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I sometimes hear stories where people compare their feelings in winter in different places in the world.

It goes like

in city X the temperature was the same as in city Y, but the humidity made me feel much colder...

or

oh well, -20°C would be cold, but the humidity was low, so it felt OK

so it implies that humidity somehow makes it feel colder. I am talking about temperatures below freezing (-30...0°C).

Does this have any physical explanation, or is it some sort of psychological phenomenon?

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up vote 2 down vote accepted

Various types of apparent temperature have been developed to combine air temperature and air humidity. For higher temperatures, there are quantitative scales, such as the heat index. For lower temperatures, a related interplay was identified only qualitatively; e.g., in a 2012 textbook:

High humidity and low temperatures cause the air to feel chilly

Or in The Columbia Electronic Encyclopedia

Cold air with high relative humidity "feels" colder than dry air of the same temperature because high humidity in cold weather increases the conduction of heat from the body.

Or in Popular Mechanics:

There has been controversy over why damp cold air feels colder than dry cold air. Some believe it is because when the humidity is high our skin and clothing become moist and are better conductors of heat, so there is more cooling by conduction.

A useful concept is thermal comfort, which considers many other factors, such as skin wetness and cloth friction. (I've started a section at Wikipedia).

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That last one seems credible: when temperature drops and reaches the dew point, water will condensate on surfaces, including clothes. Damp clothes will cause extra heat loss, not just because of better conduction, but also because that moisture can evaporate again when coming in contact with the warmer (relatively dryer) air between the layers of clothing. – Previous Jun 3 at 3:28

Higher humidity means that their is a larger concentration of water in the air. Water has a higher specific heat compared to oxygen gas, thus it is able to more easily give off heat (or take it away in the case of something feeling cold).

An everyday example of this is how diving into a pool that's just above 0 degrees Celcius feels much much colder than being outside in 0 degrees. Similarly cold air with high humidity will reduce the temperature of our body's faster than air with low humidity. And, hot air with high humidity will increase the temperature of our body's faster than air with low humidity.

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2  
water (liquid or vapor) also has higher thermal conductivity than air – hyportnex Feb 5 '14 at 16:29
1  
It seems that for 100% humidity there is about 3% water vapor in the air; it has about twice the heat capacity of air - does 3% difference in heat capacity really make such a difference? Or do we have to take thermal conductivity into account too? – user27542 Feb 6 '14 at 10:06
    
I don't think this answer is correct - (i) unless you are in a small, enclosed space the heat capacity of the air is not very relevant (motion of air currents will take any warmer air heated by your body away from you faster than it can heat up significantly), (ii) the thermal conductivity or the air is more relevant, but the thermal conductivities of moist and dry air are virtually the same at these temperatures. – Mark A Jun 3 at 1:50
    
@user27542 At 0°C and 100% humidity, there would be less than 0.5% water vapor in air ( 4.8 g/m³, and a cubic meter of air weighs about a kilogram). – Previous Jun 3 at 2:27

a lot of humidity means a lot of water vapors in air and water has a lot of thermal capacity(heat needed to raise 1'C of substance) compared to air.This means that surrounding air will take more heat to warm up and therefore will make you feel cooler.

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Also, at lower temperatures the air cannot hold as much water vapor so lower temperatures may not feel as cold as higher temperatures.

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There are a few states I consider with this question;

  1. Temperature Humid
  2. Temperature Dry
  3. Movement of molecules when Humid vs Dry

Your body sweats to release heat. That water(sweat) that comes off your skin is holding heat from your body. So, when there is humidity in the air the sweat your body is giving off is not as able to be released so you will feel hotter. As long as we are above freezing, hot and cold do not matter. You will feel warmer in humid climates because our bodies cooling system is evaporative cooling. I say above freezing because once you hit freezing the moisture begins to leave the air and it's all dry at that point. Being dry means your body cools more effectively. Hot or cold, your body will feel cooler without humidity. http://www.baltimoreaircoil.com/english/what-is-evaporative-cooling

Now let's take in to account wind. The movement of molecules. With more humidity in the air the wind is more able to remove hot molecules from your body. So a humid wind will cool you better than a dry wind. Anyone that lives in a place like Southern Arizona can tell you that wind is no more than a hot blow dryer in your face during the summer.

Case in point on water that moves vs doesn't. A wetsuit uses water to insulate your body. So more water around does not mean you will feel colder. It is the ability of those molecules to remove that heat. Ie. The molecules need to be able to transport heat away from you.

-20°C would be cold, but the humidity was low, so it felt OK

At -20°C the humidity will be leaving the air pretty fast. Meaning you shouldn't be feeling much humidity at these temps. Yes there are situations such as snowing and freezing rain but I am not currently covering those.

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