Ralph H. FoldingBoats_at_aol.com wrote: >> Matt, if I understand the above correctly then NO water can freeze until the entire body of water has been cooled to 40F, thereby stopping the convection process and leaving the increasing less dense (now dropping below 40F) water at the surface to cool until it freezes -- fresh water at 32F. Now, does the magic 40F mark apply to fresh or salt water? At what temperature is the water least dense and would therefore show the greatest tendency to rise to the surface? [snip] How does this work in DEEP water? For example Crater Lake in Oregon never freezes despite pretty low temperatures throughout the year [snip] >> Good issues, Ralph. Re: Matt's assertion (that an entire body of water has to cool to ca. 40 F (3.98 C) before the surface will freeze: in theory, yes, assuming perfect convection. In practice, however, blast freezing conditions can freeze the surface before the convection processes have established a uniform 40 F temp below a certain depth and the top "layers" gradient down to 32 F at the surface. Depends on the rate of cooling. Water is funny stuff. It does not always behave as it "should." Witness "anchor ice," which is ice frozen to a stream bed, under running water (I refused to believe this could happen -- until I saw it with my own eyes!). How does that happen? Durned if I know! I suspect perhaps nucleation is important. Many of us are familiar with "freezing rain," which is liquid water __below__ 32 F in the air, which is so super cooled than when it hits something mimicking the stricture of ice it immediately freezes. Coats trees, roads, houses, even people(!). We get it here once or twice a year, as a warm front overrides a retreating cold air mass. On to sea water: I do not have the data here, but I believe there is a similar temp for sea water, and that it also would undergo a density-temperature inversion as it cools. The salt in sea water alters the way water molecules interact, but does not completely destroy its properties. As most know, sea water freezes as pure water (except for mechanical entrainment of salty water), so that after a curing period, one can drink melted sea ice. That minimum temp in the bottommost ocean water comes from the polar regions, so I bet it is the temp of densest water. On to Crater Lake: the heat budget has to include the effects of summer, which are "stored" in the deep lake, so unless there is an enormously cold, extended period of time, not likely it will freeze over. In addition, I believe there are thermal sources in Crater Lake, remnants of its history as a volcano, which heat it year round. -- Dave Kruger Astoria, OR (long-time chemistry teacher) *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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