Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 78

an average of 3,800 mm (150 in.) is received annually as storms are funneled up valleys
oriented toward winter storm tracks (Fig. 3.4; Mass 2008).
FORCED DESCENT
Atmospheric-pressure conditions determine whether the air, after passing over a moun-
tain barrier, maintains its altitude or is forced to descend; it is then heated by com-
pression (adiabatic heating), resulting in clear, dry conditions. This characteristic phe-
nomenon in the lee of mountains is responsible for the famous foehn or Chinook winds.
The important point is that the descent of the air is induced by the barrier effect, result-
ing in clear, dry conditions that allow sunshine to reach the ground with much greater
intensity and frequency than it otherwise would. This can produce “climatic oases” in
the lee of mountain ranges, as in the Po Valley of Italy (Hoggarth et al. 2006).
Although heavy precipitation may occur on the windward side of mountains where
the air is forced to rise, the leeward side may receive considerably less precipitation
because the air is no longer being lifted (it is descending) and much of the moisture
has already been removed. This rainshadow effect creates an arid area on the leeward
or downwind side of mountains. In the lee of Mount Waialeale, Kaua'i, precipitation de-
creases at the rate of 3,000 mm (118 in.) per 1.6 km (1 mi) along a 4-km (2.5 mi) tran-
sect to Hanalei Tunnel (Blumenstock and Price 1967). In the Olympic Mountains, pre-
cipitation decreases from totals approaching 6,350 mm (250 in.) on the windward side
to less than 430 mm (17 in.) at the town of Sequim on the leeward, a distance of only 48
km (30 mi) (Fig. 3.4; Mass 2008). Since both of these leeward areas are maritime, they
are still quite cloudy; under more continental conditions, there would be a correspond-
ing increase in sunshine as precipitation decreases, especially where the air is forced to
descend on the leeward side.
Major Climatic Elements
The discussion so far has covered the more or less independent climatic controls of lat-
itude, altitude, continentality, and the barrier effect of mountains. These factors, along
with ocean currents, pressure conditions, and prevailing winds, control the distribu-
tion of sunshine, temperature, humidity, precipitation, and local winds. The climatic ele-
ments of sunshine, temperature, and precipitation are essentially dependent variables
reflecting the major climatic controls (Barry 2008). They interact in complex ways to
produce the day-to-day weather conditions experienced in different regions. In moun-
tains, these processes frequently occur on scales small enough to be invisible to stand-
ard measurement networks used in weather forecasting, though their impact can be
serious.
Solar Radiation
The effect of the sun becomes more exaggerated and distinct with elevation. The time
lag, in terms of energy flow, between stimulus and reaction is greatly compressed in
mountains. Looking at the effect of the sun in high mountains is like viewing its effects