Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 103

clouds. When the lapse rate exceeds the dry adiabatic rate of 3.05°C per 300 m (5.5°F
per 1,000 ft), a condition of absolute instability prevails. Under these conditions, even
a slight lifting of the air by a landform is enough to “trigger” it into continued lifting of
its own accord. If it then begins to feed upon itself through the release of latent heat
of condensation, it can yield considerable precipitation (Smith et al. 2009). As a result,
thunderstorms can develop, even on small hills in the path of moist unstable air (Schaaf
et al. 1988).
LOCAL CONVECTION
Clouds commonly form over mountains during the day, especially in summer, when
nights and early mornings are clear, but by midmorning clouds begin to build, often cul-
minating in thunderstorms with hail and heavy rain (Kirshbaum and Durran 2004). This
has been well documented for the base of the Colorado Rockies, where the higher peaks
of the Front Range create a “heated chimney effect” in the initiation of thunder and hail-
storms (Banta and Schaaf 1987). Mountains serve as elevated heat islands during the
day, since their surfaces can be warmed to a temperature similar to surrounding low-
lands. As a consequence, the air at a given altitude is much warmer over the mountains
than over the valleys. The lapse rate above the peaks, therefore, is considerably greater
than in the surrounding free air, resulting in actively rising air. Glider pilots have long
taken advantage of this (Ludlam, 1980). Airline pilots, on the other hand, make every
effort to avoid the turbulence associated with unstable air over mountains. Rarely, given
weak synoptic conditions, local mountain convection can become organized into a meso-
scale convective complex (Tucker and Crook 1999). These strong storms can reinforce
themselves, spawning severe thunderstorms and even tornadoes (“mountainadoes”).
Clouds and thunderstorms initiated in Colorado's Front Range frequently drift east-
ward, continuing to develop as they move onto the plains, and producing locally heavy
precipitation (Sato and Kimura 2003; Jeglum et al. 2010). A study in the San Francisco
Mountains north of Flagstaff, Arizona, suggests that clouds may increase in volume by
as much as 10 times after drifting away from a mountain source (Banta and Schaaf
1987). Most clouds observed in this area were small cumuli that eventually dissipated
once removed from their supply of moist, rising air, but a large cumulonimbus could
maintain itself independently of the mountains and result in storms at some distance
away. Fujita (1967) found a ring of low precipitation about 24 km (15 mi) in diameter
encircling these mountains, with an outer ring of heavier precipitation. During the day,
the rainfall is over the mountains, but at night it falls over the lowlands because the
mountains are relatively cold. A “wake effect” due to wave action created by airflow
over the mountains may be partly responsible for the inner ring of light precipitation
(Fujita 1967; Brady and Waldstreicher 2001; Epifanio and Rotunno 2005).
The height of the cloud base is very important to the development of convection in
mountains since, once the sun is blocked, the positive effect of solar heating is elimin-
ated. The height of the cloud base is also critical to the distribution of precipitation. If
the cloud base is below the level of the peaks, as it usually is in winter, when forced
ascent occurs, cloud growth and precipitation will take place mainly on the windward
side. In summer, however, the base of convection clouds is generally much higher.