Clackamas County lies in between northwest Oregon near the Washington Border and the northern part of the Cascade mountain range. It is within both Climate Division 2 (Willamette Valley) and Climate Division 4 (Northern Cascade) established by the National Climatic Data Center. Below is a description of the climate of Division 2 and 4, followed by specific descriptions of Clackamas County. Climate tables for various parameters, as observed at long-term climate stations in Clackamas 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 Clackamas 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 2a and 2b 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.

Climate Division 4 -- Northern Cascade

The Cascade Mountains, the dominant terrain feature in Oregon, encompass the entire length of the state from the California border to Washington. With average elevations in excess of 4,000 feet, the Cascades are crowned with a number of very high peaks. Mount Hood, near the Washington border, exceeds 11,000 feet, while Mt. Jefferson and the Three Sisters exceed 10,000 feet. Mt. McLoughlin near Medford is approximately 9,500 feet. The Cascades are a higher and more imposing topographic feature in the northern part of Oregon, however. Average elevations and the number of tall peaks (over 9,000 feet) are higher north of about 43.5 deg N latitude. The region extending northward from this latitude to the Columbia River and encompassing high elevations west of the Cascade crest is the fourth of nine Oregon climatic zones.

The northern Cascades exert a profound effect on Oregon climate and weather. Mid-latitude storms approaching from the west are forced to rise as they encounter the Cascades, resulting in large amounts of orographic (terrain-induced) precipitation on the western slopes. So effective are the Cascades in removing moisture from the Pacific air masses, however, that most of Oregon east of the Cascades lies in a "rain shadow," resulting in large areas with annual precipitation less than 12 inches. Most of the northern Cascades, on the other hand, receive an excess of 80 inches per year; the highest peaks collect more than 150 inches per year, most of it in the form of snow. As in the case of rest of western Oregon, most of the precipitation in the Northern Cascades falls during the winter months with November through March period accounting for more than 75 percent of the total annual precipitation. Spring and fall rain and snow and summer thunderstorms contribute to the annual precipitation total, but they are dwarfed by the winter precipitation totals. Table 1 lists monthly and annual normal precipitation at Zone 4 sites.

Tables 2a and 2b list the average number of days with precipitation amounts exceeding certain thresholds.

Table 4 lists average monthly and annual snowfall totals for various stations. Monthly mean snowfall totals vary significantly according to elevation. Since precipitation tends to increase with increasing elevation, more potential moisture for snowfall is available at higher elevations. Since temperatures generally decrease with increasing elevation, those high precipitation amounts are more likely to be in the form of snow. As an example, McKenzie Bridge (elevation 1400 feet) receives an average of about 42 inches snow per year, while Marion Forks (2,500 feet) receives about 150 inches and Government Camp (3,980 feet) about 300 inches per year. Figure 1 shows NOAA climate stations in Zone 4, which were in operation during the 1961-90 period. Figure 2 shows the Clackamas County region from the Oregon annual precipitation map.

Table 3 lists normal monthly temperature at stations in the area. The correlation of temperature with elevation is quite strong, with the highest station (Government Camp) having consistently lower temperatures than the other sites. McKenzie Bridge has by far the highest annual mean maximum temperatures, but its annual average temperature is only slightly higher than Detroit Dam due to lower minimum temperatures at McKenzie.

Table 5 & 6 list median frost dates and mean growing seasons, respectively, for four different temperature thresholds. Detroit Dam, at an elevation of 1,220 feet, has an exceptionally long growing season. This is probably due to the fact that its location above the valley floor prevents significant accumulation of cold air on clear nights, and the presence of nearby Detroit Lake serves to moderate any low temperatures. The growing season at higher elevation sites such as McKenzie Bridge, Marion Forks, and Belknap Springs is only about 50 percent as long as at Detroit: for example, Marion Forks at 2,480 feet has an average of only 116 days between occurrences of 32deg F temperatures compared with 244 days at Detroit Dam.

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

County Description

Established: July 5, 1843
Population: 345,150
Area: 1,879 sq. mi.
Economy: Agriculture, metals manufacturing, trucking and warehousing, nursery stock, retail services, wholesale trade and construction.
County Seat: Oregon City

Clackamas County was named for the resident Clackamas Indians and was one of the four original Oregon counties created in 1843. Oregon City, the county seat, was the first incorporated city west of the Rockies, the first capital of Oregon Territory, and the site of the first legislative session. Oregon City was also the site of the only federal court west of the Rockies in 1849, when the city of San Francisco was platted. The plat was filed in 1850 in the first plat book of the first office of records in the West Coast and is still in Oregon City. The area's early history is featured at the End of the Oregon Trail Interpretive Center, a living history museum on an 8.5 acre site with three 50-foot high covered wagon-shaped buildings, an outdoor amphitheater and heritage garden. From its 55-foot elevation at Oregon City, the county rises to 11,235 feet at the peak of Mt. Hood, the only year-round ski resort in the United States and the site of Timberline Lodge National Historical Landmark. The mountain, rivers and forests offer excellent outdoor recreation activities, from skiing and rafting to fishing and camping. The Clackamas Town Center, one of the largest shopping malls in Oregon, is the hub of eastside business.

Climate Tables (Clackamas County, Oregon)