Why does a wet towel cool beer?

07 September 2007


Beer bottles



Phillip Croll, Germany asked:

If you wrap a beer on the beach, on a hot day, in a wet towel and leave it in the sun it will cool down which seems unlikely but not impossible and I think the key to this is latent heat of vaporisation. What do you think?


Dave: I think it probably is. When water turns from a liquid into a gas, it needs lots and lots of energy, that’s why if you sweat on a windy day you cool down because the water evaporated off your skin which absorbs lots of energy and cools you down. Essentially that is what the wet towel is doing. It’s evaporating so that absorbing lots of energy, cooling everything down. Ideally you wants it to by slightly windy, you probably don’t want too much sun on the outside as it will be heating it up. Unless, the sun heating it up is causing air currents, it’s making its own winds because it gets hot and air expands so it floats upwards.

Chris: I did try this when I was in Boston, It was so hot and I was in this room, and they said they would let me borrow a fan from reception for $5 so I took this fan up to my room. I then though all this is doing is blowing hot air from outside into my room, and making it hotter. So I thought I’d apply a bit of physics. I went and got a towel, dunked it in the sink, making it really wet and draped it over the fan. For a while it worked beautifully. The cold air coming through was evaporating water off of the towel, bringing down the temperature of the air going through and cooling my room down! That is until the fan blew up and filled the room with smoke because it had gotten water in it!


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