Gilliam County lies in the north central part of Oregon along the Columbia River. It is wholly within Climate Division 6 (North Central Oregon) established by the National Climatic Data Center. Below is a description of the climate of Division 6 followed by specific descriptions of Gilliam County. Climate tables for various parameters, as observed at long-term climate stations in Gilliam County, are included below.

Climate Division 6 — North Central Oregon

North Central Oregon, climatic Zone 6, is a relatively dry region lying east of the Cascade Mountains. The Cascades serve as an effective moisture barrier, causing storms to dump much of their moisture west of the peaks and leaving areas to the east in a "rain shadow." As a result, Zone 6 is generally rather dry. The region extends from the Columbia River southward over hill country to the forested mountain areas, which border climate Zone 7. The Columbia is used in irrigation, transportation and hydroelectric power, and therefore dominates the area.

This region is Oregon's major wheat producing area. Grain production on dry land farms is the main source of agricultural income except for the Hood River Valley, which produces mostly tree fruits. Despite relatively small dimensions, the latter is one of the most important production areas in the Northwest. Its annual income of approximately $60 million derives mostly from pears, apples, and cherries. Other important commodities produced in Zone 6 include green peas, irrigated truck crops, beef cattle, sheep, alfalfa, and poultry.

Just as most of Oregon, this region has a definite winter rainfall climate. The months of November through February generally receive the most precipitation due to winter storms, which bring rain to lower elevations and snow to higher ridges and peaks. Annual totals vary greatly and are proportional to elevation; some of the lower elevations receive less than 12 inches per year, while a few of the higher areas receive more than 40 inches. Occasional summer thunderstorms bring localized, occasionally heavy showers.

Figure 1 shows NOAA climate stations in Zone 6, which were in operation during the 1961-1990 period. Figure 2 shows the Curry County region from the Oregon annual precipitation map. Table 1 lists normal monthly and annual precipitation for stations in Zone 6. Locations at the lowest elevations (adjacent to the Columbia) such as Arlington and Hermiston receive less than 10 inches per year. Precipitation increases steadily with elevation. Highest annual totals are found in the Blue Mountains along the extreme east border of the region, where totals exceeding 50 inches occur. Table 2 lists the average number of days with precipitation amounts exceeding certain thresholds.

The Columbia Gorge is a major east-west passageway connecting Zone 6 with the Willamette Valley and Oregon coast. Vigorous winds are common in and around the Gorge. During summer, wind direction is predominantly from the west, causing strong, steady winds within the Gorge and along the northern edge of Zone 6. These winds, in fact, make Hood River a world-renowned wind surfing location. Winter winds can blow from the west or the east and can reach speeds sufficient to cause widespread damage.

A major effect of the Gorge is a moderation of air temperatures near the Columbia by allowing maritime air to reach the area from the west; this can occur both in summer and winter. Occasionally, however, large-scale easterly flow brings very cold continental air to the region, resulting in extremely cold conditions. During such periods, the cold air passes westward through the Gorge, creating extreme conditions in the western valleys as well.

Table 3 lists normal monthly and annual temperatures in the region. Highest summer temperatures are observed at the low-lying points near the Columbia (i.e. Arlington, Hermiston, and Milton Freewater), while mean temperatures decrease with increasing elevation. Winter temperatures follow the same pattern with mildest temperatures at the lower elevation sites.

Table 4 lists average monthly and annual snowfall total for the various stations.

Median frost dates and length of the growing season are listed in Tables 5 and 6, respectively. These also follow the same elevation relationship evident in the temperature data: the longest growing seasons are in the mild and low elevation sites, while increasing elevation generally causes a shortening of the season. Arlington and Condon, both at nearly 3,000 feet above sea level, have much shorter growing seasons than lower sites such as The Dalles and Arlington.

Table 7 and 8 list average monthly and annual heating and growing degree days.

County Description

Established: Feb. 25, 1885
Population: 1,900
Area: 1,223 sq. mi.
Economy: Agriculture, recreation and environmental services.
County Seat: Condon

Gilliam County was established in 1885 from a portion of Wasco County and was named after Col. Cornelius Gilliam, a veteran of the Cayuse Indian War. The first county seat was at Alkali, now Arlington. At the general election of 1890, voters chose to move the county seat to Condon, known to early settlers as "Summit Springs." A brick courthouse was built in Condon in 1903 but was destroyed by fire in 1954. The present courthouse, built on the same site, was constructed in 1955. Gilliam County is in the heart of the Columbia Plateau wheat area. The economy is based mainly on agriculture, with an average farm size of about 4,200 acres. Wheat, barley and beef cattle are the principal crops. The largest individual employers in the county are two subsidiaries of Waste Management Inc., Chemical Waste Management of the Northwest and Oregon Waste Systems, Inc., two regional state-of-the-art waste disposal landfills. With elevations of over 3,000 feet near Condon in the south of the county and 285 feet at Arlington, 38 miles north, the county offers a variety of climates and atmosphere. Hunting, fishing and tourism are secondary industries. Two major rivers, the John Day and Columbia, traverse the area east-to-west, as well as Interstate 84. Highway 19 connects the county's major cities north-to-south and serves as gateway to the John Day Valley.

Climate Tables (Gilliam County, Oregon)