Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 99

The increase of precipitation with elevation is well known from every country, even if
the landforms involved are only small hills. In many regions, an isohyetal map with its
lines of equal precipitation will look similar to a topographic map composed of lines of
equal elevation (Fig. 3.17). Of course, the data on which most precipitation maps are
based are scanty, so that considerable interpolation may be necessary, particularly in
areas of higher relief (Kyriakidis et al. 2001; Scherrer et al. 2010). Precipitation does
not always correspond to landforms. In some cases, maximum precipitation may occur
at the foot or in advance of the mountain slopes (Barry 2008). In some regions and un-
der certain conditions such as topographic funneling, valleys may receive more rain-
fall than the nearby mountains (Sinclair et al. 1997; Mass 2008). In many higher alpine
areas, precipitation decreases above a certain elevation, with the peaks receiving less
than the lower slopes. Wind direction, temperature, moisture content, storm and cloud
type, depth of the air mass and its relative stability, orientation and aspect, and con-
figuration of the landforms are all contributing factors in determining the location and
amount of precipitation (Ferretti et al. 2000; McGinnis 2000; Drogue et al. 2002; Lin
et al. 2001; Houze and Medina 2005; Roe and Baker 2006). The complex topographic
arrangement and often high relief of mountains create complex meso- and microscale
three-dimensional circulation and cloud formations, leading to complex spatial patterns
of precipitation (Garreaud 1999; Germann and Joss 2001, 2002; Roe and Baker 2006).
Great variations in precipitation occur within short distances; one slope may be excess-
ively wet while another is relatively dry. The terms “wet hole” and “dry hole” may be
used in this regard. For instance, while the Grand Tetons of Wyoming receive 1,400 mm
(55 in.), Jackson Hole, in a protected site at their base, only 16 km (10 mi) away, re-
ceives 380 mm (15 in.).
The most fundamental reason for increased precipitation with elevation is that land-
forms obstruct the movement of air and force it to rise, due to the orographic effect.
Forced ascent of air is most effective when mountains are oriented perpendicular to the
prevailing winds; the steeper and more exposed the slope, the more rapidly air will be
forced to rise. As air is lifted over the mountains, it is cooled by expansion and mixing
with cooler air at higher elevations (i.e., adiabatic processes). The ability of air to hold
moisture depends primarily upon its temperature: Warm air can hold much more mois-
ture than cold air. The temperature, the pressure, and the presence of hygroscopic nuc-
lei in the atmosphere tend to concentrate the water vapor in its lower reaches. This is
why most clouds occur below 9,000 m (30,000 ft), and why those that do develop higher
than this are usually thin and composed of ice particles, yielding little or no precipita-
tion.
When the air holds as much moisture as it can (i.e., relative humidity is 100 percent),
it is considered saturated. The temperature at which condensation takes place is called
the dew point. Ground forms of condensation (i.e., fog, frost, and dew) are caused by
cooling of the air in contact with the ground surface, but condensation in the free atmo-
sphere (i.e., clouds) only results from rising air. This may occur in one of several ways.
Convection (thermal heating) takes place where the sun warms the Earth's surface and
warm air rises until clouds begin to form. Such clouds may grow to great size since
they are fed from below by relatively warm, moist rising air. Convectional rainfall is best
displayed in the humid tropics, where water vapor is abundant, but it occurs in all cli-
mates. Air may also be forced to rise by the passage of cyclonic storms, where warm