label this part of Washington “the banana belt.” Port Townsend directly downwind of
the Olympic Mountains receives a bit over 65 cm (26 in) of precipitation. To the east
of Puget Sound, Seattle receives nearly 100 cm (40 in) of precipitation because the
air streams that have flowed around the Olympic Mountains are able to converge
and cause the rise of air.
The largest rain shadows occur in locations where topographic blockage forces
air to sink through distances of thousands of meters so that the adiabatic warming
is pronounced. In the United States, the crest of the Cascade range of the Pacific
Northwest is oriented at right angles to the moist westerly flows coming from the
Pacific Ocean. Air must ascend over 3,000 m in some places and then sink over
2,000 m into the continental interior. The maritime air ascends the windward sides
of the mountains and produces copious amounts of precipitation in some places.
On the leeward side of the mountains the strong descent of air inhibits precipita-
tion so that the climate is markedly drier. Paradise Ranger Station on the windward
slopes of Mt. Rainier in Washington State averages more than 1,700 cm (675 in)
of snow a year, while Yakima, Washington—some 100 km (60 mi) downwind—
averages a mere 50 cm (20 in). In practical terms this means the difference
between a relatively moist climate capable of supporting a significant forest cover
and a climate supporting a semiarid grassland.
There are numerous major rain shadows on the planet. Some notable examples
include:- The Patagonian Desert in Argentina east of the Andes Mountains.
- The southwestern (leeward) slopes of Haleakala volcano on Maui, Hawaii are
semiarid compared to the rainforest on the northeast (windward) slopes. - The western side of Madagascar is shadowed from moist air from the Indian
Ocean to the east. - The Thar Desert of eastern India made dry from mountain ranges to the north-
east west and southeast. - The presence of the Gobi Desert of Mongolia is partially attributable to block-
age of moist tropical air by the Himalayas and Tibetan Plateau.
Although it is tempting to conceive of a rain shadow location as having an arid
or semiarid climate, this is not always the case. In Great Britain, the mountains in
Wales and western Scotland put leeward areas to the east in rain shadows, yet
these rain shadows had natural forest cover in the lowlands because there is still
enough precipitation to support a forestbiome.
Unquestionably, the largest of the rain shadows increase the difficulty of pro-
ductively using the landscape for human habitation and agriculture. However, the278 Rain Shadow Effect