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Climates of the Puget Sound Lowland M-III., Ii!. CHURCH University of Washington, Seattle, Washington The lowland in western WashCasington between the Olympic and Cascade mountains is an area having a temperate marine climate. Three-fourths of the moisture adequate for most agricultural crops comes during the long winter rainy season. The coldest month is well above freezing. The sky is overcast four-fifts of the time. The summer is cool, sunny, and nearly rainless. The frost-free period is nearly eight months long. Though all the lowland has these general climatic features, the mountains to the east, north and westv the sources and trajectories of the visiting air masses, the location of the mountain passes, and the moderate relief of the valley floor itself all conspire to produce wide climatic differences within short distances. The Olympics cast an intense rain shadow which reflects the general altitude of this great precipitation barrier. Near the northern end of the valley there is less moisture than at the southern end. This is illustrated by the average precipitation of a series of stations, from north to south, in the middle of the lowland. Anacortes has 27 in., Everett 32 in., Seattle 31.5 in., Tacoma 40 in., Olympia 52 in., and Castle Rock 54 in. Sequim, only 40 miles airline from Mt. Olympus but to the leeward, has a mean of less than 16 in. With reference to the mountains, this series shows that the shielding effect is dominantly toward the east northeast, or the major rain bearing wind is from the west southwest . The low hills of southwestern Washington produce an area of less rainfall in the valley as compared to regions both north and south of it. The Cascades, too, have their effect on the precipitation though they lie to the leeward of the winter wind. Eastward from the middle of the valley there is a sharp increase in the total yearly amount. The isohyets closely parallel the mountains. This is the result of of the gradual lifting of the moist winter Polar Pacific air which blocks itself on the western slope of the mountains. This same principle operates west of the Olympics producing heavy rain well to the windward of the actual slope of the mountains. There is a general increase of moisture amounts with higher altitude, for Seattle (alt. 125 ft.) has 31.5 in., Startup (alt. 560 ft.) 56 in., Monte Cristo (alt. 2900 ft.) an indicated 118 in., Mt. Baker Lodge (alt. 4,200 ft,) 104 in., and Paradise Inn (alt. 5,500 ft.) 100 in. This series further indicates that the zone of maximum precipitation is somewhere between 3,000 and 4,000 ft. above sea level. The yearly amounts in the lowland are substantially less than those of corresponding altitudes on the west slope of the Olympics. Here it has been calculated from the discharge of the Wynoochee River that the upper reaches receive more than 270 in. The Cascade range further acts as an effective temperature barrier in winter, for the relatively even 6,000 ft. crest prevents the westward spreading of the cold Polar continental air which frequently 1-19) floods the Columbia Plateau. Two low passes, the Columbia gap on the south and the Fraser gap on the north permit this cold air to flow westward unhindered and fill the adjacent portions of the valley. The southern end has recorded lower minima than the northern end owing to the greater width of the Columbia River gap. The central part of the lowland is notably warmer owing to the absence of low mountain passes to the east, However, when the cold Polar air is of sufficient depth to pour westward over the top of the Cascades, the descending air is warmed rapidly by descent and the lowland enjoys a true foehn. The Physiography of Western Washington HOWA UI) ?. COOMBS University ol Washington, Seattle. Washington Western Washington is divided into three topographic units all trending in a north-south direction. The westernmost of these is an extension of the Coast ranges culminating in the mile high Olympic mountains. The known rocks of the Olympics are essentially Tertiary sediments and volcanics which have been thrown...

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Additional Information

ISSN
1551-3211
Print ISSN
0066-9628
Pages
p. 19
Launched on MUSE
2014-10-01
Open Access
No
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