Four coordinated reports on recent trends in glacial activity in the State of Washington, U.S.A., have been prepared by three authors and three of these appear in this issue of the Journal of Glaciology. The fourth report will appear in a later number. Other reports regarding the condition of glaciers in this state and in other regions of the western United States have appeared in various mountaineering journals. It is the purpose of this paper to serve as a reference to the bibliography in this field and show how the detailed story included in the companion papers is part of a widespread change in trend. Results reported by other observers are reviewed briefly. In addition, some data on observations by the author in the Sierra Nevada are included in order to compare the magnitude of the activity in California and Washington.

A simple equation is developed for ablation in a snow cover on the basis of the principle of conservation of mass. Sinking of the snow surface as measured with ablation stakes or ablatographs is commonly accepted as a measure of ablation. In so doing, a principal term associated with density changes in the snow with time in the above-mentioned equation is neglected. In a few investigations, ablation was determined from sinking of the snow surface alone, and also by other methods. These investigations demonstrated errors in the calculated ablation values resulting from neglect of density changes in the order of 15–20 per cent for long observation periods, and as great as 65 per cent for short observation periods. The terms are also discussed which must be measured to determine short period values of melting and evaporation of the snow for analyses of the heat exchange at a snow surface.

The measurements by seismic sounding of the long profile of the Unteraar Glacier reported by Mercanton and Renaud are compared with theoretical calculations. By taking the valley to be a cylinder of parabolic cross-section a theoretical curve for the surface is deduced which depends on one unknown parameter only, the average shear stress on the bed. If this parameter is given the value 0.77 × 106 dynes/cm.2 the theoretical curve agrees well with the measurements.

Between 1953 and 1955, 73 glaciers in the Olympic and Cascade Mountains of Washington State have been investigated to determine their present activity. 50 of these glaciers are now advancing at rates from 3 to 100 m. or more per annum. Of the remaining 23, 22 glaciers either demonstrate clear evidence of increasing thickness, or have remained so heavily snow-covered at the end of the ablation season that it has not been possible to locate their limits.

The present glacier growth, which appears to have started about 12 years ago, represents a radical change from conditions during the previous 20 years when glaciers of the Olympics and Cascades without exception were shrinking rapidly. An analysis of local climatic data demonstrates a present trend toward a cooler, wetter climate in western Washington. The ten year running mean annual temperature at Tatoosh Island off the Washington coast has decreased approximately o 8° C. from the period 1934–1943 to the period 1945–1954. In the same interval of time the ten year running mean annual precipitation at Tatoosh has increased about 38 cm., and during the last decade has reached its highest value since the period 1898–1907.