Crater Lake National Park's Frequently Asked Questions
Crater Lake Institute - http://www.craterlakeinstitute.com/
Answer: Although snow occupies Crater Lake National Park throughout 8 months of the year (average annual snowfall is 14 m, or 533 in), the lake rarely freezes over. There have been only two reports of this ever happening. It was completely covered by ice from March 14 to mid May during the winter 1949 and four days in February, 1924. In April, 1983 about 95 percent of the lake froze. The immense depth of Crater Lake acts as a heat reservoir that absorbs and traps sunlight, maintaining the lake temperature at an average of 12.8 °C (55 °F) on the surface and 3.3 °C (38 °F) at the bottom throughout the year. The surface temperature fluctuates a bit, but the bottom temperature remains quite constant.