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Inland Winds
Foehn, German Föhn, warm and dry, gusty wind that periodically descends the
leeward slopes of nearly all mountains and mountain ranges. The name was first
applied to a wind of this kind that occurs in the Alps, where the phenomenon was
first studied.
A foehn results from the ascent of moist air up the windward slopes; as this air
climbs, it expands and cools until it becomes saturated with water vapour, after
which it cools more slowly because its moisture is condensing as rain or snow,
releasing latent heat. By the time it reaches the peaks and stops climbing, the air is
quite dry. The ridges of the mountains are usually obscured by a bank of clouds
known as a foehn wall, which marks the upper limit of precipitation on the
windward slopes. As the air makes its leeward descent, it is compressed and warms
rapidly all the way downslope because there is little water left to evaporate and
absorb heat; thus, the air is warmer and drier when it reaches the foot of the
leeward slope than when it begins its windward ascent.
Foehn winds in various parts of the world have local names: chinook in the North
American Rockies, ghibli in Libya, and zonda in the Andes of Argentina.
From: http://www.britannica.com/EBchecked/topic/211886/foehn
Creating a Desert
The Great Basin Desert exists because of the "rainshadow effect" created by the Sierra Nevada
Mountains of eastern California and . When prevailing winds from the Pacific Ocean rise to go over the
Sierras, the air cools and loses most of its moisture as rain. By the time the winds cross over the
mountains and sweep down the far side, they are very dry and absorb moisture from the surrounding
area. This drying effect is responsible for creating the Great Basin Desert.
Chinook Wind (an inland wind)
The winds are caused by moist weather patterns, originating off the Pacific
coast, cooling as they climb the western slopes, and then rapidly warming as they
drop down the eastern side of the mountains. The Chinook usually begins with a
sudden change in wind direction towards the west or southwest, and a rapid
increase in wind speed.
As moist weather patterns blow ashore on the coast, they run into a barrier of
mountains. As they are forced to climb upwards to crest the mountains, they cool
down at a specific rate. Weather patterns cool at rates of .54°C/100 m for moist
systems, and 1°C/100 m for dry systems. For example, a coastal weather pattern
beginning at -1°C near Vancouver, Canada will cool at the dry rate until it becomes
saturated with moisture. From that point on, it will cool at the slower moist rate. If
the saturation level is reached at 1,000 m, it will cool to -11°C to this point, and then
slow to .54°C/100 m. When it crests the summit of a 3,050 m (10,000 ft) peak, it will
have dropped to -22°C. During this process of rising and cooling, it will release most
of its moisture in the form of snow or rain. This results in rainforest conditions on
the western side of the mountains, while the eastern side of the divide remains quite
dry.
Once the now dry weather system crests the summit, it begins to move downhill.
Dry weather patterns warm up with drops in elevation at almost twice the rate of
moisture laden patterns. (1° C/100 m). This means that the above example, in
dropping from 3,050 m (10,000 ft) to the valley bottom at 1,370 m (4,500 ft) will
rise to -5.2° C. If the ambient temperature prior to the Chinook was -25° C, the site
would see a rise of 19.8° C over a very short period.
From: http://www.mountainnature.com/climate/Chinook.htm
Gorge Winds
The Columbia River wind corridor straddles the Oregon-Washington state border
from just east of Portland, Oregon, to Boardman, Oregon (which is about 70 km or
40 mi west of Pendleton, Oregon). Goodnoe Hills, the site of three MOD-2 wind
turbines, is located on a ridge in the eastern part of the Columbia River corridor.
The Columbia River gorge provides a low-elevation connection between continental
air masses in the interior of the Columbia Basin east of the Cascade Range and the
maritime air of the Pacific coast. Especially strong pressure gradients develop along
the Cascades and force the air to flow rapidly eastward or westward through the
gorge. Summer winds blow eastward from the cool, dense maritime air west of the
Cascades to the hot, less dense air in the Columbia Basin. In winter, the
comparatively cold air in the Columbia Basin frequently blows westward through
the gorge.
Air flowing through a restriction, like the gorge, has a tendency to accelerate. Such is
the case here. When high pressure is prevalent east of the Cascade Mountains,
especially in winter, winds can gust to 100 mph at the west end of the gorge near
Crown Point. In summer, as the Pacific high pressure becomes settled off the coast
and lower pressures prevail east of the mountains, westerly winds replace the
easterly winds, creating excellent wind surfing conditions. This change is manifest in
the trees. At the west end of the gorge, trees exposed to the wind have few or
stunted branches on the east side. At the east end of the gorge, the reverse is true.
Although the Columbia River corridor is generally an area of high wind resource,
terrain variations cause considerable local variability in the wind resource. The
wind resource has been measured at numerous sites throughout the Columbia River
corridor, and the annual average wind resource at exposed areas ranges from class
3 to class 6 (wind speeds of 10 to 20 miles per hour). Spring and summer are the
seasons of maximum wind power, except for the extreme west end where the
maximum resource is in winter.
Coastal Winds
Sea Breeze
Land Breeze
Sea breezes are the result of differential heating of the land and the sea. Sea breezes
occur by day, when the land becomes warmer than the sea. Warm air from the land
cannot expand into the sea as the air is cooler and more dense, so it expands up into
the atmosphere. Cumulus clouds tend to form as the warm air rises over the land to
about 500-1500m. The diagram below shows the whole sea breeze process.
Air in sea breezes is cool and moist compared to the air over the land. Sea breezes
can move 70km inland in temperate climates by 9pm in the evening. Sea breezes can
be noticed several kilometres out to sea. In the tropics they can be felt 20km from
the land. Wind speeds from sea breezes can be about 4-8m/s but can be even
stronger.
Land breezes occur at night and in the early morning, when the land is cooler than
the sea. This is because as the air cools in the night time (as there is less heating
from the sun) it contracts. Pressure is higher over the land than the sea. This causes
the air to flow from the land to the sea, which is known as a land breeze. The
circulation is completed by air rising and moving towards the land at 100-300m.
This is shown on the diagram below.
Cumulus clouds form where there is rising air. Land sea breeze fronts tend to only
affect a small area of 10-15km out to sea, in comparison to the much larger effect of
sea breezes. Wind speeds are also lower at 2-3m/s.
Oregon and Washington Coast
The estimated annual average wind power for exposed coastal areas of Oregon
and Washington is class 4 at 50 m above the ground (164 ft up). Specific sites
that experience terrain-induced acceleration of the wind may have greater than
class 4 power. “Class 4 power” means the winds typically flow at about 14
miles per hour. The abrupt increase in surface roughness inland from the
coastline, because of vegetation and topography (land forms), rapidly reduces
the wind inland.
During winter, the season of maximum wind power at sites well-exposed to the
prevailing south and southeasterly winds, high wind speeds are usually
associated with storms and fronts moving in from the Pacific Ocean. However,
during the summer, wind power is high along the central and southern Oregon
coast at sites well-exposed to northerly winds and is associated with the strong
surface pressure gradients created by the cold water and relatively warm
interior.
Mountain Range Winds
Anabatic winds
Katabatic Winds
Anabatic wind is caused by thermal (heat) processes. Anabatic (upslope) winds occur over slopes that are
heated by the sun. Air in contact with slopes that are warmed expands upward and cool and sinks over
neighboring valleys (see diagram). Anabatic winds are usually slow, at only 1-2m/s and are rarely
importance expect near coasts where they can increase the strength of sea breezes.
Katabatic (downslope) winds occur over slopes which are cooled. Katabatic winds occur where air in
contact with sloping ground is colder than air at the same level away from the hillside over the valley (see
diagram below). Katabatic winds are nocturnal phenomena in most parts of the world (i.e. they tend to
happen at night) as there is surface cooling, especially when there is little cloud and due to lack of heating
by the sun.
Katabatic wind speeds do not typically not exceed 3 or 4 m/s. However, where the ground is covered with
snow or ice, katabatic winds can occur at any time of day or night with speeds often reaching 10 m/s, or
even more if funnelling through narrow valleys occurs. Katabatic winds may lead to the formation of frost,
mist and fog in valleys.
Cascade Gap Winds
From May through September, gap winds are a common phenomenon in places like
Wenatchee and Ellensburg. The temperature difference between the West Side and
East Side of Washington state is one of the primary causes of gap winds. Cooler
maritime air west of the Cascades becomes banked up the mountain range. During
the warmest part of the day, the East Side of the state can become quite hot setting
up a strong west to east temperature across the mountains. If the depth of the cold
air west of the Cascades is deep enough, this chilly/dense air spills through the
Cascade gaps bringing breezy evenings.
Strong fronts (like the cold front we had today) can strengthen the gap winds. Since
mountain passes like Snoqualmie and Stevens Pass are low spots in the mountain
chain, cold air spills through these gaps making Ellensburg and Wenatchee windy.
This gap wind pattern frequently occurs in June and July when some of the strongest
temperature difference from west to east occur. Often the strongest winds of the day
occur early in the evening as the cold/dense air initially spills over the Cascade
Crest.