Name: ________________
Period: ______________
Washington State Climate and weather. Read the passage below and answer the
questions about weather and climate in Washington.
Climate
Weather
Climate is what you expect.
Weather is what you get.
Climate is about long-term records, trends and
averages.
Weather is the day to day
experience.
Climate is the sum or synthesis of all the
weather recorded over a long period of time. It
Weather is a description of
conditions over a short period of
tells us the average or most common
conditions, or extremes, or counts of events, or
frequencies.
time—a "snap shot" of the
atmosphere at a particular time.
Weather
The Basics of Pacific Northwest Weather
If one could use a single phrase to describe Pacific Northwest weather, "wet and
mild" would be a start, but not a particularly exact one. Although the region west of the
Cascade crest is considered "wet" by many, it enjoys some of the driest summers in the
nation and receives less annual precipitation than much of the eastern United States.
East of the Cascades, where arid conditions dominate, "wet" is certainly not an apt
description, and east-side temperature extremes, ranging from -48° to 119° F, makes
"mild" a misnomer at times. Northwest weather and climate are dominated by two
main elements: (1) the vast Pacific Ocean to the west and (2) the region's
mountain ranges that block and deflect low-level air. Together, these factors explain
many of the dominant and fascinating aspects of the region's weather. The ocean
moderates the air temperatures year-round and serves as a source of moisture,
and the mountains modify precipitation patterns and prevent the entrance of
wintertime cold-air from the continental interior.
Why is Pacific Northwest Weather Generally Mild?The Pacific Northwest is
located in the northern hemisphere mid-latitudes, a zone stretching from approximately
30° to 60° north latitude where winds generally blow from west to east. This eastward
movement of air is usually not uniform in strength, but is typically strongest in a
relatively long, narrow current a few hundred miles across and a few miles deep, known
as the jet stream. Before reaching the Pacific Northwest, the eastward-moving air
traverses thousands of miles over the Pacific Ocean. The air near the surface is
profoundly modified, moistening, and taking on the temperature of the underlying ocean
surface. Thus low-level air reaching the Pacific Northwest during the winter is generally
mild and moist, resulting in typical wintertime air temperatures west of the Cascades
rising into the mid-40s. Summer sea-surface temperatures off the Northwest coast vary
little during the year and rarely rise above the lower 50s °F.
As the jet stream and associated storms weaken and retreat northward during the
warm season, high pressure builds northward over the eastern Pacific (Figure 2.2). With
higher pressure offshore, cool air from the ocean is pushed inland, ensuring that
summertime temperatures west of the Cascades remain moderate, rarely exceeding 90
°F along the coast and over the Puget Sound lowlands. Only when the wind direction
reverses and air moves westward from the warm continental interior can temperatures
reach the upper 80s °F and beyond over the west side of the Cascades.
Figure 2.2—Topographic map of the Pacific Northwest
The other major element of Northwest weather is the terrain, ranging from the
formidable Rocky and Cascade mountains, which reach 5,000 to 14,000 feet, to the low
coastal mountains, which attain only 3,000 to 4,000 feet (Figure 2.3). East of the
Cascades a topographical "bowl" encompasses the lower Columbia valley, including the
Tri-Cities in Washington and Pendleton, Oregon, while eastern Oregon is an elevated
plateau, with some higher peaks and several major valleys.
In the winter the Rockies and Cascades form a double barrier to the cold air of the
continental interior (Figure 2.3). The Rockies act as the Northwest's first line of defense,
blocking the cold air that develops over the snowfields of the Canadian Arctic and that
subsequently moves southward into the interior of the continent. If the cold air becomes
deep enough, some can push over the Rockies, but since air warms as it descends, the
air moving down the western slopes of the Rockies reaches eastern Washington and
Oregon considerably warmer than air at similar elevations east of the continental divide.
Next come the Cascades, which block the westward movement of most of the cold,
dense air that does manage to reach eastern Washington and Oregon. However
Eastern Washington is much cooler during the winter than Western Washington, which
is warmed by the Pacific Ocean and the Puget Sound.
Figure 2.3—the major mountain ranges of the Northwest protect the region from the frigid
air of the interior of North America. At low levels, the coldest air is found east of the
Rockies, within the continental interior. Air that makes it across the Rockies warms as it
descends into eastern Washington and Oregon. Any air that crosses the Cascades is further
warmed as it descends over the west slopes and is compressed by the higher pressure at
lower elevations. Illustration by Beth Tully/Tully Graphics.
Why Does Precipitation Vary So Much Across the Pacific Northwest?
Nowhere in North America are precipitation contrasts great than in the Pacific
Northwest. Driving east on Interstate 84 through the Columbia River gorge, one
transitions from rain-forest conditions near Cascade Locks (80 inches per year) on the
western side to an arid environment in the Dalles (13 inches), only 45 miles to the east
On the southwest side of the Olympics there is a sodden Hoh rain forest, which receives
140-160 inches a year, while 40 miles to the northeast the town of Sequim in the
Olympic rain shadow enjoys a relatively dry, sunny climate with about 15 inches a year
(The distribution of precipitation over the Pacific Northwest is greatly influenced by the
region's mountain ranges (Figure 2.6, below). Clouds and precipitation are associated
with rising air, while clearing occurs as air descends. Since air typically moves across
the region from the southwest to the northeast during the wet, winter season,
precipitation generally increases on the southwestern or western slopes of the
Northwest Mountains where air is forced to rise. In contrast, precipitation typically
decreases over the northeastern or eastern slopes where air descends. Meteorologists
typically refer to the slope facing the wind as the windward side, while the drier,
downwind slope where the air descends is known as the leeward side.
Figure
2.6—Mean Annual Precipitation (in inches) for Washington and Oregon.
Moist air moving eastward from the Pacific Ocean is first forced to rise by the coastal
mountains of Oregon, Washington, and Vancouver Island, producing substantial
precipitation on their western slopes. Annual totals range from 60 to 80 inches on the
lower coastal mountains to 90 to 160 inches on the western slopes of the highest
coastal terrain, the Olympics. As the air crosses the coastal mountains and descends
into the lowlands of western Washington and Oregon (Puget Sound through the
Willamette Valley), the annual precipitation decreases to around 35 to 45 inches a year.
Thus, the lowland urban corridor stretching from Bellingham to Eugene is in the rain
shadow of the coastal mountains and enjoys a far drier climate than the coastal zone. In
west side locations where the air frequently descends from higher terrain, such as
northeast of the Olympic Mountains, precipitation is further reduced - as it is in Sequim.
As air continues to move eastward it is forced to rise by the Cascades, and 60-120
inches of precipitation typically fall each year. Although precipitation initially increase
with elevation over the western side of the Cascades, it appears that precipitation
begins to drop off at the highest elevations, above approximately 7,000 feet. Why? At
such heights there is less precipitation formation. After crossing the Cascade crest, air
descends rapidly over the eastern slopes of the Cascades, producing a sharp decrease
of clouds and precipitation. Air descending into the bowl of eastern Washington
produces extreme aridity, with annual precipitation decreasing to less than 10 inches a
year.
Orographic Precipitation: Why Do Mountains Influence Precipitation?
The key reason for the complex variations of precipitation around the Northwest is the
effect of terrain. In discussing the impact of mountains on weather, meteorologists like
to use the term "air parcel" to signify an identifiable portion of air - think of a balloon
filled with air. As air parcels approach the slopes of a mountain they are forced to rise
(Figure 2.7). Since air pressure decreases with height, rising air parcels experience
decreasing pressure and expand as they climb the windward slopes of a mountain or
hill. This expansion is evident in real balloons, which increase in size as they ascend to
regions of lower pressure. Eventually the expansion is so great the balloons bust!
Expanding air parcels cool, a fact evident in the coolness of the spray from pressurized
aerosol cans or the chill air escaping from a pressurized tire. It takes energy to expand,
and that energy is supplied by lowering the air parcel's temperature.
Figure 2.7 - Schematic of air flow over a mountain barrier. On the windward side, air is
forced upward by the mountain and encounters lower pressure as it rises. Lower pressure
causes the air to expand and cool. Eventually the air cools sufficiently to become saturated.
As it ascends farther, clouds and often precipitation form. On the leeward side the air sinks,
compresses, and warms, causing clouds to dissipate. Illustration by Beth Tully/Tully
Graphics.
All air parcels contain some water vapor and that is particularly true of air approaching
the Northwest coast, having traversed thousands of miles of the Pacific Ocean. The
amount of water vapor that can be contained in an air parcel depends on temperature,
with warmer air able to contain more water vapor than cooler air. Imagine a situation
where an air parcel holds a certain amount of water vapor and no clouds or precipitation
are apparent. As the air parcel is forced to rise by a mountain, the surrounding pressure
decreases and thus the air parcel expands and cools. As the air cools it can hold less
and less water vapor. Eventually, if the air parcel rises and cools enough it can only
hold the amount of water vapor it already carries and no more. This is called saturation,
and at this point the relative humidity is 100 percent. If the air parcel rises and cools
further, it will have more water vapor than it can hold, so some of the water vapor (a
gas) must condense out into liquid form and cloud droplets appear (Figure 2.8).
Figure 2.8 - Upslope clouds form as air moves up mountain slopes. This picture shops
upslope clouds on high terrain above Kennedy Lake near Tofino, Vancouver Island. Photo
courtesy of Michael Hanna.
In contrast to the windward slopes, air parcels descend on the leeward slopes of
mountain barriers. As the air parcels descend, the pressure increases, the air parcels
are compressed, and their temperature increases. As the air warms its ability to hold
water vapor is enhanced and condensation no longer occurs. In fact, as the descending
air parcel warms, water and ice particles tend to evaporate, producing a dramatic
reduction in clouds and precipitation. The descent of air over the eastern slopes of the
Cascades explains often rapid decrease of clouds and precipitation east of the Cascade
crest when the regional winds are from the west.
Summary
Washington weather is very diverse in its nature, with many diverse climate zones and a
wide variety of weather. There are many factors to our weather patterns, such as the jet
stream, landforms, and air pressure. These can be seen in our three main climate
zones. Rainforest, temperate and arid.
The Olympic peninsula has rainforests and a temperate climate, whereas
Western Washington is a temperate zone with a mild climate and weather and Eastern
Washington being an arid climate with less rainfall and warmer and colder weather than
Western Washington. When living in Washington your weather can change by just
getting in your car and driving.
Name: ____________________
Washington’s Climate and weather
1) Using figure 1, what are the differences between climate and weather?
2) What are the two main elements that dominate weather in the Northwest?
3) Using a quote from the reading, what is the reason for Washington’s mainly
mild weather?
4) In Eastern Washington what are the 2 major land features that effect its
weather?
5) Why do the physical features of the Northwest have such a profound impact
on Eastern Washington?
6) How much precipitation does the. Hoh rainforest receive annually?
7) What influences the distribution of precipitation over the Pacific Northwest?
8) Meteorologist refer to the slope facing the wind as the
_______________side? While the drier downward slope where air descends
is known as ________________ side?
9) Why does the Western slopes of our mountains get more rain annually than
its Eastern slope?
10) Why is there less precipitation at higher levels (top of mountains) than at
lower levels?
11) What is the orographic effect and how does mountains of our state influence
it?
12) What does the word saturation refer to?
13) Describe the three climate zones of Washington State?