In the Yakima Valley Area, arid conditions prevail. The average yearly precipitation at Yakima is 6.67 inches. Only in November, December and January may more than one inch of precipitation be expected monthly. Snow may be expected in the winter months and the yearly average snowfall is 22.1 inches. Winter temperatures are low, the average for December and January being but slightly above freezing. Summer temperatures are extreme; the July average is 73.1° and the average maximum for the same month is 89°. The highest temperature recorded is 111°.
The open pine forests of the eastern Cascades give way to grasslands. Grasses of several species are common but the bunchgrass (Agropyron spicatum) is most important. Other plants include the primrose (Oenothera pallida), lupines (Lupinus), and Mertensia. In ravines and near watercourses such shrubs as hawthorn (Crataegus douglasii), service-berry (Amelanchier cusickii, Amelanchier utahensis), aspen (Populus tremuloides), syringa (Philadelphus lewisii), snowberry (Symphoricarpos albus), choke-berry (Prunus melanocarpa) and elderberry (Sambucus caerulae) form thickets. Lower in the valley the vegetation is xerophytic, similar to that of the Columbian Plateau. Sagebrush (Artemisiae tridentata) is dominant. Other shrubs include rabbit brush (Chrysothamnus nauseosus, Chrysothamnus viscidiflorus), hop sage (Grayia spinosa), black sage (Purshia tridentata) and greasewood (Sarcobatus vermiculatus).
Fig. 9. Mount Rainier. Washington: Cowlitz Chimneys from base of Dege Peak, July 19, 1933. (Fish and Wildlife Service photo by Victor B. [Scheffer], No. 859.)
Climatic conditions in the Olympic Mountains are, in general, similar to those of the higher Cascade Mountains. There is a limited area of Arctic-alpine Life-zone on Mount Olympus. The principal life-zone is the Hudsonian. The Canadian merges with the lowland Humid-Transition and is difficult to ascertain as a separate zone. In the Hudsonian Life-zone the average temperatures are low. Winter climate is bitter and the snow lies deep. In the absence of government weather stations in the Olympics, detailed descriptions of climatic conditions can not be given. Vegetation of the Hudsonian Life-zone of the Olympic Mountains is, in general, similar to that of the same life-zone of the Cascades (see Jones, 1936, Botanical Survey of the Olympic Peninsula).
The lowlands of western Washington have a cool, humid climate. The average annual temperature of the area varies little from 50°. In winter the temperature, especially in January and February, commonly drops below the freezing point at night. Summer temperatures are moderate, rarely reaching 90°. Snowfall is light, averaging about 10 inches. The prevailing winds are from the west and are moisture-laden. They rise over the Olympic Mountains and loose heavy rains along the coastal area of the lowlands. The average annual precipitation at La Push, Clallam County, is 97.9 inches; at Clearwater, Jefferson County, 124.98 inches; at Aberdeen, Grays Harbor County, 81.58 inches; at South Bend, Pacific County, 83.35 inches. In contrast, the more inland areas receive less than half as much precipitation. The yearly average at Bellingham is 31.09 inches; at Seattle, 30.07 inches; at Tacoma, 39.53 inches; at Vancouver, Clark County, 37.24 inches.
The outstanding feature of the vegetation of western Washington is the coniferous forest. Previous to the logging activities a dense cover of Douglas fir, western hemlock and red cedar spread almost unbroken over the area. The openings in the forest and the marshy ravines and river valleys supported growths of underbrush and deciduous trees so dense and luxuriant as to compare with a tropical jungle. In the dense rain forests along the coast, mosses and lichens develop an understory vegetation many inches deep and clothe the branches of the forest trees. The mild temperature and excessive rainfall cause some species that usually are of bush or shrub size to reach the proportions of small trees. In some places one can climb twenty feet from the ground in a huckleberry tree, the trunk of which is five inches in diameter. The coniferous forest is made up of several species of trees. Most important among these are the western hemlock (Tsuga heterophylla), Douglas fir (Pseudotsuga taxifolia), and red cedar (Thuja plicata). Locally the western yew (Taxus brevifolia), lodgepole pine (Pinus contorta) and spruce (Picea sitchensis) may be common. Deciduous trees are numerous and include several willows (Salix sp.), aspen (Populus tremuloides), hazel (Corylus californica), alder (Alnus oregona), oak (Quercus garryana), broadleaf maple (Acer macrophyllum), vine maple (Acer circinatum), and flowering dogwood (Cornus nuttallii). Mosses and ferns are abundant. The sword fern (Polystichum munitum) and bracken (Pteridium aquilinum) are especially common. Space prevents listing all but a fraction of the typical shrubs but these include huckleberry (Vaccinium parvifolium, Vaccinium ovatum), Oregon grape (Berberis nervosa), salal (Gaultheria shallon), rose (Rosa gymnocarpa), thimbleberry (Rubus parviflorus), salmonberry (Rubus spectabilis), blackcap raspberry (Rubus leucodermis) and wild blackberry (Rubus macropetalus).
The higher parts of some of the ranges of northeastern Washington are in the Hudsonian Life-zone, but most of the mountains are in the Canadian Life-zone. The valleys are in the Transition Life-zone. Climatic conditions are similar to those of the eastern slopes of the Cascades. Winter temperatures are low, the average for December, January and February being below freezing. Summer temperatures are high, the July average for Colville being 67.2° and the July average maximum being 87.4°. Vegetation consists principally of coniferous forests in the mountains and deciduous woods in the valleys. Among the interesting features of the vegetation are the extensive stands of almost pure larch (Larix occidentalis). In most respects the flora closely resembles that of the Blue Mountains.
The Blue Mountains of southeastern Washington differ from other ranges in Washington in their relative aridity. There are few streams and a single river drains the area. There are no government weather stations in the Blue Mountains. The winter temperatures are low and the snow deep and lasting. Summer temperatures are high and humidity and precipitation low. Coniferous forests of the type of arid regions form the principal tree cover. Typical plant species include the white fir (Abies grandis), alpine fir (Abies lasiocarpa), larch (Larix occidentalis), spruce (Picea columbiana), and such shrubs as fool huckleberry (Menziesia ferruginea), Pachystima myrsinites, dogwood (Cornus canadensis), wild current (Ribes petiolare), mountain mahogany (Cercocarpus ledifolius), spirea (Spiraea sp.), lupines (Lupinus) of several species, maple (Acer douglasii), buckbrush (Ceanothus sanguineus), sticky brush (Ceanothus velutinus), and huckleberry (Vaccinium membranaceum).