So I was wondering, when I said that I felt colder when the humidity increased for the same temperature... was that true? I mean physically true or psychologically true?
Googling provided much speculation and little solid facts. Thus, from my knowledge of thermodynamics and relying on a couple of published papers on the changes in physical properties of humid air from 0 to 100°C, I set out to answer the seemingly confusing question.
At first glance it seems to be not true. Humid air has lower density, thermal conductivity, and thermal diffusivity than dry air. However, I'm not convinced. The differences are at most all less than 5% which is equivalent to calculation and approximation errors here. I did rough calculations and while it increases by about 1-2% in humid air, there doesn't seem to be a significant difference in the heat transfer coefficient. So that was a dead end.
I swear it feels colder. I know that 7°C in Arizona feels cold, but 7°C in Beirut absolutely feels much colder.
What else. Oh yes. Thermal conductivity of skin: My only remaining hypothesis. If it's not hot and we are not sweating (otherwise evaporation becomes the dominant mode of heat transfer and humidity would make seem hotter because nothing can evaporate), then an increase in air humidity mean our skin is less dry. I have not found much solid data, but might I make the slight assumption that moist skin transfers heat better than dry skin.
I've run out of ideas :/ Now I see why there wasn't a solid answer on google. Although one person in one forum mentioned they were cooling something in relatively extreme conditions in an experiment, and the object cooled faster in humid air versus dry air.
I should probably ask khalto about this... her field of specialty and all that.
Humid air has less conductivity than dry air? Are you sure?
ReplyDeleteHumidity makes temperatures feel extreme. Hot feels hotter, cold feels colder. 10-15 degrees in Aachen is not that cold and around 15 degrees you can start getting away with a T-shirt and a jacket. In Beirut or Nice (also on the sea), it feels very cold that you need a sweater. Same temperature but feels a lot colder.
You can last a long time in 18 degree weather (not so cold) but you will die from heat loss if you were to sit a long time in 18 degree water (very cold!). I'd double check the conductivity. Water definitely makes you lose more body heat.
Yes.
ReplyDeleteFirst, there's a difference between water as a liquid and water as vapor in the air. Liquid water is more densely packed and way more viscous. So more skin contact. Heat transfer coefficient is waaaaay higher, even if thermal conductivity of H2O is lower than that of N2 and O2. Air is a lousy thermal conductor because it's so close to vacuum... very little density.
Second, while thermal conductivity may be lower, it means that conduction in air is lower. It says little about convection which is how we really lose heat to fluids. Convection is affected by a whole host of parameters, which is why this gets confusing. Density, viscosity, Prandl numbers, thermal conductivity of skin and so on.
I've actually sent a small email to khalto.. I wonder if I'll get an answer = ) I hope so!
Alright. After digging more into this, I have found what seems to be the most plausible explanation:
ReplyDeleteOur bodies perspire and exude moisture all the time, even in cold weather. When the cold air is dry, the moisture evaporates without us noticing much about it, as the cooling occurring is not really significant because water easily evaporates and mixes with the dry air.
However, when the cold air is humid, the moisture will condense in tiny particles on our skin and on the fibers of our clothing. This will do two things. First it will compromise the insulation value of our clothing and skin because liquid water has a much higher thermal conductivity than air. Second, the micro-environment that now exists between your skin and the outer layer of your clothing and even just between your skin and body hair, is now even more humid than the room air because of all the tiny condensed water particles, and the temperature drops in that small layer to be closer the wet bulb temperature (or dew point temp).
Also, just a note. It is not the same for hot air, because hot air has a much higher capacity to hold water vapor than cold. So 50% relative humidity in hot weather means a lot more water vapor present than 50% RH in cold.
ReplyDeleteAlso, in hot weather, we perspire a lot more, so we want the dry air to carry off the water vapor to make room for more evaporation. Otherwise, we're stuck with a layer of water (or sweat) that little further cooling and heats up to body temperature.