How can humidity make us both hot and cold?

31 October 2010

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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 the air that will drop on your skin and then evaporate, and that makes 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, 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 a bigger [effect] when it's low humidity than the high humidity, but I think it [tends to be] windier when it's damp.

Comments

"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."

Huh? This makes no sense.

I agree that the wording isn't ideal. I've added some extra words [in brackets] to help clarify. What might be confusing you is the difference between wind chill and windy weather. They are not the same thing...

Dave if the humidity is so high that there is liquid water in he air, there will be little or no evaporation. The air is already saturated! Drops on your body will not evaporate and cool you.

He points out that there are contrasting effects in winter and summer, thus accounting for the difference.

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