Good evening.

19:26, Friday. It is ungodly cold here in Moscow. I don't mean the temperature itself, -15C is quite bearable, but 80% humidity and wind turns even short walks into a torturous experience.

And because I'm a nerd, here is an explanation: heat capacity of humid air is higher than it is of a dry air, so it draws more heat from your clothes.

@academicalnerd Maybe stupid question, but I always relate humidity to warm climate, how can you explain humidity in cold climate?

@doltamalte @academicalnerd Absolute humidity in cold weather tends to be lower however it is still there. The problem is that in cold weather humidity has greater effect and what wouldn't be felt in warmer weather feels freezing on cold day.

Currently we have 48% humidity so it can get even wetter - and that would feel really cold!

@alex @academicalnerd
What factors contribute to humidity in cold weather?

@doltamalte @alex
So you have absolute humidity and relative humidity. Let me explain.

At any temperature air can only hold on to so much water (the excess amount condensates into water). This is the maximum absolute humidity, which you use to calculate relative humidity (in %). And the higher the temperature is, the lower the maximum humidity goes, while the relative one is between 0 and 100% and the absolute value is between 0 and maximum for said temperature.

So for higher temperatures air can get almost wet because it holds a lot of water (that's what you mentioned). At lower temperatures the air is quite dry and you can't feel it like usual, but it plays role in how cold you feel.

I'm not sure what contributes to high humidity at low temperatures but I'm pretty sure salt that is used here to melt ice on the roads increases it quite a bit.

Hope this makes sense.
<good lord it's lengthy, phew>

@academicalnerd @alex

I quite get it, it's just not very intuitive, in a cold climate the air holds little mass of water especially at those negative temperature so even at 100% Relative Humidity, am not sure if the thermal mass of water in the air plays a major role in the feeling of coldness, my guess would be wind has a more significant role.

https://chicagowindowexpert.com/wp-content/uploads/2017/04/chart3.png

Interesting read:

https://chicagowindowexpert.com/window-condensation-absolute-humidity/

@doltamalte @alex
So I ran some math and you seem to be right, at these temperatures (-15 C) it's about 1% heat capacity increase at 100% humidity. Which is weird because it feels really cold. Maybe the wind does it, indeed.