The Rain Shadow
Oregon’s location equidistant from the equator and the North Pole subjects it to weather from both tropical and polar air flows. This makes for a pattern of changeability in which calm often alternates with storm, and extreme heat and extreme cold seldom last long.
Oregon’s weather system is best understood as a series of valley climates separated from each other by mountain ranges that draw precipitation from the eastbound weather systems. Moving west to east, each of these valley zones records progressively lower rainfall levels until one encounters a desert on the eastern side of the state.
Moisture-laden westerlies off the Pacific slam into the Coast and Klamath Ranges. As the mountains push the clouds higher, they drop their moisture in the form of rain or snow. That’s because rising air cools 3°F for every 1,000 feet of altitude gain, and cooler air can’t hold as much moisture as warm air. As a consequence, rainfall at the coast often exceeds 80 inches per year. In parts of the coastal ranges, yearly totals of well over 100 inches aren’t uncommon.
By contrast, the Willamette and other inland valleys on the east side of the mountains usually record half that total.
The rain shadow effect is repeated when the Cascades catch precipitation from eastward-moving cloud masses, wringing the moisture out of the storms; consequently, the eastern side of this range often records annual rainfall totals below 10 inches.
Wet but mild, average rainfall on the coast ranges from a low of 64 inches per year in the Coos Bay area to nearly 100 inches around Lincoln City. The Pacific Ocean moderates coastal weather year-round, softening the extremes. Spring, summer, and fall generally don’t get very hot, with highs generally in the 60s and 70s, and seldom topping 90°F. Winter temperatures only drop to the 40s and 50s, and freezes and snowfall are quite rare occurrences.
If there is one constant in western Oregon, it is cloudiness. Portland and the Willamette Valley receive only about 45 percent of maximum potential sunshine; more than 200 days of the year are cloudy, and rain falls an average of 150 days. While this might sound bleak, consider that the cloud cover helps moderate the climate by trapping and reflecting the earth’s heat. On average, fewer than 30 days of the year record temperatures below freezing. Thus the region, despite being on a more northerly latitude than parts of Canada, has a milder climate. Except in mountainous areas, snow usually isn’t a force to be reckoned with. Another surprise is that Portland’s average annual rainfall of 40 inches is usually less than totals recorded in New York City, Miami, or Chicago.
The southern end of the Willamette Valley can be affected by temperature inversions. In winter, for example, warm air above the valley walls holds in the colder air below, resulting in fog. In the southern valleys, fog helps to counterbalance the region’s long dry season: Ashland and Medford sometimes record only half the yearly precipitation of their neighbors to the north, as well as higher winter and summer temperatures. At the same time, these inversions can cause unwelcome pollution to linger.
While it’s difficult to predict daily weather patterns in western Oregon, there are definite seasonal climatic shifts. In winter, arctic and tropical air masses collide over the Pacific, producing much of the state’s rain. During the summer the clashes are less frequent. At that time, Oregon weather is more affected by Pacific Ocean temperatures and air pressure differences between inland and coastal areas.
By contrast, the scorching deserts of eastern Oregon can give way to cold temperatures at night. This is because clear skies and a dearth of vegetation facilitate the escape of heat. Consider that on May 2, 1968, the difference between the high and low temperatures at Juniper Lake, north of the Alvord Desert in southeastern Oregon, was 81 degrees.
Mountain areas also experience extreme diurnal temperature fluctuations. Thin mountain air does not filter out ultraviolet radiation as effectively as the denser air at lower elevations, so the sun’s force is accentuated at higher elevations. At night, chill spreads quickly through this thin air.
by Judy Jewell and W. C. McRae from Moon Oregon, 8th Edition, © Elizabeth & Mark Morris and Avalon Travel