Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 77

FIGURE 3.4 The influence of the Olympic Mountains on the wind field and precipitation. The arrows
are flow lines indicating wind direction. Distance between the flow lines indicates relative speed:
The closer they are to one another, the faster the wind in that region. Notice that the flow lines
are evenly spaced over the Pacific Ocean. As they are deflected through the Strait of Juan de
Fuca, the wind speed increases. Also notice that winds are funneled up the western valleys of
the Olympics, concentrating moist air and increasing precipitation at the Hoh Rain Forest (3,800
mm), while Sequim only receives 430 mm in the rainshadow. (A. J. Bach.)
FORCED ASCENT
Air that blows perpendicular to a mountain range is forced to rise, and is cooled adia-
batically. Eventually the dew point is reached, condensation occurs, clouds form, and
precipitation results. This increased cloudiness and precipitation on the windward slope
is known as the orographic effect (Lin et al. 2001). The rainiest places in the world
are mountain slopes in the path of winds blowing of relatively warm oceans. There are
many examples from every continent, but the mountainous Hawai'ian Islands serve as
an illustration. The precipitation over the water around Hawai'i averages about 650 mm
(25 in.) per year, while the islands average 1,800 mm (70 in.) per year. This is largely
because of the presence of mountains, many of which receive over 6,000 mm (240 in.)
peryear (Nullet and McGranaghan 1988). At Mount Waialeale on Kaua'i, the average
annual rainfall reaches the extraordinary total of 12,344 mm (486 in.), that is, 12.3 m
(40.5 ft.), one of the highest recorded annual averages in the world (Blumenstock and
Price 1967). In the continental United States, the heaviest precipitation occurs at the
Hoh Rain Forest on the western side of the Olympic Mountains in Washington, where