Weather & Climate

The Monument lies within the semi-arid shrub-steppe Pasco Basin of the Columbia Plateau in southeastern Washington State. The region's climate is greatly influenced by the Pacific Ocean, the Cascade Mountain Range to the west, and other mountain ranges located to the north and east. The Pacific Ocean moderates temperatures throughout the Pacific Northwest, and the Cascade Range generates a rain shadow that limits rain and snowfall in the eastern half of Washington State. The Cascade Range also serves as a source of cold air drainage, which has a considerable effect on the wind regime on the Hanford Site. Mountain ranges to the north and east of the region shield the area from the severe winter storms and frigid air masses that move southward across Canada.

Prevailing wind directions near the surface on most of the Monument are from the northwest in all months of the year; however, winds from the northwest occur most frequently during the winter and summer. Winds from the southwest also have a high frequency of occurrence on the Monument, especially in the southeastern quadrant. During the spring and fall, there is an increase in the frequency of winds from the southwest and a corresponding decrease in winds from the northwest.

Monthly average wind speeds at 50 feet above the ground are lower during the winter months, averaging six to seven miles per hour (mph), and faster during the summer, averaging eight to nine mph. The fastest wind speeds on the Monument are usually associated with flow from the southwest. However, the summertime drainage winds from the northwest out of the Cascade Mountains frequently exceed speeds of 30 mph. The maximum speed of the drainage winds (and their frequency of occurrence) tends to decrease as one moves toward the southeast across the Monument.

Temperatures on the Monument vary considerably from season to season. The average monthly temperatures on the Monument’s lower levels range from a low of 31° F in January to a high of 76° F in July. The highest winter monthly average temperatures were 44° F in February 1958 and February 1991, and the lowest average monthly temperature was 12° F in January 1950. The highest monthly average temperature was 82° F in July 1985 and the lowest summer monthly average temperature was 63° F in June 1953. Daily maximum temperatures on the Monument vary from an average of 35° F in late December and early January to 96° F in late July. There are, on average, 52 days during the summer months with maximum temperatures of 90° F and 12 days with temperatures greater than or equal to 100° F. The greatest number of consecutive days on record with maximum daily temperatures of 90° F is 32. The record maximum temperature of 113° F occurred on July 13, 2002 and August 4, 1961. From mid-November through early March, the average daily minimum temperature is below freezing; the daily minimum in late December and early January is 21° F. On average, the daily minimum temperature of 0° F or below occurs only three days per year; however, only about one winter in two experiences such low temperatures. The greatest number of consecutive days on record with minimum daily temperatures of 0° F or below is 11. The record minimum temperature of -23° F occurred on both February 1 and 3, 1950.

The Monument is generally quite arid. The annual average relative humidity is 55%. It is highest during the winter months, averaging about 76%, and lowest during the summer, averaging about 36%. The annual average dewpoint temperature on the Monument is 34° F. In the winter, the dewpoint temperature averages about 27° F, and in the summer it averages about 43° F.

Average annual precipitation on the Monument’s lower levels 6.8 inches. In 1995, the wettest year on record, 12.3 inches of precipitation was measured; in 1976, the driest year, only 3.0 inches was measured. The wettest season on record was the winter of 1996-1997 with 5.4 inches of precipitation; the driest season was the summer of 1973 when only 0.03 inches of precipitation was measured. Most precipitation occurs during the late autumn and winter, with more than half of the annual amount occurring from November through February. Days with greater than 0.50 inches of precipitation occur on average less than one time each year.

Average snowfall ranges from 0.1 inch in October to a maximum of 5.2 inches in December, decreasing 0.5 inches in March. The record monthly snowfall of 23.4 inches occurred in January 1950. The seasonal record snowfall of 56.1 inches occurred during the winter of 1992-1993. Snowfall accounts for about 38% of all precipitation from December through February.

Fog has been recorded during every month of the year on the Monument; however, 89% of the occurrences are from November through February, with less than 3% from April through September. The average number of days per year with fog (visibility of six miles or less) is 48, while those with dense fog (visibility of 0.25 miles or less), is 25. The greatest number of days with fog was 84 days in 1985-1986, and the least was 22 in 1948-1949. The greatest number of days with dense fog was 42 in 1950-1951, and the least was nine days in 1948-1949. The greatest persistence of fog was 114 hours in December 1985, and the greatest persistence of dense fog was 47 hours in December 1957.

Other phenomena causing restrictions to visibility (i.e., visibility of six miles or less) include dust, blowing dust, and smoke from field burning. There are few such days; an average of five days per year have dust or blowing dust, and less than one day per year, on average, have visibility reduced from smoke to less than six miles.

Concerns about severe weather usually center on hurricanes, tornadoes, and thunderstorms. Fortunately, Washington does not experience hurricanes. In addition, tornadoes are infrequent and generally small in the northwestern portion of the United States. The National Climatic Data Center maintains a database that provides information on the incidence of tornados reported in each county in the United States. This database reports that in the ten counties closest to the Monument (Benton, Franklin, Grant, Adams, Yakima, Klickitat, Kittitas, and Walla Walla Counties in Washington and Umatilla and Morrow Counties in Oregon), there have been only 22 tornados recorded since 1950. Of these, 15 tornadoes had maximum wind speeds estimated to be in the range of 40 to 72 mph, four had maximum wind speeds in the range of 73 to 112 mph, and three had maximum wind speeds in the range of 113 to 157 mph. There were no deaths or substantial property damage (in excess of $50,000) associated with any of these tornadoes.

For a five-degree block centered at 117.5° west longitude and 47.5° north latitude (the area in which the Monument is located), the expected path length of a tornado is five miles, the expected width is 312 feet, and the expected area is about one square mile (Ramsdell and Andrews 1986). The estimated probability of a tornado striking a point on the Monument is less than one thousandth of one percent per year. However, the Monument could be impacted by extreme winds generated by a tornado in the nearby vicinity.

The average occurrence of thunderstorms in the vicinity of the Monument is 10 per year. They are most frequent during the summer; however, they have occurred in every month. Thunderstorms can generate high-speed winds and hail. Using the National Weather Service criteria for classifying a thunderstorm as “severe” (i.e., hail with a diameter of 3/4 inch or greater or wind gusts of 58 mph), only 1.9% of all thunderstorm events surveyed on the Monument have been “severe” storms, and all met the criteria based on their wind gusts. High-speed winds on the Monument are more commonly associated with strong cold frontal passages. In rare cases, intense low-pressure systems can generate winds of near hurricane force.