Although snow covers Crater Lake National Park for eight months of the year (average annual snowfall is ), the lake rarely freezes over due in part to a relatively mild onshore flow from the Pacific Ocean. The last recorded year in which the lake froze over was in 1949, a very long, cold winter. A 95% surface freeze occurred in 1985. The immense depth of Crater Lake acts as a heat reservoir that absorbs and traps sunlight, maintaining the lake temperature at an average of on the surface and at the bottom throughout the year. The surface temperature fluctuates a bit, but the bottom temperature remains quite constant.