How can humidity make us both hot and cold?

31 October 2010

Question

In summer, air humidity has us feel hotter as there's less evaporation. In winter, it works to feel colder. Why is it opposite?

Answer

Dave - Quite a lot of this effect is down to the fact that if the air is very humid, it's likely to also be drizzling - there's actually droplets of water in there that will drop on your skin and then it'll evaporate, and that make you feel very, very cold. Also, I think water vapour will have a slightly higher specific heat capacity, it will be able to absorb slightly more energy per unit volume than normal air, because it's a more complicated molecule so it can vibrate in more exciting different ways so it can absorb more energy as it warms up. Whereas when it's warm, if it's very, very high humidity, sweating doesn't work anymore because water doesn't evaporate very well and you feel very warm.

Chris - So on a cold day, if you've got air which has got a bit of water in it and it's cold, you got to supply a lot of energy to that wet air to make it get any hotter. Therefore, it's always going to feel a bit colder, isn't it...

Dave - Yes.

Chris - ...Then in summer when the water is already saturating the air because of high humidity, and you're trying to sweat to lose the energy, and you can't lose the heat quick enough. So it's two slightly different things going on which is why you've actually got the difference.

Dave - Of course in this country as well, it tends to be windier when it's wet, just because of the way the weather works. So, you get wind chill added on top of that. I think windchill is probably bigger when it's low humidity than the high humidity but I think it's windier when it's damp.

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