Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 90

no wind to mix and equalize the temperatures, and the transparent sky allows the sur-
face heat to be rapidly radiated and lost to space (Rucker et al. 2008; Daly et al. 2010).
Consequently, the surface becomes colder than the air above it, and the air next to the
ground flows downslope. The cold air will continue to collect in the valley until equi-
librium between the temperatures of the slopes and the valleys has been established.
The speed of the winds is controlled by valley width, increasing through narrows (i.e.,
Bernoulli's Theorem), as well as by differential heating along the valley (Rucker et al.
2008). If the valley is enclosed, a pool of relatively stagnant colder air may collect, but,
if the valley is open, there may be continuous movement of air to lower levels, leading
to air pollution (Gohm et al. 2009). The depth of the inversion depends on the charac-
teristics of the local topography, airshed size, mixing with air aloft, and general weather
conditions, but is generally not more than 300-600 m (1,000-2,000 ft) (Whiteman et al.
2001; Smith et al. 2010).
Figure 3.11 demonstrates a temperature inversion in Gstettneralm, a small enclosed
basin at an elevation of 1,270 m (4,165 ft) in the Austrian Alps, about 100 km (62 mi)
southwest of Vienna. Because of the local topographic situation and the “pooling” of
cold air, this valley experiences some of the lowest temperatures in Europe, even lower
than the high peaks (Schmidt 1934). The lowest temperature recorded there is −51°C
(−59.8°F), while the lowest temperature recorded at Sonnblick at 3,100 m (10,170 ft) is
−32.6°C (−26.7°F).
As might be expected, distinct vegetation patterns are associated with these extreme
temperatures. Normally, valley bottoms are forested and trees become stunted on high-
er slopes, being replaced by shrubs and grasses still higher up, but the exact oppos-
ite occurs here. The valley floor is covered with grass, shrubs, and stunted trees, while
the larger trees occur higher up. An inversion of vegetation matches that of temper-
ature (Schmidt 1934). A similar vegetative pattern has been found in the arid moun-
tains of Nevada, where valley bottoms support sagebrush, while higher up is a zone
of pinyon and juniper woodland. Higher still, the trees again disappear (Bradley and
Fleishman 2008). The pinyon/juniper zone, the thermal belt, is sandwiched between the
lower night temperatures of the valley bottom and those occurring higher up.
Human populations have taken advantage of thermal belts for centuries, particularly
to cultivate frost-susceptible crops such as vineyards and orchards. In the southern Ap-
palachians of North Carolina, the effect of temperature inversions is clearly displayed
by the distribution of the fruit orchards (Dunbar 1966). During winter, the valleys are
often brown with dormant vegetation, while the mountain tops at 1,350 m (4,430 ft) may
be white with snow. In between is a strip of green that marks the thermal belt. Frost is
common in the valley but, in the thermal belt, the sensitive Isabella grape has appar-
ently grown for years without danger from frost (Peattie 1936). A similar situation exists
in the Hood River Valley of Oregon, on the north side of Mount Hood, where cherries
are grown in a sharply delimited thermal belt between the river and the upper slopes.
TEMPERATURE RANGE
The temperature difference between day and night and between winter and summer
generally decreases with elevation (Fig. 3.12). This is because of the relatively greater
distance from the heat source: the broad level of the Earth's surface. Temperature in