Why does water cool more quickly outdoors, even when the weather is hot?

12 September 2010

Question

We did an experiment where we had water that was 117 degrees Fahrenheit. We placed two containers inside in the air conditioning at 78 degrees and two containers of the same temperature, same quantity of water outside in the sun and two outside in the shade. We were shocked to find that the water outside in 90 degree weather in the shade cooled much quicker than the water in the cool classroom.

Someone suggested it was evaporation and so we covered the tops the next day and re-did the experiment and still got the same results. We’re wondering why is the water in the hotter air cooling quicker than the water in the cool, air-conditioned classroom?

Answer

Dave - That's very interesting. The answer I was going to give immediately was evaporation because the maximum amount of heat which water looses is generally due to evaporation and therefore if you've got wind blowing across the top, all those hot molecules evaporating off the top will get blown away quicker, you can get more evaporation and it's going to lose heat quicker. It could still be: I would have thought the only big effect is going to be the wind and it could just be that even just losing heat to the air around it is a lot faster if wind is blowing past something because otherwise, if the air is stationary, the hot object just heats up the air around it and then that insulates it from the rest of the air. If there's a wind blowing past, it will keep moving that hot air away and so you can keep losing heat much more quickly. Certainly the only thing I can think of is that it's to do with the wind but possibly due to conduction as well as evaporation.

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