Located in the rainshadow produced by the Cascades to the east and the Olympics to the south, the San Juans currently have an average rainfall of 29 inches per year (based on the recording station at Olga on Orcas Island, 1890 to present). However, being an average, various parts of the archipelago tend to differ from other parts, with the general rule being wetter in the north and drier in the south. On San Juan Island itself, for instance, precipitation ranges from a low of 19 inches per year at Cattle Point to the south to 29 inches per year at Roche Harbor to the north, with San Juan Valley averaging 25 in between. Elevation, of course, also makes a difference: the lowest lying lands on Lopez average 19, while 2407-foot high Mt. Constitution on Orcas has been known to get 45. During the historic record, however, average annual precipitation has varied from almost 38 inches in the wettest year, 1917, to 15 inches during the driest year of 1929. On average, the thirty-year period from 1891-1921 was wetter (31.04 inches) than the succeeding twenty-four years, 1921-1945 (26.33 inches). The period following World War II indicated a trend toward wetter and cooler years (Russell 1975; Schlots et al 1962).
The climate in the San Juans is generally mild, tempered by the surrounding sea waters and westerly winds. Due to the rainshadow, the number of days of sunshine is high compared to the surrounding region. Temperatures range from an average of 40 degrees F in the winter to 59 in the summer. San Juan Island has an historic average of 226 frost-free days (the ‘growing season’), although low-lying pockets have been known to experience freezing in July. However, because the majority of precipitation occurs during the winter months (70% from October through March), farms usually experience drought conditions during the summer, favoring either crops that need little water, farming in water-retentive soils, or irrigation.
Prevailing winds are generally from the south or southeast in the summer and west or northwest in the winter (Phillips 1966:12-13). However, the combination of mountain ranges to the east and west and sea level passages between Puget Sound and the ocean contribute to varying wind conditions in the county. One of the more prominent ‘exceptions’ are wintertime northeasters that bring frigid air down the Frazer River Valley and over the islands. Wind conditions in specific locales may also vary depending up the nearby topography of hills, valleys, and waterfront.
The climate was altered by the Native American, and later EuroAmerican, practice of burning prairies and forests to clear land. The Indians principally burned the open, savannah-like areas of prairie grass and Garry oaks to keep them open for cultivation of camas lilies and ease of hunting game such as deer. EuroAmericans burned standing trees, as well as stumps and slash, to clear the land for farming acreage.