Type Curves and Duration of Snow Cover in Washington

Abstract

TYPE CURVES AND DURATION OF SNOW COVER IN WASHINGTON P. E. Church University of Washington. Seattle IN THE past few years there has been an increasing demand for more detailed studies of various characteristics of snow cover in areas where the snow lies on the ground for some time during the winter. In this country studies of certain features of snow, particularly depth on ground and duration, are being pursued by R. G. Stone for New England and New York(l) and by Eric Miller for Wisconsin (2). A preliminary report on the snow cover of Washington was presented before the International Commission of Ice and Snow of the International Union of Geodesy and Geophysics at its Seventh Assembly, Washington, D. C, 1939. The present report on types of seasonal curves of snow depth and duration of snow cover is also preliminary. Data. The report is based on the records of snowfall kept at first order and cooperative stations of the United States Weather Bureau. Nearly 100 stations have been used, a majority of which have had unbroken records during the 20-year period 1918-19 to 1937-38. Less than 10 stations had records shorter than 10 years. No adjustment of short records has been made as yet. Though the snowfall records include amount of fall each day, depth on the ground on the 15th and last day of each month, water content (melted), and total snowfall during the season, only depth on ground on the 15th and last day of each month are used here. The duration of the snow cover was computed from the curves of depth on the ground on the 15th and last day of each month. Snotv on Ground during Winter Season. The situation of the state of Washington and its complex relief result in almost innumerable combinations of temperature and precipitation. The controls that have the greatest effect on the distribution of snow, its depth on the ground, and its duration are latitude, position to leeward of a relatively warm ocean, occasional westward spread of continental air, and relief. Briefly, the depth on ground and the duration of snow cover are largely dependent on altitude. In the higher mountains above 6,000 feet the snow season begins about the middle of October. By the first of November there is a continous blanket of snow above 3,000 feet on the western side of the Cascades, though there is none at that altitude in the Blue Mountains or in the Okanogan Highlands . By the middle of November, snow covers the surface down to about 1,500 feet on the western side of the Cascades and a thin cover extends down to 2,000 feet on their eastern side. In early December the snow cover is continuous above 1,000 feet on the eastern side of the Cascades. During the first two weeks of December all of the Puget Sound area becomes covered, and all the "Inland Empire" is also under snow. Only the immediate coast along the Pacific is free of snow. By December 15 the area covered is as great as in January, and above 3,000 feet the average depth is in excess of 30 inches. By the first of the year the Puget Sound Lowland has no snow below about 1 ,000 feet. The "Inland Empire" generally has experienced a little increase in depth, but about the confluence of the Columbia and Snake Rivers, the lowest altitude in eastern Washington, the depth has decreased to about one inch. The depth on the ground above 1,500 feet, both east and west of the mountains, has 21 Yearbook of tlic Association Vol. 6, increased materially. By January 15 the pattern has again changed; all the Puget Sound Lowland, except along the immediate shore and the San Juan Islands, now has a snow cover, although it averages less than one inch in depth. On the last ot January the pattern is the same as on the 15th except that the snow is deeper over the whole state. More than half the stations report their deepest cover at this date. These stations are below 1000 feet elevation on the western side, and between 2...