The physical geography of Oregon and Washington is unique, and gives a great variety of climates. Approaching from the Pacific, we find, first, a narrow skirting of coast, from one to six miles in width. Back of this rises the Coast Range of mountains, from three to five thousand feet high. Beyond this range are fine, level prairies, extending for from forty to sixty miles eastward. Beyond these prairies rises again the Cascade Range, from five to eight thousand feet in height, and having to the east of them high, rolling prairies, extending to the base of the Blue Mountains, which trend south-westwardly, leaving plains and small valleys, to the east, between themselves and the Snake River, which forms the eastern boundary of Oregon and a portion of Washington
These differences in altitude would, of themselves, produce differences in temperature. But the great reason why the change is so great from the coast to the Snake River lies in the arrangement of the mountain-ranges; and in the fact that the north-west shore of the American continent is washed by a warm current from the China seas. The effect of this current is such that places in the same latitude on the Atlantic and Pacific coasts are several degrees—sometimes twenty degrees—warmer on the latter coast, than on the former. This gives a temperature at which great evaporation is carried on. The moisture thus charged upon the atmosphere by day, is precipitated during the cooler hours of night in fog, mist, or rain.
In summer, the prevailing wind of the coast is from the north-west, thus following the general direction of the shore-line. It naturally carries the sea-vapor inland; but the first obstacle encountered by these masses of vapor is a range of mountains high enough to cause, by their altitude and consequent lower temperature, the precipitation of a large amount of moisture upon this seaward slope. Still, a considerable portion of moisture is carried over this first range, and through the gaps in the mountains, and falls in rain or mist upon the level prairie country beyond. Not so, however, with the second, or Cascade Range. These mountains, by their height, intercept the sea-fog completely; and while great masses of vapor overhang their western slopes, on their eastern foot-hills and the rolling prairies beyond not a drop of dew has fallen. This is the explanation of the difference in climate, as regards dryness and moisture, between Eastern and Western Oregon and Washington. All other differences depend on altitude and local circumstances.
Notwithstanding the great amount of moisture precipitated upon the country west of the Cascades, the general climate may be said to be drier than on the Atlantic Coast. The atmosphere does not seem to hold moisture, and even in rainy weather its drying qualities are remarkable. Taken altogether, the stormy days in this part of Oregon and Washington are not more numerous than in the Atlantic States; but the rainy days are, because all the storms here are rain, with rare exceptions. The autumn rains commence, usually, in November—sometimes not till December—and the wet season continues until April, or possibly till May; not without interruptions, however, sometimes of a month, in midwinter, of bright weather. About the middle of June the Columbia River is high, and during the flood there are generally frequent flying showers. After the flood is abated, there is seldom any rain until September, when showers commence again, and prove very welcome, after the long, warm, but wholly delightful summer. The annual rain-fall of the Wallamet Valley ranges from thirty-five to fifty inches. In the Umpqua and Rogue River valleys it is less; and at the mouth of the Columbia, and along the coast, both north and south, it is more. At Steilacoom, on the Sound, it is fifty-three inches. From a weather-record of over ten years, kept at Portland, the lowest point in the Wallamet Valley, we clip the following summing up:
"Sixty-five per cent. of the above days were without rain or snow.
"Notes.—Ice formed December 2d, 1858. In 1859, ponds were frozen over at times till March 1st—ice never over two inches thick; very little cold weather in December, 1859; no ice to speak of.
"January 24th, 1860, the ground froze for the first time this winter—first ice January 26th. Ice and frost all gone February 1st. I planted potatoes February 6th; on the 17th planted onion-sets and onion-seeds; April 26th, planted corn.
"January 2d, 1862, Columbia River frozen so that the ocean steamer could not run; thermometer sixteen degrees below freezing-point. January 8th, snow a foot deep; excellent sleighing. On the 17th, Wallamet frozen hard enough to cross on foot. On 24th, ice gone out of Wallamet River. March 10th, snow all disappeared.
"January 7th, 1868, Columbia River closed with ice. On the 11th Wallamet closed over so as to stop the steamers running to Oregon City, until the 28th. No rain fell from the 1st of July to September 3d—63 days—and then none again till October 23d."
The librarian of the Portland Library Association furnishes us with the following, for the year 1870:
"Number of rainy days, 97; number of snowy days, 4; total rain-fall, 33.50 inches—equal to 2 feet 9 5-10 inches: total snow-fall, 10 inches."
The mean annual temperature at Corvallis, eighty miles south of Portland, is fifty-three degrees. At the mouth of the Columbia, it is fifty-two degrees two minutes; and at Steilacoom, fifty degrees eight minutes. Mean winter temperature of the Sound, forty-one degrees; mean summer temperature, sixty-two degrees.
East of the Cascades, the arrangement of the seasons is somewhat different. There is much less rain, which comes in showers, rather than in a steady fall; and is confined to the months between September and June. Occasionally, snow falls to the depth of a few inches; and in some winters has remained on the ground a number of weeks. The heat of summer and the cold of winter are each more extreme, but not at their highest or lowest degrees so trying as the same amount of heat or cold would be in a moister atmosphere. The autumn months in this portion of the county are most delightful, with the thermometer ranging from fifty-five degrees to seventy. The phenomenon of the plains is the periodical warm wind which comes from the south—perhaps from the California valleys—and blows over their whole extent, up to the forty-ninth parallel. The following is the mean temperature for the different seasons, as well as for the year:
A country like Eastern Oregon and Washington, without marshes or any local causes of miasma; with a clear, dry atmosphere, warmed by the sun, and cooled by the vicinity of snow mountains, could never be unhealthy. Only the most reckless disregard of health can occasion disease in a climate so naturally free from miasmatic poison as this. Western Oregon and Washington may be said to be equally healthful; with this difference, that during the rainy season those who are already invalids are liable to greater depression of the vital powers by reason of the continuous wet weather. Even a well person, of a peculiarly sensitive temperament, may suffer from the same cause. But the age attained by the original settlers of the country, shows well for the general salubrity of the climate of every portion of it. The rate of mortality for Oregon in 1860 was one in 173—the lowest death-rate in any of the States. In 1870 it was one in 155, or about the same as Minnesota.
One peculiarity of the climate of every part of Oregon and Washington, is, the comparative coolness of the nights. No matter how warm the days may have been, the nights always bring refreshing sleep, usually under a pair of blankets, even in summer. Nor does the heat, however great, have that fatal effect which it does in the Atlantic States. Not only men, but cattle and horses, can endure to labor without exhaustion in the hottest days of summer; and sun-strokes are of very rare occurrence.