PACIFIC NORTHWEST
   
Weather
The climate of the Pacific Northwest is profoundly affected by
air masses from both the Pacific Ocean and the North American
continent. Pacific air masses (mild in winter, cool in summer)
dominate along the coast, while continental air (cold in winter,
hot in summer) prevails east of the Cascade Mountains. However,
the boundary between oceanic and continental air is always in
motion, and at times the entire region can be engulfed by either
kind of air.
Wind Patterns
Earth's atmosphere is driven into motion as hot tropical air rises
and spreads toward the poles and cold polar air sinks and flows
toward the equator. Earth's rotation warps this north-south exchange
of warm and cold air into vast wind patterns, including the prevailing
westerlies, a broad west-to-east air current that flows over most
of the United States and southern Canada. Embedded in the prevailing
westerlies are a succession of whirls and eddies: systems of high
pressure (fair weather) and low pressure (cloudiness, high humidity,
stormy weather) that form and dissipate along fronts, which are
the boundaries between warm and cold air masses. Winds blow in
a circular pattern around the center of weather systems: In the
Northern Hemisphere they blow counterclockwise (as seen from above)
in a low-pressure system and clockwise in a high-pressure system.
Aleutian Low
During autumn, winter, and spring, a huge semi-permanent low-pressure
system, the Aleutian Low, develops over the Gulf of Alaska. The
counterclockwise flow around its southern perimeter intensifies
the strength of the prevailing westerlies blowing into the Pacific
Northwest coast, and the Aleutian Low spawns smaller low-pressure
systems that come ashore with almost monotonous regularity.
North Pacific High
In summer, the Aleutian Low all but disappears, and the North
Pacific High, a massive semi-permanent high-pressure system that
occupies most of the Pacific Ocean north of the equator, expands
northward, keeping Washington and Oregon in sunshine for days
at a time as it shunts storms and cold fronts to the north. West
of the Cascades, the ocean-cooled air keeps temperatures comfortable,
but persistent sunshine farther inland can raise temperatures
east of the Cascades into the 90s and 100s Fahrenheit. Occasionally,
easterly winds send such heat into the Puget Sound and Willamette
Valley areas. However, the humidity is usually quite low, making
the heat bearable and virtually eliminating the chance of thunderstorms.
Upslope and Downslope Winds:
Rain Shadow Effect
The mountain ranges of the Northwest provide a formidable barrier
to the flow of air across the region and profoundly influence
local climates. When moist westerly winds are forced to ascend
a ridge to get to the other side, the air cools to the condensation
temperature (dew point), and clouds, rain, and snow develop. Because
of this upslope effect, the western slopes of the Olympic, Coast,
and Cascade Ranges are among the wettest spots in North America.
As the westerlies, already reduced of moisture, descend the eastern
slopes of the ranges, they warm up as a result of compressional
heating, and humidity is even further reduced. These mild downslope
winds, called "chinooks," create a rain shadow effect to the east
of the larger ranges, and are responsible for the desert climate
of parts of central and eastern Washington and Oregon.
The Progress of a Storm
As a storm system spins off the Aleutian Low, a warm front, usually
on the eastern side of the storm center, marks the leading edge
of northbound warm air. As it rises over the colder, denser air
to its north, the warm air produces moderate but steady rain.
Meanwhile, west of the storm's center, arctic air plunges south
behind a cold front, along which heavier air shoves like a wedge
beneath the warm, and usually moist, air. Forced upward, the warm
air expands and cools, its moisture condensing into clouds and
rain. As the center of low pressure passes, generally to the east,
the trailing cold front may sweep the entire region, setting off
brief but heavy showers, squalls, and thunderstorms. After the
storm departs, high pressure moves in, bringing clearing, cooler
weather, perhaps a brief interval of fair skies, and eventually
southerly winds ahead of the next storm.
Drawing of progress of storm by Howard S. Friedman
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