Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 106

al. 2011). The tiny fog droplets are intercepted by the leaves and branches and grow by
coalescence until they become heavy enough to fall to the ground, thereby increasing
soil moisture and feeding the groundwater table. If the trees are removed, this source
of moisture is also eliminated.
Many tropical and subtropical mountains sustain “cloud forests,” which are largely
controlled by the abundance of fog drip (Jarvis and Mulligan 2011). Along the east coast
of Mexico in the Sierra Madre Oriental, luxuriant cloud forests occur between 1,300
and 2,400 m (4,300-7,900 ft). The coastal lowlands are arid by comparison, as is the
high interior plateau beyond the mountains. Globally, 477 cloud forests have been iden-
tified, at elevations between 220 and 5005 m (720-16,420 ft), with an average elevation
of 1,700 m (5,600 ft) (Jarvis and Mulligan 2011). Such forests were once much more
extensive, but they have been severely disturbed by people and are now in danger of
being eliminated. The climate of cloud forests is highly variable from site to site, with
an average rainfall of ∼2,000 mm per year and an average temperature of 17.7°C (Jar-
vis and Mulligan 2011). On the northeast slopes of Mauna Loa, Hawai'i, at 1,500-2,500
m (5,000-8,200 ft.), above the zone of maximum precipitation, fog drip is a major eco-
logical factor in the floristic richness of the forests. Over 28 weeks, fog drip was found
to provide 638 mm (25.3 in.) of moisture at an elevation of 1,500 m (5,000 ft) and, at
2,500 m (8,200 ft), 293 mm (11.5 in.), 65 percent of the direct rainfall (Fig. 3.18; Juvik
and Perreira 1974). The study was replicated in 2006-2008 with automated instrument-
ation, measuring annual fog drip totals of 901-2883 mm (35.5-113.5 in.): 181-462 per-
cent of measured rainfall (Juvik et al. 2011).
The contribution of fog drip on middle and upper mountain slopes at lower latitudes
is clearly a major factor in the moisture regime. The relationship between the cloud
forest and fog drip is essentially reciprocal. The trees cause additional moisture in the
area. At the same time, they need the fog drip in order to survive, especially in areas
with a pronounced dry season, when fog drip provides the sole source of moisture for
the plants. In middle latitudes, fog drip is less critical to the growth of trees, but can still
be important (Jarvis and Mulligan 2011). This can be seen in the mountains of Japan,
where there is heavy fog at intermediate altitudes (Fig. 3.19).
RIME DEPOSITS Rime is formed at subfreezing temperatures when supercooled cloud
droplets are blown against solid obstacles, freezing on contact (Krzysztof et al. 2002).
Rime deposits tend to accumulate on the windward side of objects. The growth rate is
directly related to wind velocity. Rime deposits can reach spectacular dimensions and,
by their weight, cause considerable damage to tree branches, especially if followed by
snow or freezing rain. Trees at the forest edge and at timberline frequently have their
limbs bent and broken by this process; power lines and ski lifts are also greatly affected.
One study in Germany measured a maximum hourly growth of 230 g per m (8.1 oz per
3.3 ft) on a power-line cable (Geiger 1965). The stress caused by this added weight may
cause a power failure if the supporting structures are not properly engineered. Rime
accumulation is a severe obstacle to the maintenance of mountain weather stations be-
cause instruments become coated, making accurate measurements extremely difficult.
Some instruments can be heated or enclosed in protected housing, but the logistic-
al problems of accurately monitoring the alpine environment are very great. The U.S.
Weather Bureau Station on Mount Washington, New Hampshire, where rime-forming
fogs are frequent and the wind is indefatigable, exemplifies the problems encountered.