing station data using generalized
elevation-precipitation relationships (Kyriakidis et al. 2001; Minder et al. 2010; Scher-
rer et al. 2010). Additionally, seasonal and interannual variability of storm tracks and
storm intensities can create non-elevational precipitation patterns.
The movement of air up a mountain slope, creating clouds and precipitation, may be
due simply to the wind, but is usually associated with convection and frontal activity.
Rising air cools at a rate of 3.05°C (5.5°F) per 300 m (1,000 ft) (dry adiabatic rate) until
the dew point is reached and condensation occurs (Fig. 3.28). Thereafter, the air cools
at a slightly lower rate (wet adiabatic rate) because of the release of the latent heat
of condensation. If, upon being lifted, the air has a high relative humidity, it may take
only slight cooling to reach saturation but, if it has a low relative humidity, it may be
lifted considerable distances without reaching the dew point. Conversely, if the air is
warm, it often takes considerable cooling to reach dew point but then may yield copious
amounts of rainfall; cool air usually needs only slight cooling to reach dew point but also
yields far less precipitation. After the air has passed over the mountains, precipitation
decreases or may cease as the air descends. Descending air gains heat at the same rate
at which it was cooled initially: 3.05°C per 300 m (5.5°F per 1,000 ft), since it is being
compressed and is moving into warmer air (Fig. 3.28). Such conditions are not condu-
cive to precipitation.
In relation to precipitation, the orographic effect involves several distinct processes:
(1) forced ascent, (2) blocking (or retardation) of storms, (3) the triggering effect, (4)
local convection, (5) condensation and precipitation processes, and (6) runoff.
FORCED ASCENT
Forced ascent is the most important precipitation process in mountains. Steeper slopes
produce more precipitation than gentle slopes (Lin et al. 2001). The process may be
most clearly seen in coastal mountains, like the Olympics, that lie athwart moisture-
laden winds. Other processes contribute to the total precipitation, and differentiation
among them is difficult. To explain the amount and distribution of rainfall caused strictly
by forced ascent, it is necessary to consider the atmospheric conditions from three dif-
ferent perspectives.
The first perspective involves the large-scale synoptic pattern that determines the
characteristics of the air mass crossing the mountains: its depth, stability, moisture con-
tent, wind speed, and direction (McGinnis 2000). Second is the microphysics of the
clouds, the presence of hydroscopic nuclei, the size of the water droplets, and their tem-
perature, which determine whether the precipitation will fall as rain or as snow or evap-
orate before reaching the ground (Uddstrom et al. 2001). Third, and most important, is
the motion of the air with respect to the mountain (Tucker and Crook 1999). Will it blow
over the mountain, or around it? This determines to what depth and extent the air mass
at each level is lifted. It is not realistic, for example, to assume that the air will be lif-
ted the same amount at all levels. The solution to these problems involves atmospheric
physics and the construction of dynamic models (Drogue et al. 2002).