Author

SUE A. FERGUSON is an atmospheric scientist, Forestry Sciences Laboratory, 

4043 Roosevelt Way NE, Seattle, WA 98105.
United States Department of 

Agriculture

Forest Service

Pacific Northwest Research Station

Portland, Oregon

General Technical Report 

PNW-GTR-434

August 1998

Air Quality Climate 

in the

Columbia River 

Basin

Sue A. Ferguson

Interior Columbia Basin Ecosystem

Management Project: Scientific Assessment

Thomas M. Quigley, Editor

Abstract

Ferguson, Sue A. 1998. Air quality climate in the Columbia River basin. Gen. Tech. 

Rep. PNW GTR-434. Portland, OR: U.S. Department of Agriculture, Forest Service, 

Pacific Northwest Research Station. 23 p. (Quigley, Thomas M., ed. The Interior 

Columbia Basin Ecosystem Management Project: scientific assessment).

Aspects of climate that influence air quality in the Columbia River basin of the 

Northwestern United States are described. A few, relatively simple, analytical tools 

were developed to show the spatial and temporal patterns of mean-monthly mixing 

heights, precipitation scavenging, upper level and surface trajectory winds, and drought 

that inhibit pollution uptake. Also, potential changes in air quality from the effects of 

increasing greenhouse gases are discussed.

Keywords: Air quality, climate, air pollution, mixing heights, trajectory winds, 

pollution trajectories, pollution scavenging, drought, global change, Columbia River 

basin.
Summary

Several components of climate influence air quality in the Columbia River basin of the 

Northwestern United States. For example, stagnant air trapped below a persistent temperature inversion is common in the basin during winter. Stagnant conditions remain 

possible but are less frequent in summer. Midwinter rain in the basin occurs 

occasionally and allows effective scavenging of pollutants by liquid precipitation 

throughout the year. Pollution from the basin that reaches upper levels of the 

atmosphere most often will be transported by prevailing westerly winds toward states 

east of the basin. Northerly upper level winds, which transport pollutants to the Grand 

Canyon, occur less than 10 percent of the time. Surface winds usually transport 

pollution toward the west (offshore) in winter and toward the east (onshore) in summer. 

Also, during summer a diurnal pattern allows pollution to be lofted to upper levels 

during the day and follow downslope winds at night. Drought events in the basin, which 

reduce the susceptibility of many plants to pollution damage, are common but usually 

mild. Finally, a potential change in climate caused by increasing greenhouse gases 

may affect air quality in the basin. For example, higher mixing heights in the 

atmosphere may improve dispersion during summer but a decrease in storms may 

allow winter stagnation to persist for longer periods than normal. Increased summer 

drought in the central basin but less frequent drought in southeast portions of the basin 

may change the susceptibility of some plants to pollution damage. Also, a decreased 

snow cover may cause early discharge of pollutants held in the snowpack.
Although air quality in the basin generally is 

considered good, effects of air pollution on 

natural resources have been observed 

(Schoettle and others, in press). Also, rapidly 

growing population centers in and around the 

basin are increasing the need for abundant new 

energy resources, which will compound current 

pollution levels. Because the transport and 

deposition of pollutants are controlled to a great 

extent by atmospheric conditions, understanding 

related spatial and temporal climatic patterns 

plays a key role in developing effective means 

of managing air quality.

Climate patterns in the Columbia River basin 

are dominated by topographic features. 

Surrounding mountain ranges prevent frequent 

intrusion from progressing air masses and 

create an isolated, sometimes stagnant, basin 

atmosphere. Gaps through the mountains, 

however, allow a striking pattern of mixing that 

is unique to the basin. Figure 1 shows key 

features of the basin.

Components of Air Quality

Climate

Mixing Height

Mixing height is a level in the atmosphere above 

which vertical exchange of air is inhibited. Low 

mixing heights mean that the air is generally 

stagnant with very little vertical motion; 

pollutants usually are trapped near the ground 

surface. High

mixing heights allow vertical mixing within a 

deep layer of the atmosphere and good 

dispersion of pollutants.

When a parcel of air is warmer than its 

surroundings, it will rise. When it is cooler than 

its surroundings, it will sink. One way to 

determine mixing height is to consider a parcel 

of air near the ground, whose temperature 

equals the daily maximum surface temperature. 

The parcel is lifted dry-adiabatically until it 

reaches the ambient air temperature, which is 

measured from a radiosonde observation 

(RAOB). The level at which the temperature of 

the lifted parcel equals the ambient air 

temperature is defined as the mixing height. 

Above the mixed layer, further lifting of the 

parcel will cause it to become cooler than 

ambient air temperatures.

Afternoon RAOBs were used to approximately 

coincide with the time of maximum daily surface 

temperature. Afternoon RAOBS occur near 0000 

Greenwich Mean Time (GMT), which coincides 

with 1600 Pacific Standard Time (PST) and 

1700 Mountain Standard Time (MST) in the 

Columbia River basin. Data included about 

1,000 RAOBs taken from 1966 to 1989. Mixing 

heights were calculated by the Western 

Regional Climate Center. The stations analyzed 

included Quillayute, Washington (58 meters 

elevation), the only station on the Pacific coast; 

Salem, Oregon (61 meters), and Medford, 

Oregon (421 meters), between the coast and 

Cascade Ranges; Spokane, Washington



(722 meters) and Boise, Idaho (871 meters), 

inside the basin; and Winnemucca, Nevada 

(1310 meters), just south of the basin in northern 

Nevada.

In winter, mixing heights remain below about 

300 meters in most places (fig. 2). During 

spring, Winnemucca begins to "mix out" first 

because the air is not constrained by basin 

topography (fig. 3). During summer (fig. 4) 

Winnemucca experiences consistently high 

mixing heights because the summer sunshine 

efficiently warms the inland continent. Within the 

basin, the mean summer mixing height is about 

1800 meters at Spokane and Boise. The range 

of mixing heights, however, includes levels 

below 900 meters, especially at Boise where 

topographic constraints from the

Snake River valley are even more dominant 

than the overall basin topography. Stations west 

of the Cascade crest are influenced by frequent 

intrusions of marine clouds that increase 

atmospheric stability. This mostly is observed at 

Quillayute on the Washington coast. In autumn, 

mixing heights frequently drop below 900 meters 

at west-side and basin interior stations (fig. 5). 

They also begin to lower at Winnemucca.

To illustrate potential areal extent of stagnant 

air, constant height levels of 1000 meters and 

1400 meters were plotted over the basin 

topography. The first plot (fig. 6) shows where 

stagnant air may occur if the mixing height is 

constant at 1000 meters, near the lowest 

summer mixing heights. Stagnant air is confined 

to the central plateau of

Figure 3-Distribution of mixing 

heights in April for Quillayute 

(uil); Salem (sle); Medford (mfr); 

Spokane (geg); Boise (boi); and 

Winnemucca (wmc). Height 

level categories are in 

increments of 150 meters above 

ground level (AGL). Mixing 

heights were calculated in feet 

AGL then converted to 

approximate values in meters.

Figure 4-Distribution of mixing 

heights in July for Quillayme 

(ml); Salem (sle); Medford 

(mfr); Spokane (geg); Boise 

(boi); and Winnemucca (wmc). 

Height level categories are in 

increments of 150 meters 

above ground level (AGL). 

Mixing heights were calculated 

in feet AGL then converted to 

approximate values in meters.
Figure 5–Distribution of mixing height in October for Quillayute (uil); Salem 

(sle); Medford (mfr); Spokane (geg); Boise (boi); and Winnemucca (wmc). 

Height level categories are in increments of 150 meters above ground level 

(AGL). Mixing heights were calculated in feet AGL then converted to 

approximate values in meters.

the basin and the lower Snake River valley. If 

the mixing height is constant around 1400 

meters (fig. 7), there is relatively good 

dispersion in the central plateau of the basin. 

High basins and some high valleys in Oregon 

and western Montana may trap air, however, 

and cause pockets of stagnation.

Rain Days per Month

Airborne pollutants may fall out of the 

atmosphere by attaching to precipitating 

particles. Liquid precipitation (rain) is more 

efficient at scavenging gas and particles than is 

solid precipitation (snow, hail, etc.). A simple 

analysis of rain days per month therefore was 

conducted to help determine the frequency of 

wet deposition onto plants and into soils and 

snow cover.

To determine rain days in wildland regions, data 

were selected from all National Weather Service 

(NWS) cooperative observation station sites 

above 900 meters elevation. Days of rain were 

defined as those days with measured 

precipitation and mean temperatures greater 

than 5 °C. When near-surface air temperatures 

are above 5 °C, 100 percent of observed 

precipitation is rain (Ferguson 1994, Ferguson 

and Breyfogle 1994). Because mean 

temperature was used, there may be some 

periods during the day when temperatures are 

lower and snowfall may occur, but these should 

be rare. This definition omits days with cold rain, 

mixed periods of rain and snow, and purely 

snow.





cance of frontal and orographic precipitation 

begins to increase but snow also may occur. In 

1982, most of the mountain sites showed more 

than 25 percent of the days with rain. Cooler 

than normal seasonal temperatures also may 

have caused days with snow. In 1988, a dry 

year, few stations measured more than a few 

days of rain. Most of those occurred in Idaho 

and western Montana where convection 

probably remained important. During a typical 

year (fig. 11), most stations observe about 25 

percent of days with rain in October.

Upper Level Winds

Winds in the upper atmosphere may carry 

buoyant pollutants long distances. The 

Columbia River Basin Scientific Assessment 

team were interested specifically in the 

possibility of pollutants from the basin reaching 

the Grand Canyon. This could occur if upper 

level winds over the basin are strong from the

north.

The distribution of wind speed and direction 

was calculated by the Western Regional 

Climate Center. Winds at the 

700-hectoPascals (hPa) level were analyzed, 

because that level usually is above the 

influence of terrain surrounding the basin 

(about 3000 meters) and winds there are most 

likely to carry pollutants out of the basin.

At Spokane, the mean winter 700-hPa wind 

direc tion is westerly with a normal variation 

between SSW-W-NNW (fig. 12). Nine percent of 

the winds have a northerly component with 

speeds greater than 11 meters per second. 

Spring winds are highly variable with mean 

directions between SW and WSW. Twelve 

percent of the winds have a northerly 

component but all are less than 9 meters per 

second and most are less than 2 meters per 

second. A similar wind distribution occurs 

during summer (fig. 13) with a little more preference to SW and WSW directions. In autumn, 

prevailing winds begin to shift back to westerly. 

Four percent of autumn winds have northerly 

components greater than 11 meters per 

second.

Only data from 1982, 1988, and 1989 were considered. These years were chosen by the 

Columbia River Basin Scientific Assessment 

Team as charac teristic of cool and wet, warm 

and dry, and normal climatic conditions, 

respectively. A more complete analysis would 

benefit from a long period of spatially distributed 

observations, which were not available at the 

time of this study.

In January, there are few days of midwinter rain 

at elevations above 900 meters. Although 

significant precipitation occurred during winter 

1982, most fell as snow with rain confined to 

lower elevations in eastern Oregon and western 

Idaho. Only a small amount of precipitation 

occurred in January 1988, so a similar pattern 

of rain days occurred, with some rain at higher 

elevations in central Idaho and western 

Montana. The normal year of 1989 (fig. 8) again 

showed few midwinter rain days; those that 

occurred were mostly at lower elevations.

The number of rain days per month becomes 

greater than snow days per month as seasonal 

temperatures increase. Slightly fewer stations 

observed rain during April in 1982 than in other 

years because snowfall continued through 

spring that year. In 1988, several stations 

throughout the basin experienced 50 to 75 

percent days with spring rain. In 1989, most 

stations experienced at least 25 percent days 

with rain, which is typical of spring (fig. 9).

Summer precipitation in the basin is 

dominated by atmospheric convection. During 

wet years, such as 1982, a large number of 

stations experience more than 50 percent 

days with rain, especially in places where 

summertime convection dominates 

precipitation. In 1988, no station measured 

significant precipitation as dryness pervaded 

the basin. During a normal year (fig. 10) 

typical summer patterns of precipitation 

prevail with most rain days occurring in 

places where convection is common, such as 

eastern Idaho and western Montana.

During autumn, convection remains important in 

precipitation distribution. In addition, the signifi-
Upper level winds at Boise have a slightly 

different distribution. This may be because the 

Snake River valley, which is oriented northwest 

to southeast, may influence the winds, even 

those well above the surrounding topography. 

Over Boise, winter winds prevail from the WNW

and 15 percent have a strong (greater than 11 

meters per second) northerly component (fig. 

14). Spring winds prevail from the W, WNW, to 

NW, and 9 percent have strong northerly 

components. In summer, winds over Boise 

prevail from the WSW, and 14 percent of the 

winds have a northerly component, but all are 

less than 9 meters per second (fig. 15). In 

autumn, westerly winds dominate. Seven 

percent have a strong northerly component.

These results suggest that strong northerly 

winds are most common during winter when 

there is very little biomass burning. The 

northerly winds could scour away pollution 

trapped under the basin's frequent winter 

inversion, however. Whether the scoured 

components can reach the Grand Canyon or 

deposit along the way is unknown. Strong

northerly winds are possible during spring and 

autumn burning seasons but are rare. Summer 

northerly winds usually are too weak to 

transport material for long distances.

Surface 

Winds near Earth's surface are most efficient at 

transporting nonbuoyant or neutrally buoyant 

pollutants. They can carry smoke from burning 

biomass into nearby towns and cities. In 

addition, surface winds can carry pollutants from 

industrial sources into wildland areas.

Only a few observations are needed to 

determine the characteristics of upper level wind, 

because atmospheric patterns usually change 

slowly over space and time above the influence 

of topography. Near the ground surface, 

however, winds are strongly influenced by small 

undulations in topography. There are not enough 

observations of surface wind to show the true 

variation in wind.

 A simple mesoscale wind model therefore was 

adapted to analyze the effect of surface wind on 

pollution transport.'

 Surface wind during winter is controlled 

primarily by pressure gradient forces between a 

persistent region of high pressure over the 

continent and frequent low centers from 

approaching Pacific storms (fig. 16). This 

pressure gradient causes strong easterly winds 

to persist through the Cascade mountain 

passes. It also causes much of the stagnant air, 

which is trapped under the frequent winter 

temperature inversion, to be pulled against the 

eastern slopes of the Cascades. The passing 

storms also cause winds to accelerate over the 

higher ridge tops, especially in the Rocky 

Mountains. Converging winds are common, as 

in the lee of the Blue Mountains and around the 

head of the Snake River. Stagnant winds also 

occur, usually below the persistent inversions in 

the central Snake River valley and central 

Columbia plateau.

During summer (fig. 17), surface winds are 

dominated by downslope flows caused by 

cooling air that drains into the valleys at night. 

An onshore flow also prevails, causing weak to 

moderate westerly winds through the Cascade 

passes. Areas of weak convergence or 

stagnation occur most frequently in valleys and 

basins where the downslope winds are trapped 

by surrounding topography.

Drought

Drought stress causes plants to close stomata 

and become less susceptible to pollution. 

Drought in the Columbia River basin is common 

but tempered by an abundant source of 

moisture that spills over the Coast Ranges and 

through mountain gaps from the Pacific Ocean. 

Because most rainfall accumulates during 

winter and spring in the basin, and because 

much of the summer water resource relies on 

winter snowpack, the most severe droughts 

occur with abnormally low winter and spring 

rainfalls (Namias 1983).

'Ferguson, S.A.; Peterson, M.R.; Hayes, P.S.; Ruthford, J.: Surface wind 

patterns in the Pacific Northwest. Manuscript in preparation.























To help determine the spatial pattern of drought 

events, Palmer drought severity index (PDSI) data 

(Heddinghause and Sabol 1991, Palmer 1965) 

were obtained from the National Climate Data 

Center (NCDC). The index generally ranges from 

-6.0 to +6.0, with negative values denoting dry 

spells and positive values indicating wet spells. 

Drought frequency patterns for each season were 

created by counting the number of times from 1895 

to 1994 that the PDSI was less than -3 (severe 

drought) in each climate division, a region specified 

by the National Weather Service. Maps of the 

drought frequency for each season are shown in 

figure 18.

During winter (fig. 18a), drought is most common in 

places having heavy winter precipitation only when 

the jet stream and related storm tracks are 

positioned optimally. For example, winter drought is 

common at the east end of the Columbia Gorge 

(the coastal barrier's most significant gap), the Blue 

Mountains (which rely on abundant moisture from 

the southwest), and some of the lower elevation 

inland areas in Idaho, western Montana, and 

Wyoming.

During the spring-and-summer growing season 

(fig. 18b and c), convective precipitation becomes 

important. Because convection is highly variable 

from year to year, those places that depend on 

convective precipitation have more frequent 

periods of drought. This includes most of the 

basin, except for some of the mountain areas near 

the Cascades, which benefit from spillover Pacific 

moisture, and places in northern Washington and 

Idaho, which collect moisture from a few storms 

that pass over Canada during summer. Winds at 

the head of the Snake River commonly converge, 

and this helps to enhance lifting and precipitation 

there.

In autumn (fig. 18d), the winter storm tracks begin 

to establish. This helps to relieve much of the 

northwestern and mountainous parts of the basin 

from further drought. The south and southeast 

portions, however, retain some reliance on the 

highly variable convective precipitation; these 

regions of the basin experience drought whenever 

autumn convection is minimal.

Implications of Global Change 

on Air Quality in the Basin

Atmospheric concentrations of greenhouse gases 

are increasing. Major carbon constituents that 

contribute to the greenhouse effect are increasing at 

the following known rate: carbon dioxide (CO2)—-0.4 

percent or 1.5 parts per million (ppm) per year; 

methane (CH4)—-0.9 percent or 0.015 ppm per year; 

chlorofluorocarbons (CFCs)—4.0 percent or 0.015 

ppm (Houghton and others 1990, 1992). A recent 

report by the USDA Forest Service, Pacific Global 

Change Research Program, summarizes the 

effective sources and sinks of greenhouse gases 

and discusses possible implications for management 

(Bytnerowicz 1997). For example, major sources for 

CO2 are listed as decomposition, respiration, 

biomass fires, volcanic emissions, fossil fuel burning, 

and changes in land use. It also is recognized, 

however, that more nitrogen can produce a 

cofertilization effect in the terrestrial biomass, which 

leads to sequestration of additional carbon. In 

addition, a discrepancy in the global nitrous oxide 

budget is attributed to emissions from forest soils, 

but proper management practices could minimize 

this. The report also explains that sources of 

atmospheric methane include cattle, wetlands, and 

termites. Like CO2 and nitrous oxides, CH4

emissions can be changed by altering management 

practices.

Another implication of increasing greenhouse gases 

is their possible effect on global climate. Although 

uncertainty remains in the rate and magnitude of 

expected change, a general warming trend due to 

increasing concentrations of greenhouse gases is 

almost certain. A climate change scenario that 

considers the regional effect of doubling the global 

atmospheric concentrations of carbon dioxide 

(2xC02) on regional climate is discussed in Ferguson 

(1997). This report and
Conclusion

The Columbia River basin experiences air 

pollution patterns similar to typical basins. For 

example, temperature inversions, which can 

trap pollution in stagnant air near the ground, 

are common. The basin's topography and the 

influence of three distinct air mass types, 

(marine, arctic, and continental), however, 

create a unique pattern of climate that can 

influence the trajectories of polluting gases and 

particles. Winds through the mountain gaps 

can transport pollution into and out of the 

basin, or scour away stagnant air trapped in 

the basin. Rainfall is common enough to cause 

relatively efficient scavenging of airborne 

pollution during all seasons.

A few, relatively simple, analytical tools were 

used to show the general pattern of climate in 

the basin that influences air pollution. More 

comprehensive analyses are possible with 

mesoscale meteorological models and 

atmospheric dispersion and deposition models. 

These tools could help to show greater details in 

the timing and extent of stagnation periods, 

distance and concentration of pollution gases 

and particles, and pollution distribution patterns 

under different climate and management 

scenarios. Until more comprehensive tools become available at landscape-level scales, 

however, this cursory analysis may provide a 

reasonable foundation for understanding the 

strategic role of climate in controlling the impact 

of air pollution on natural resources.

reports from the Intergovernmental Panel on 

Climate Change (Houghton and others 1990, 

1992) are summarized below in a list of 

possible effects of global climate change on 

regional air pollution:

1. Greatest warming over high-latitude continents: less intense arctic influence and 

thus weaker temperature inversions during 

winter.

2. Continents could warm more than oceans:

higher summer mixing heights.

3. Decreased snow cover: less intense 

winter stagnation and earlier seasonal 

discharge of pollutants held in the 

snowpack.

4. Increased convection over continents: more 

wet deposition in spring and summer with 

less frequent drought in southeast portions of 

the basin.

5. Decreased soil moisture during summer: 

greater summer drought, especially in 

the central basin.

6. Fewer, but stronger, winter cyclones: less 

frequent but more significant disruption of 

winter stagnation.
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assessment. Cambridge, England: Cambridge University Press. 364 p.

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