Oregon enjoys a mild, though varied climate with only a rare occurrence of devastating weather elements such as cloudbursts, tornadoes, or hailstorms severe enough to cause serious widespread damage. The single most important geographic feature of the climate of Oregon is the Pacific Ocean whose coastline makes up the western border. Because of the normal movement of air masses from west to east, most of the systems moving across Oregon have been modified extensively in traveling over the Pacific. As a result, winter minimum and summer maximum temperatures in the west, and to a lesser extent in the eastern portion, are greatly moderated. The occurrence of extreme low or high temperatures is generally associated with the occasional invasion of the continental air masses. The unlimited supply of moisture available to those air masses that move across the Pacific is largely responsible for the abundant rainfall over western Oregon and the higher elevations of the eastern portion.
Beginning near and following the coast the full length of the state, the Coast Range is the farthest west of the three mountain ranges that exert an important influence on Oregon's climate. This range rises to between 2,000 and 3,000 feet above sea level in the northern part of the state and between 3,000 and 4,000 feet in the southern portion with occasional peaks rising another 1,000 to 1,500 feet. This range, athwart the path of the moisture laden marine air moving in from the Pacific, forces it to rise as it moves eastward. The resultant cooling and condensation produces some of the heaviest annual rainfalls in the United States along the higher western slopes, and materially reduces the available moisture in the air.
The Cascade Mountains parallel the Coast Range about 75 miles to the east and to within 50 to 75 miles of the California border where the two ranges merge, forming a fairly broad, rugged mountain chain known as the Rogue River Mountains. The Cascades rise from the broad valley of the Willamette eastward to an average height of about 5,000 feet, with a few peaks over 10,000 feet. One of these, Mount Hood, at an elevation of 11,245 feet, is the highest point in the state. Once again, the air masses from the west are forced to ascend causing them to give up additional moisture. The rain potential of the marine air, however, was greatly reduced by passage over the Coast Range; therefore, the rainfall on the west slopes of the Cascades at a corresponding elevation is only about one-half to two-thirds as great as on the Coast Range. Precipitation amounts decrease rapidly once the crest is crossed and descent down the eastward side begins.
The Blue Mountains extend from the northeast corner southwestward to the valleys of the John Day and the Deschutes Rivers in central Oregon. Part of the chain projects southeast to the Snake River Valley, while in the northeast a separate branch is known as the Wallowa Mountains. These mountains, roughly between 5,000 and 6,000 feet with peaks from 7,000 to 9,000 feet, also exert an influence on the climate in the immediate area including several sizable valleys, particularly those of the Umatilla and Grand Ronde Rivers. However, the overall effect is much less than that of either the Coast of the Cascade Range. These mountains are the source of a number of small streams used for irrigation and power production. They are also in an area of fairly heavy snowfall which provides excellent skiing. The snowmelt maintains the steady and reliable water flow necessary for fine trout and salmon streams.
The Steens Mountains are a short range in the southeast part of the state less than 25 miles in length and only a very local climatic significance. The main crest is slightly more than 8,000 feet above sea level, with one peak of 9,354 feet. They serve as a snow shed that feeds several small streams useful to local irrigation. Most important of these is the Donner and Blitzen River.
The Columbia River is of vital economic importance to the state, since the large dams along its course generate most of the hydroelectric power in the Northwest. Its waters are used to irrigate several thousand acres of rich agricultural land in the Boardman and Portland areas. As a major waterway it carries millions of tons of shipping hundreds of miles inland each year. The ports of Vancouver on the Columbia, and Portland on the Willamette at its confluence with the Columbia, are among the finest and largest fresh water ports in the world. Cutting through both the Cascade and the Coast Ranges, the Columbia Gorge offers ready passage of marine air from the Pacific. Temperatures are moderated to the east in both summer and winter. Continental air occasionally passes in reverse and produces the more extreme low temperatures in the western valleys.
Winding through the rugged terrain that makes up much of Oregon are the Columbia and Snake River Basins, the valleys of the many streams that head in the mountains and several very wide plateau regions. The valleys, particularly those of the Columbia, Snake, Willamette, Rouge, and Hood Rivers, produce most of Oregon's agricultural wealth; however, the mountain and plateau regions are used extensively for livestock grazing and dryland farming. The Columbia Plateau covers about two-thirds of the state's total area and extends from the eastern border westward to the eastern slopes of the Cascade Mountains and from the southern boarder north to the Columbia River. Its elevations ranges from 4,000 to 6,000 feet and because of its arid nature and scant vegetation, summer heating and winter cooling often becomes extreme.
The state is divided into six major agroclimatic areas:
Few states have greater temperature extremes than Oregon where they have ranged from a low of 54° F below zero to a high of 119° F. Seldom, however, do daily extremes occur even closely approaching these absolute records. In 80 percent of recent years the highest temperature recorded in the state has not exceeded 114° F, nor was the absolute minimum lower than -37° F. In 50 percent of those years no temperature was recorded higher than 110° mark. Here the mean of the coldest month, January, is 45° F only 15° less than that of July, the warmest month. In the Willamette Valley few stations have had a maximum temperature greater than 98° F, or a minimum temperature lower than 16° F for over half of their years record. Temperature of 90° F or more, occur only about six to eight days a year and those below zero occur on an average of once every 25 years. Here the mean temperatures average 38° F in January and 66° F in July. In the inland valleys of the southwest the average summer temperatures are about 5° F higher than in the northwest and maximums of 90° F or more occur 40 to 50 days a year. In south-central Oregon the median annual maximum temperatures over a period of years have been between 95° F and 100° F, varying, of course, with the different stations; in most other areas east of the Cascades this variance is between 100° and 105° F. Median annual minimum temperatures for eastern Oregon vary from near zero in the more protected areas of the Columbia Basin to -26° F in the high mountain and plateau regions. The minima for majority of these stations, however, lies in the range of —1 to -10° F. The normal mean January temperature in southeast Oregon is 25° to 28° F and in the northeast 29° to 33° F; July normal means range between 65° and 70° F in the central valleys and plateau regions and 70° to 78° F along the eastern border.
The average annual rainfall in Oregon varies from less than eight inches in drier Plateau Regions to as much as 200 inches at points long the upper west slopes of the coast Range. Accordingly, vegetation ranges from the heavily wooded Coast Range and west slopes of the Cascades with their dense undergrowth to only a very sparse growth of sagebrush and desert type grasses over the wide plateau areas of central Oregon. This variation in precipitation also produces a similar irrigation projects in recent years have converted thousands of acres of semi desert areas into highly productive farmland.
The state as a whole has a very definite winter rainfall climate. West of the Cascades about one-half of the annual total precipitation falls from December through February; about one-fourth in the spring and fall and very little during the summer months. East of the Cascades the differences are not as pronounced with slightly more precipitation in winter than in spring and fall, while only about 10 percent falls during the summer. Along the coast the normal annual total is from 75 to 90 inches, and increases up the west slopes of the Coast Range to almost 200 inches near the crest. Amounts decrease on the eastern slopes and in the Willamette Valley. On the western slopes of the Cascades there is again a marked increase in precipitation with elevation as annual averages range up to 75 inches. Amounts decrease rapidly on the east side. The annual average precipitation with elevation as annual averages range up to 75 inches. Amounts decrease rapidly on the east side. The annual average precipitation for the great plateau of the state is often less than eight inches. In the Columbia River Basin and the Blue Mountains, totals are about 15 to 20 inches; however, some of the mountain regions receive as much as 35 inches.
In the high Cascades, where the state's heaviest snowfalls occur, there are few official observing stations. Considerable reliance must be placed on measurements obtained on various snow courses. It appears that annual average totals can range from 300 t 550 inches. A maximum annual snowfall of 879 inches and a snow depth of 242 inches has been officially recorded at Crater Lake National Park headquarters. Winter precipitation along the Coast Range, due to its lower elevations, occurs largely in the form of rain, although it too is occasionally subject to very heavy snows. In the Blue Mountains, seasonal totals range between 150 to 300 inches and depths on the ground may occasionally exceed 120 inches, but during most years the greatest recorded snow depths are less than 100 inches. The periods of continuous snow cover very with elevation. On the peaks of the Cascades higher than 7,000 feet above sea level it persists in glacial form the year around. In most mountain areas above 4,500 feet snow cover lasts from early December until the latter part of April. Snow courses aberages show that above 4,500 feet, snow depths (again varying with elevation) are approximately 50 to 100 inches in the Cascades, 25 to 65 inches in the Blue Mountains at the end of January; 60 to 125 inches and 25 to 70 inches, respectively, at the end of February; 75 to 135 inches and 25 to 80 inches at the end of March; 40 to 120 inches and five to 45 inches at the end of April.
Along the coast the average annual snowfall is only one to three inches, with many years in which there is no measurable amount. In the inland western valleys most yearly totals average between 10 to 15 inches, with snow on the ground seldom lasting more than two to three days at a time. In north-central Oregon the annual average is 15 to 30 inches, while over the higher plateau region that makes up the south-central portion snowfall ranges up to as much as 60 inches. In the valleys of the northeast 40 to 75 inches is normal, while in the Snake River Basin which makes up most of the southeast it is only 15 to 40 inches. Every few years some part of the state, (with the possible exception of the coastal areas), may be visited by heavy snowstorms which even in the Willamette Valley can produce 20 to 25 inches in a 24-hour period.
Hailstorms occur each year, but are generally light and cover very small areas. They cause thousands of dollars damage annually to crops with much lesser damage to buildings. Practically all of these storms occur east of the Cascades. In the western part of the state thunderstorms occur in the valleys an average of four to five days a year and are not usually severe. In the eastern part, they occur on 12 to 15 days with heavier precipitation and greater wind damage; however, total losses are not extensive. It is in the mountain areas that these storms occur most frequently and each year many forest fires are started by the accompanying lightning.
Several times each year winds of hurricane force (74 m.p.h. and over) strike the Oregon coast. They sometimes move inland to the western valleys and up the Columbia Gorge. At Portland, gusts with instantaneous speeds of 75 to 80 miles per hour are occasionally observed. Damage is usually confined to power and communication lives, to some crops, and to outdoor signs, and timber. Very rarely does loss of life or major structural damage to buildings result. The few tornadoes reported have been short lived. The prevailing wind direction is influenced by the surrounding terrain. In the Columbia Gorge, for example, the prevailing direction of the wind follows the orientation of the gorge at that point. Similarly, in the Willamette Valley prevailing directions are aligned north-south with the valley. The very strong winds, of course, are determined by the directions of the major storm movements. In reviewing the fastest wind on record for each month for a number of Oregon stations it was found that 60 percent of them were from the south or west. Light winds greatly outnumber the strong storms winds, and mountain slopes and other topographic features influence their direction.
Most of the state is drained into the Pacific Ocean through the Columbia River. Major tributaries to the Columbia include: the Willamette draining both the east slope of the Coast Range and the west slope of the Cascades; the Deschutes; the John Day; the Umatilla and the Snake Rivers. The Snake makes up more than half of Oregon's eastern border and drains practically all the state east of the Blue Mountains. The west slope of the Coast Range and all areas south of the Willamette Basin and west of the summit of the Cascades are drained directly into the Pacific Ocean by three large river systems — the Umpqua, Rogue and Smith Rivers along with a number of smaller coastal steams. The only major river draining south central Oregon is the Klamath. The remainder of the area lying south of the Deschutes and John Day Basins and between the Cascades and the Blue Mountains has only internal drainage into brackish lakes. Many of these lakes become dry during the summer months.
Major flooding in the Willamette Basin and the coastal steams usually results from several days of moderate to heavy rain extending over the entire Basin. When combined with sharply rising air temperatures and a warm southerly wind, the melting of a heavy snow pack on the middle and upper slopes of the Coast Range and/or the Cascades greatly increases the flood potential. Until recently, major flooding in the Willamette had an average frequency of about on in four years, and in the Coastal steams once in every two years. The construction of a number of large multiple-purpose dams in recent years on many of the larger tributaries of the Willamette has significantly reduced flooding.
Flooding in the main channel of the Columbia River usually occurs during late spring and early summer when snowmelt in the mountains is most rapid. Simultaneous occurrences of heavy, warm rain over large parts of the Columbia Basin have, on occasion, produced some very damaging floods. The Columbia, like the Willamette, has many large multiple-purpose dams as an aid in flood control and the danger of severe flooding is greatly reduced.
During the early morning hours the relative humidity is greatest and there is little variation at this time between winter and summer readings in eastern and western Oregon. The 4:30 a.m. average for practically every station in the state for which relative humidity figures are available is between 82 and 92 percent in January and only about five percent less in each case in July. In contrast, the 4:40 p.m. averages, when the relative humidities are least, show a very marked difference between summer and winter and also between the areas east and west of the Cascades. The afternoon average relative humidity ranges between 75 and 85 percent in January, while in July this drops to 25 or 30 percent east of the Cascades and slightly higher on the western side. Relative humidities of 10 to 20 percent often occur under extreme conditions during the summer and early fall.
The average dates of the last 32° F temperature in the spring and the first in fall are used to determine the average length of the growing season. A temperature that is very critical to one type of vegetation may not be damaging to others. The length of the growing season is extremely variable and depends primarily on elevation and latitude. This period ranges from less than 50 to almost 300 days.
Several evaporation stations are in operation in Oregon, primarily at agricultural experiment stations and at reservoir installations. Most of these records are seasonal (May-September) when about 90 to 80 percent of the annual evaporation takes place. A summary of the average evaporation for these five months at representative stations shows a range from 18 to 46 inches.
The north coastal area has the least sunshine, while the southeast corner of the station the most, based on limited records. The sun will shine about 20 percent of the time possible in the coastal area and 45 percent of the time possible in the southeast. These values increase in April to values of 50 to 70 percent; in July to 55 percent the northwest and 90 percent in the southeast; and by October have declined to 40 and 65 percent respectively.
Oregon's economy is very closely tied to its climate, with regard to its industry, agriculture and recreation. The mild temperatures and abundant rainfall on the middle and upper slopes of the Coast Range and the Cascade Mountains make this one of the fastest tree growing areas in the nation. Many billions of board feet of Douglas fir and pine are harvested each year. Lumbering and the manufacture of related forest products are the principal industries for the state. The heavy sustained runoff from the high elevations makes it possible to generate vast quantities of hydroelectric power. The many large dams throughout the Columbia Basin testify to this. Numerous industries dependent upon electricity have been developed in this area.
Agriculture in Oregon is a varied as the climate that sustains it. Although annual precipitation ranges from less than 10 inches to more than 60, the principal crop areas have one thing in common — their very predominate winter rainfall climate. There are years when the combined July and August total over the entire sssse averages only a quarter of an inch or less and the normal in the eastern part of the state for these two months is less than an inch. Despite this, a very wide range of crops is grown here. Because of the frequency of dry summers, agricultural crops and practices are planned accordingly.
The most damaging drought condition is a very marked deficiency of rainfall during spring and early summer. It is needed then to complete maturity of hay and grain, and to bring on range forage. Seldom has the lack of moisture been severe enough to cause a complete crop failure over any sizable area. It does occasionally reduce crop yields and quality and force ranchers to put livestock on the market before they stock has reached prime conditions.
The heavy winter precipitation in the mountain areas also provides an abundant water supply for irrigation in areas able to take advantage of it. In recent years there has been a tremendous growth in land brought under irrigation. This has been done in a number of ways. The dams that generate hydroelectric power, and others as well, are used to store winter rainfall for summer irrigation. Several private irrigation districts have been formed and an extensive system of canals built to bring water from reservoirs and streams to previously semi-desert land. In many areas deep wells have been dug, and sprinkler systems have been put into operation. There is a great deal of land in Oregon in the low rainfall areas not being utilized to its full potential that is not now being fully realized. Techniques for utilizing these assets are progressing at an accelerated pace. Ample rainfall, mild temperatures and a fairly long growing season in many areas makes it feasible to develop the irrigation systems necessary to develop additional croplands.