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Lesson Two: Weather and Climate—The Weather and Climate of the Pacific Northwest

How To Proceed in This Chapter

  • Read the Chapter Two in the text;
  • Return to this page with your textbook as a reference and read specifically how weather and climate affect the Pacific Northwest.
  • At the bottom of this page is a link to Assignment 2A: Weather, and Assignment 2B: Climate. Both assignments rely on the concepts and ideas of the text, and the specific facts of this Lesson Two about the weather and climate in the Pacific Northwest.
  • Assignment 2A (Weather) questions will be posted by the instructor the week prior to when they are due. Questions will use specific current weather conditions. Responses to the questions will come from your understanding and interpretation of Chapter Two from the text and Lesson Two online readings. Because climate is about long-term records, trends and averages, Assignment 2B (Climate) questions will not vary substantially from quarter to quarter - and therefore are posted now.
  • Finally, post your responses to questions in Assignments 2A and 2B, separately, to the online course Discussion Boards.


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 tells us the average or most common conditions, or extremes, or counts of events, or frequencies. Weather is a description of conditions over a short period of time—a "snap shot" of the atmosphere at a particular time.
If weather is the watch, then climate is the calendar.


The Basics of Pacific Northwest Weather

Portions of the following are excerpted from: Mass, Cliff. The Weather of the Pacific northwest. University of Washington Press: Seattle, WA, 2009.

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 midlatitudes, 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. Usually 5-8 miles above the surface, jet-stream winds often reach 100 to 200 miles per hour during the winter. Weather systems, such as the low-pressure systems that bring rain and wind, then to follow the jet stream, and thus the jet stream can be considered at atmospheric "highway" for storm and precipitation. The jet stream undulates north and south like a sinuous snake but is not continuous around the globe, since there a longitudes where is is broken or weak.

Before reaching the Pacific Northwest, the eastward-moving air traverses thousands of miles over the Pacific Northwest. The air near the surface is profoundly modified, moistening, and taking on the temperature of the underlying ocean surface. The surface temperature typically ranges from 45 to 50 °F between Japan and the Northwest coast (Figure 2.1). 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.

Sea-Surface Temperature - north Pacific Ocean

Figure 2.1—Sea-surface temperatures over the northern Pacific Ocean (Click to enlarge).

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.

Topographic map of the Pacific Northwest

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. During the unusual circumstances when the cold air east of the Cascade crest becomes deep enough to push westward across these mountains, it is warmed further as it descends the western side. In short, because of the blocking effect of the Rockies and Cascades, eastern Montana is colder than eastern Washington and Oregon, which in turn are colder than western Washington and Oregon.

Major mountain ranges of the Pacific Northwest

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.

A Survey of Pacific Northwest Temperatures

Temperatures across the Pacific Northwest are controlled by proximity to water and by elevation, the amount of clouds, and the position of major mountain barriers. Figure 2.4 illustrates the typical surface air temperatures over the region of summer (July) and winter (January).

Minimum and maximum temperatures for January and July

Figure 2.4—Climatological (1971-2000) maximum and minimum temperatures for January and July (°F). Graphics courtesy of Chris Daly and Mike Halbelb of the Oregon State University PRISM group. (Click to enlarge)

Clouds play an important role in producing the winter temperature distribution. West of the Cascades, incessant wintertime clouds reduce the maximum temperatures and increase the daily lows. During the day, clouds reflect a great deal of incoming solar radiation, which is why they look white in visible weather-satellite pictures shown on television. Reflecting the sun's rays back into space produces cooling on the ground. In contrast, clouds can warm the surface at night, since they lessen the ability of the ground to emit infrared radiation to space. Thus, cloud nights generally are warmer than clear ones.

Summer brings not only much warmer temperatures, but a very different pattern of temperature variation across the region. July minimum temperatures are relatively uniform west of the Cascades, with lows in the mid- to lower 50s °F. Warmer temperatures are found east of the Cascades in the lower elevations of the Columbia River basin, particularly in the topographic bowl encompassing the Tri-Cities and Pendleton, where temperatures decline to about 60 °F. Over the higher elevations of the Cascades and the highlands of eastern Oregon nighttime temperatures are chilly even in midsummer, with typical minimum temperatures in the 40s °F. For maximum summer temperatures, there are significant variations over the Willamette Valley, ranging from the 80s °F to the north to the 90s °F to the south; in contrast, over the western Washington lowlands, temperatures reach only into the mid-70s °F. These temperature differences are caused by terrain and proximity to water. While the western Washington lowlands are flooded with air from Puget Sounds, the Straits of Juan de Fuca and Georgia, and the Pacific Ocean, the Willamette Valley is landlocked on three sides, limiting access to air tempered by a cool water surface.

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 (Figure 2.5b). 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 (Figure 2.5a).

Annual total precipitation

Figure 2.5—Annual total precipitation (1971-2000) over northwest Washington State (a) and the Columbia River gorge area of Oregon (b). Graphics courtesy of Chris Daly and Mike Halblelb of the Oregon State University PRISM group. (Click to enlarge)

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.

Mean Annual Precipitation (in inches) for Washington and Oregon.

Figure 2.6—Mean Annual Precipitation (in inches) for Washington and Oregon. (Click to enlarge)

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 westside locations where the air frequently descends from higher terrain, such as northeast of the Olympic Mountains, precipitation is further reduced - as it is in Squim.

As air continues to move eastward it is forced to rise by the Cascades, and 60-120 inches of precipitation typically fall each year from southern Oregon into southern British Columbia on the barrier's western slopes. 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.

Infrequently, the region's winds blow from the east. On such occasions, the distributions of clouds and precipitation make a corresponding shift, with the eastern slopes of the Cascades and Olympics becoming enshrouded in clouds and showers, while the usually wet western slopes turn warm and dry. The mountain crests remain in cloud for either wind direction. When the winds are from the southeast, the southeastern portions of the Olympics can receive extraordinarily heavy precipitation, sometimes collecting 5-10 inches per day of water and, if temperatures ar cool enough, heavy snow can fall over the Kitsap Peninsula to the southeast of the barrier. Such heavy upslope precipitation on the southeastern side of the Olympics can cause Olympic Peninsula rivers such as the Skokomish to overflow their banks, flooding nearby communities.

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.

Schematic of air flow over a mountain barrier.

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).

Upslope clouds form as air moves up mountain slopes.

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 lee 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.


Climate change and the Pacific Northwest

Despite its reputation for rain, the Pacific Northwest (PNW) experiences dry summers, and irrigated agriculture, urban users, and ecosystems rely on snowmelt for summer water.  This fact is critical in understanding how the region responds to climate.

Year-to-year and decade-to-decade variations in PNW climate are influenced by two patterns of Pacific climate variability: El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). ENSO and PDO each tend to push PNW climate toward one of two main patterns: cool-wet or warm-dry. 

Pacific NW Weather patterns

Fig. 1 - Impacts of climate variations (ENSO, PDO) on the Northwest.  The top panel shows the impacts of sea surface temperature (SST) conditions over the North Pacific associated with the warm phase of ENSO or PDO, and the bottom panel shows the impacts of the cool phase. Click image to enlarge.

The warm-dry winters have thinner snowpack and lower spring and summer streamflow, with generally negative impacts on salmon and forests.  The cool-wet winters have the opposite effects.  Even though the annual temperature and precipitation fluctuations associated with the PDO (Figure 2) are fairly small, these small changes in climate have large impacts on the region's natural resources.

Natural Columbia River flow at present (dashed), and in 2050 as simulated under future climate conditions from four climate models.

Fig. 2 - Natural Columbia River flow at present (dashed), and in 2050 as simulated under future climate conditions from four climate models.

During the past 100 years, the PNW has become warmer and wetter.  The region's average temperature has increased 1.5°F, and average precipitation has increased about 3° (15%).  Scientists cannot be sure what has caused these increases, but the increases are consistent with trends generated by climate models using observed increases in carbon dioxide.  Climate models project continued increases in temperature and winter precipitation.  Summer precipitation could go up or down.

What effects will these climatic changes have? 

For the PNW, the most significant consequence of climate change is likely to be the reduction in all-important summer water supply.  As the climate warms, snowpack will shrink and summer streamflow will drop considerably.  This and other climate changes will have a wide range of consequences, most of them negative, for humans and ecosystems.

Water resources
The benefits to dryland agriculture of a longer growing season and greater precipitation may be offset by the losses to irrigated, high-dollar-value crops. Past experience offers some lessons: In the dry Yakima Valley of Washington, a string of years with below-average snowpack (1991-1994) led to selective water shortages and economic losses that reached $140 million in 1994.  Even though water will become less plentiful in summer, higher winter precipitation (as occurred during the winter of 1998-99) will probably also increase wintertime flooding in many rivers. 

Climate variations have clearly played a role in PNW salmon history (Figure 2), with low summer streamflow and warm coastal ocean temperatures tending to reduce salmon production.  Unfortunately, these conditions are likely to become more common in a warming climate, adding to the already long list of human-caused problems that now threaten the survival of salmon in the PNW.

Some types of  trees grow better with more carbon dioxide in the air, but for most Northwestern coniferous forests, growth tends to be lower (and forest fires more extensive) during warmer, drier years (Figures 1 and 2).  It is not yet clear how forests will change in the future, but some changes in forest composition, area, and density are likely.

Both the physical landscape and the ecosystems of the coasts will be affected by climate change and rising sea level.  Changes in wave direction may increase coastal erosion, as often happens during El Niño events.  Increased winter precipitation will probably lead to more frequent landslides; recent wet winters have shown that thousands of homes are at risk from landslides around Puget Sound and on the Oregon coast, and climate models consistently project wetter winters. 

What would we have to do to prepare for a changing climate?

Climate scientists agree that further climate change may be inevitable and will therefore require adaptation, although most scientists also think that the pace of climate change can be slowed by substantially reducing greenhouse gas emissions. This would give governments, businesses, and ecosystems around the world more time to respond and adapt to climate change as well as reducing the overall severity of climate change-related impacts, thereby buying "insurance" for an uncertain future.  Another way to buy insurance now is to incorporate climate change into all long-term decisions about natural resources, thereby providing greater resilience.

Climate change is sure to occur in some form.  Though the details are not yet clear, we know enough already to begin planning.  With few exceptions, natural resources are managed as if climate were constant. Recent experience with year-to-year climate variations, like those associated with El Niño, provides some practice at dealing with years when climate is different from "normal."  In years ahead we will see a change in the definition of "normal."  The single most important thing that the region can do to prepare for a changing climate is to develop a dialogue between scientists and decision-makers.  An increased awareness of how climate affects the region will increase resilience to climate variations and change. In addition, we can reduce local pressures on our resources and ecosystems.

Water resources
Three basic strategies should be examined: to increase supply, to decrease demand, and to increase institutional flexibility.

Salmon and forests
Ecosystems are more resilient to climate variations and change when they include a high degree of biodiversity, that is, a wide range of different habitats, species, and genetically different types within the same species.  For salmon, ensuring biodiversity mostly means increasing available healthy and connected habitat while continuing to control harvests.  For forests, ensuring biodiversity means avoiding single-species plantations.

A wide range of coastal problems could be dealt with by changing land-use controls, construction setbacks, and zoning.  Public funds could be better spent in ways other than subsidizing coastal development (especially re-development after damage) in obviously hazardous places.

Adapted from:
The Potential Impacts of Global Warming on the Pacific Northwest, 2000.

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