Oregon Climate Zone Summary

Zone 2 -- The 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, but 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.

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. Figure 1 shows a plot of 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.

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).

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.

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. During the afternoon, humidities are generally lowest, ranging from 70-80 percent during January to 30-50 percent during summer. Annual pan evaporation is about 35 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.

Table 1 is a summary of average precipitation conditions at selected stations in the Willamette Valley, while temperature conditions are summarized in Table 2. NOAA stations in operation during the 1961-1990 period are shown in Figure 1. Median dates of low temperature thresholds in spring and fall are listed in Table 3, and Table 4 gives the length of the growing season at each station.