Polk County lies along just off the northern part of the Oregon Coast. It is wholly within Climate Division 2 (Willamette Valley) established by the National Climatic Data Center. Below is a description of the climate of Division 2 followed by specific descriptions of Polk County. Climate tables for various parameters, as observed at long-term climate stations in Polk County, are included below.

Climate Division 2 -- Willamette Valley

The Willamette Valley is the most diverse agricultural area in the state of Oregon, and also the home of the majority of the population. Oregon's three largest cities, Portland, Salem, and Eugene, are located in the north, central, and south portions of the Valley, respectively. The urban areas are surrounded by varied and productive ranches, orchards, and farms. Among the crops grown in significant quantities are tree fruits, nuts, berries, mint, grains, and hay. Livestock operations are also common, including the dairy and poultry industries.

The climate of the Valley is relatively mild throughout the year, characterized by cool, wet winters and warm, dry summers. The climatic conditions closely resemble the Mediterranean climates, which occur in California, although Oregon's winters are somewhat wetter and cooler. Growing seasons in the Willamette Valley are long, and moisture is abundant during most of the year (although summer irrigation is common).

Like the remainder of western Oregon, the Valley has a predominant winter rainfall climate. Typical distribution of precipitation includes about 50 percent of the annual total from December through February, lesser amounts in the spring and fall, and very little during summer. Rainfall tends to vary inversely with temperatures -- the cooler months are the wettest, the warm summer months the driest. Figure 1 shows NOAA climate stations in Zone 2, which were in operation during the 1961-1990 period. Figure 2 shows the Polk County region from the Oregon annual precipitation map

There is considerable variation in precipitation in the Valley, ranging from annual totals below 40 inches in the Portland area to upwards of 80 inches in the Cascade and Coast Range foothills. Elevation is the single most important determinant of precipitation totals. Table 1 shows a plot of monthly & annual average precipitation versus elevation for stations in the Valley, and indicates a strong correlation between the two. Even in the lower sections of the Valley the effects of elevation are pronounced. Portland, for example, at 21 feet above sea level, receives an average of 37.4 inches (30-year normal), while Salem (196 feet) receives 40.4 inches and Eugene (359 feet) receives 46.0 inches. Thus, a change of only 338 feet of elevation produces an increase of 23 percent above Portland's total. Table 2 list the average number of days with precipitation amounts exceeding certain thresholds.

Table 3 lists normal monthly temperature at stations in the area. Extreme temperatures in the Valley are rare. Days with maximum temperature above 90 deg F occur only 5-15 times per year on average, and below zero temperatures occur only about once every 25 years. Mean high temperatures range from the low 80's in the summer to about 40 deg F in the coldest months, while average lows are generally in the low 50's in summer and low 30's in winter. The mean growing season (days between 32 deg F temperatures) is 150-180 days in the lower portions of the Valley, and 110-130 days in the foothills (above about 800 feet). Table 6 lists the mean growing season for Zone 2.

Although snow falls nearly every year, amounts are generally quite low. Valley floor locations average 5-10 inches per year, mostly during December through February, although higher totals are observed at greater elevations in the foothills. Table 4 lists average monthly and annual snowfall totals for various stations.

Table 5 lists the median frost dates for Zone 2. Severe storms are rare in the Valley. Ice storms occasionally occur in the northern portions of the Valley, resulting from cold air flowing westward through the Columbia Gorge. High winds occur several times per year in association with major weather systems.

Relative humidity is highest during early morning hours, and is generally 80-100 percent throughout the year. Humidity is generally lowest during the afternoon, ranging from 70-80 percent during January to 30-50 percent during summer. Annual pan evaporation is about 40 inches, mostly occurring during the period April - October.

Winters are likely to be cloudy. Average cloud cover during the coldest months exceeds 80 percent, with an average of about 26 cloudy days in January (in addition to 3 partly cloudy and 2 clear days). During summer, however, sunshine is much more abundant, with average cloud cover less than 40 percent; more than half of the days in July are clear.

Tables 7 and 8 list average monthly and annual heating and growing degree days, respectively.

County Description

Established: Dec. 22, 1845
Population: 63,600
Area: 745 sq. mi.
Economy: Agriculture, forest products, manufacturing, electronics and education.
County Seat: Dallas

Polk County was created from the original Yamhill district in 1845, by the Provisional Legislature. It was named for then President James Knox Polk. The first county seat was at Cynthia Ann. City officials later changed its name to Dallas, after Vice-President George M. Dallas, and moved the community about a mile to improve its water supply. The first courthouse was at Cynthia Ann. A second courthouse burned in 1898 and was replaced with the present building, built with sandstone quarried three miles west of Dallas. A three-story office annex was completed in 1966. Polk County Human Services was consolidated in the newly acquired Academy Building in 1989. Traveling back roads in Polk County will reveal many attractions, from covered bridges and pleasant parks to vineyards, wineries, and bed and breakfast lodgings spotting the surrounding hills. Many roads meander through beautiful fertile valleys from the Willamette River to the timbered foothills of the Coast Range. Polk County was the primary destination of early wagon trains which took the southern route to Oregon. Cities located in Polk County include Dallas, Independence, Monmouth, Falls City and portions of Salem and Willamina.

(County information obtained from Oregon Blue Book)

Climate Tables (Polk County, Oregon)