Thermodynamics of Spring

Rumor on the playground this evening was that the weather is warming (they're predicting 80F for Monday!). As Art Carey's recent article in the Philadelphia Inquirer points out, water temperatures will lag behind. Chuck Sutherland, an avid sea-kayaker, points out the perils of cold water. The article notes that cold water will lower your body temperature 25 times faster than cold air, as anyone who's ever jumped in a cool pool can attest!

So, why is cold water "colder" than cold air (at the same apparent temperature)?

A little statistical thermodynamics is a help here. What we sense on the macroscopic level as heat, at the microscopic level is the result of molecular motion. The faster a herd of molecules moves, the "hotter" they are. To move energy from one set of molecules (say, those in your skin) to another (those in the water) requires a collision. This is why a thermos works, by making a near vaccum, where there are very few molecules to collide, you reduce heat transfer. The more collisions, the more heat gets transfered. The density of molecules in liquid water is about 1000 times higher than that in air, leading to more collisions in any given period of time and so increased heat transfer.