Submitted by MindTheReddit t3_zo23xk in askscience
Comments
thewrongequation t1_j0kt6ki wrote
High humidity means that a higher proportion of air is made up of water molecules than air of low humidity. Water has a higher heat capacity than, and is a better thermal conductor than, the other things that make up air (mostly nitrogen and oxygen). So it gives and takes away thermal energy much more efficiently. Just like how, if you touch a piece of metal and a piece of plastic that are at room temperature, the metal will feel colder than the plastic, even though they are the same temperature, because the metal is a better thermal conductor and so is better able to take the thermal energy away from your hand.
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kalod9 t1_j0l0shs wrote
It's important to note that because vapour pressure rises with temperature 100% humidity at 0/32 degrees equals 9.25 times less vapour than 100% humidity at 35/95 degrees. That's why the effect of humidity on perceived temperature is far less pronounced at lower temperatures.
StandardSudden1283 t1_j0l11fe wrote
Question about those aerogels that can be placed in a furnace, get red hot, then be handled with bare hands.
I understand they have low thermal conductivity but is there a/what is the relation to their thermal capacity?
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Ninjaromeo t1_j0l4o14 wrote
Funny. I didn't think much about it for hot climates. When it is cold and you are wet, you definitely feel more cold.
fuzzygondola t1_j0l54f0 wrote
Thermal conductivity and specific heat capacity don't go hand in hand. But heat capacitance depends on the material and its mass. Aerogel is extremely light and that why it doesn't burn you even when it's red hot.
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Step845 t1_j0l6hg1 wrote
Wouldn't it be exothermic if energy (heat) exits your body?
luisgdh t1_j0l6qmm wrote
It is worth noticing that thermal feeling for cold temperature depends much more on the wind speed than on humidity, so high humidity mostly impacts hot days makes, but doesn't change much the perception on cold days
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auntanniesalligator t1_j0l7u74 wrote
Endothermic with respect to the water that is evaporating. In that sense your body is part of the surroundings, not the system.
Step845 t1_j0l897v wrote
Right, I feel like I got it. Water gains energy (heat) making it endothermic, and it exits your body as a result of evaporation. Thanks!
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m31td0wn t1_j0l8gl9 wrote
That's just evaporative cooling, same as sweating. Although it is possible to cool the air by increasing relative humidity--that's how "swamp coolers" work. Basically humidifiers that spray a fine mist of water into the air, which in extremely arid environments evaporates quickly and lowers air temperature by raising relative humidity. It takes energy for water to phase shift from a liquid to a gas, and that energy comes directly out of the air it evaporates into.
Dani_924 t1_j0l8xoa wrote
It’s like being in a steam sauna but you’re outside and can’t escape it. High heat and humidity are brutal. Also see ‘wet bulb effect’. Sweating does absolutely nothing to cool you off. Swimming can help as long as the water is cooler than the air temperature.
kilotesla t1_j0l97vf wrote
However, at 40 F = 4.4 C, 100% relative humidity means only 0.5% of the air is water vapor, so its impact on the heat capacity of the air is negligible.
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kilotesla t1_j0lawaq wrote
>Moist air is a better conductor than dry air, so moist cold air feels colder.
There are two problems with this explanation.
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If we are talking about temperatures below about 50 F or 10 C, the amount of water vapor in the air, even at 100% humidity, is very small, a fraction of a percent, and so the impact it can have on heat transfer is very low.
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According to this analysis, the impact, which is only about 3% at 50 F, is in the other direction: higher heat transfer with dry air. Perhaps that is not the best high quality source, so I would be open to revising this point if people have better sources.
I think we still need someone to come and provide a better explanation for the common perception that a cold day feels colder when the humidity is high. There may be other correlations. For example, it may be that dry, cold days often have more solar radiation, making surfaces around you warmer as well as warm and you directly, so you feel warmer because of heat transfer by radiation even while the heat transfer by convection is very similar to what it would be on a wet day. And on a wet day, the wet ground may be cooled by evaporation, making that difference even bigger. I hope someone can cite a study that includes that affect—the mean radiant temperature.
kilotesla t1_j0lb1tj wrote
When you are wet is a little different from the air having high humidity. If you are wet, evaporative cooling has an impact, and if your clothes are wet, their insulation capability is degraded.
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banksy_h8r t1_j0lcs3n wrote
> So humidity makes extremes of temperature more extreme - hot gets hotter, cold gets colder.
Is there a cross-over point where high humidity of a given temperature feels like that temperature? A subjective measure, I know, but the seems like something that would have been researched with a statistically significant number of participants.
celo753 t1_j0lct5m wrote
Since you mention comfort, I would like to add that while this isn’t related to the perception of temperature, extremely dry and hot environments can feel more uncomfortable than humid and hot ones.
Because the dry hot air dehydrates you very quickly, especially your throat and nose get dry in a matter of minutes, it feels very uncomfortable and is (in my own opinion) worse than a hot humid climate.
_Kartoffel t1_j0ld2hu wrote
This is the correct awnser, but I have one isse: Evaporation isn't a reaction. It's a physical process
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Lame4Fame t1_j0lge9x wrote
Similar to how you can stick your hand into an oven that is at 200+ celsius and not get heavy burns.
s0rce t1_j0lgnkp wrote
Is moist air a better conductor? Do you have a citation I've been trying to find data on this for a while
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Ashmedai t1_j0lgqbb wrote
> Funny. I didn't think much about it for hot climates.
Not so fun fact. There is a rare environmental condition that can occur where the heat index around you can go up higher than the body's cooling ability can handle. If you are caught in such an environmental condition and cannot find shelter, you'll die. No amount of water will help, as sweating cannot cool you.
s0rce t1_j0lgqxm wrote
Then the humidity would make you warmer by reducing evaporative cooling
s0rce t1_j0lgxsm wrote
I've looked into this and haven't found good data but agree with you it's not the air thermal conductivity. I think the issue is largely the effectiveness of most clothing insulation is reduced significantly by humidity
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CapOnFoam t1_j0lknvk wrote
Same! I always wondered why cold weather in dry climates felt "warmer" than the same temps in the Midwest. I've always associated humidity with "feels warmer". Very cool.
Gastronomicus t1_j0ll04u wrote
They also use humidifiers because the dry air is very uncomfortable for your skin and mucous membranes. I get nose bleeds and cracked skin in the winter without a humidifier.
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Olbaidon t1_j0lli5o wrote
Humidity levels change with temperature.
So 100% humidity is at hotter temperatures is different than 100% at lower temperatures. That’s simply because there is more space for water vapor at higher temperatures.
100% humidity at 55 degrees (f) will feel comfortable, but 100% humidity at 75 degrees (f) will feel hot and muggy.
This is due to dew-point, the point (temperature) at which water vapor condenses and become dew. Or what the temperature would have to be cooled to to be at 100% humidity.
Generally speaking a dew point of 55 degrees (f) and under is perceived as comfortable, which would also mean 100% humidity and any temperature 55 degrees (f) and under would also be comfortable (with in reason of course).
Outdoor athletes will often use dew point to judge if their activity will be comfortable, uncomfortable, or even down right dangerous because temperature and humidity on their own don’t tell the whole story until looked at together.
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ibonek_naw_ibo t1_j0lnl0y wrote
What about specific heat? I always thought it was because drier air in our immediate area changes more quickly toward our body temp, and more humid air more slowly. Kind of like sticking exposed skin into an oven is more tolerable than brief exposure to hot water.
m31td0wn t1_j0lo7hj wrote
Haha well temperature is just kinetic energy of molecules. Picture the atmosphere as a giant bag of marbles being shaken around. The faster it's being shaken, the more heat it contains. But if you add more marbles to the air, it becomes heavier, and harder to shake. So the shaking slows down. Keep adding more marbles, it's heavier and heavier, so it gets shaken slower and slower.
Not a perfect analogy but I think it gets the job done. The energy required to turn water into a gas is drawn out of the air, causing the air to lower in temperature.
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Dr_Vesuvius t1_j0lqxuw wrote
No, I don’t think so.
Firstly the UK’s humidity is one of the reasons why winters are relatively mild compared to Poland (for example).
The reason the UK struggles when snow falls is because we’re not so cold that snow is inevitable. People, business, and the government mostly aren’t prepared for snow and ice.
I’m in London which is one of the warmest parts of the country (southern + eastern + urban heat island) and we haven’t got anywhere close to -10. Our coldest nights might have got as low as -5 but every day has been above 0 except maybe Monday. I’d guess you’re probably in Scotland or Northern Ireland, maybe NW England or North Wales?
If we were like Canada, Northern Europe, or inland US, then local government, railway operators, and motorists would always be prepared for low temperatures and would react accordingly. The UK is not.
I will say that this week’s snow was met by the best response I can remember. There was a big snowstorm about ten years ago (maybe 2010?) that made normal life very difficult. By the time I left the house for work Monday morning, the main roads were clear, the pavements were gritted, trains were running as normal, and most Tube lines were partially open.
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Dr_Vesuvius t1_j0ltj95 wrote
Ah, gotcha.
So I think there are three issues wrt. sensory sentiment.
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we’re sheltered by our humidity and the Gulf Stream, so we aren’t toughened against cold the way that Minnesotans (for example) are. We don’t wrap up as warm as those people do. I don’t own thermal underwear or insulated trousers, for example.
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our homes are generally poorly insulated. Right now it is 12 degrees in my house. The walls, loft, and floor are not insulated and the curtains have limited thermal properties.
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humid air takes longer to warm up. If you are drying laundry inside then you need to run the heater for longer to feel warm.
Suspect if you dropped a British person moaning about the cold somewhere dry and cold, they’d still moan about the cold. We like a very narrow temperature range!
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TonyJPRoss t1_j0lukfp wrote
Something with a high specific heat capacity needs more heat to increase its temperature by 1 degree.
Conductivity directly describes how much heat flow you get per difference in temperature.
They're related concepts but I think conductivity is the relevant one here. You feel colder when heat is drawn out of your body.
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kilotesla t1_j0lv6y5 wrote
Yes, the effect of humidity on insulation is a very real effect, perhaps better documented for building insulation then for clothing, although it's well known among outdoors enthusiasts that cotton loses its insulation value rapidly when it's wet to the point of being soggy. Of course, that's complicated by the fact that the local environment between the shell of your outer layer and your skin might have different humidity than the outdoor air, especially when you first step outside and form the impression of how cold it feels.
kilotesla t1_j0lvkj4 wrote
I think a better (but still sloppy and qualitative) analogy is that if the marbles bouncing around hit a layer of stationary marbles held together with pudding, and they knock loose some more marbles when they hit, that process will absorb use up some of their kinetic energy.
omgu8mynewt t1_j0lvxc6 wrote
Same in UK, hearing that from Norwegian and Russian colleagues. And that the summer 30C heat (not this year lol, a more normal year) feel worse than 45C in Libya because the dry heat there means sweat actually cools you down unlike here.
roosterjack77 t1_j0lw5ld wrote
Serious add-on question. I like a hot cup of tea. I'll leave the kettle boiling on the stove a little longer. Sounds like it takes a lot more energy to increase the temperature of water and most of the energy is lost to steam, phase change. (no jokes about my stove kettle, Canada)
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kilotesla t1_j0lwdxg wrote
What would be relevant here is the convection coefficient, which is a result of heat capacity, thermal conductivity, and viscosity, as well as the geometry and the wind, if any. But it doesn't change significantly with humidity at cool or cold temperatures.
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kilotesla t1_j0m2cbk wrote
I'm not sure exactly what your question is, but once you have reached boiling, leaving it sitting there boiling does not change the temperature of the water, which is already at 100° C. So all the energy that you are using while you are leaving it sitting there boiling is being wasted, except to the extent that it's useful in heating the house and, if you need added humidity, humidifying it.
kilotesla t1_j0m3ajh wrote
Here's some data, a regression model based on a large data set of actual comfort surveys in different conditions. The results are summarized in figure 8, where you see that the highest temperature considered comfortable is dramatically affected by humidity, but the effect on the coldest temperature considered comfortable is tiny, and perhaps not statistically significant.
This is all indoors, however, whereas I think the question is more focused on outdoors. It's worth noting that the temperature considered there is the operative temperature including the effect of the mean radiant temperature as well as the air temperature.
Wretched_Brittunculi t1_j0m4n6n wrote
I remember some Polish friends struggling with the mild Liverpool winter because it felt colder (or more uncomfortable) than the subzero temps at home.
TWeaKoR t1_j0m5qza wrote
It's also why you should use air con when blowing hot air to demist your windows in your car. The air con cools the air, which lowers the humidity and condenses out some of the moisture, then heating it up leaves air with relatively lower humidity. This is because the air's capacity to hold water goes up with temperature, so if you have the same amount of water in cool air as hot, the hot will feel drier.
iamveryDerp t1_j0m8z3x wrote
What you are looking for is called the wet bulb temperature, which accounts for humidity and it’s effect on “perceived temperature.” It’s why those of us who live in the desert say “At least it’s a dry heat!”
So a “wet bulb” temperature is the reading on a thermometer when the bulb is wet and in the shade. In a less humid environment the rate of evaporation will be greater, and therefore the cooling effect will also be greater.
This is why a dry heat feels less intense, because the less humid environment makes our bodies self cooling method (sweat which evaporates) more effective.
Dani_924 t1_j0mesr1 wrote
Yeah that’s exactly what happens. Where I live in Canada, in the summer there is the air temperature, and then there is the ‘humidex value’. So it can be 25 degrees Celsius but with the humidity it will “feel like” 30 degrees Celsius. It gets dangerous when the air temperature is already in the 30 degree range and the humidity makes it feel closer to 40 or higher. Then we get heat warnings for people to be careful because you can get heat stroke pretty easily.
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votedbestcomment t1_j0mkbpd wrote
Boiling water a little longer will increase relative humidity inside the room. Just like sweat has trouble evaporating while the air is saturated, the tea will also let off less steam after it’s made, and stay hot longer than it would if the air was totally dry. Don’t let the others fool you, when you already have discovered this yourself. It is hotter when you take the first sip when you boil it longer. It probably has cooled down a little bit from 100 degrees while on the stove, but not as much as it would if you took it off the stove right away.
roosterjack77 t1_j0mnz1e wrote
Thank you
roosterjack77 t1_j0mo02r wrote
Thank you
iowamechanic30 t1_j0mq3qd wrote
This is actually how air conditioning and refrigeration work too. The difference is we force a refrigerant to evaporate using a pressure change witch draws heat out of the air; we then force it to condense using pressure and temperature changes to make into a continuous process.
iowamechanic30 t1_j0mqux2 wrote
This is done automatically in almost all cars on the road today and vintage cars aren't set up to work like that so don't turn the ac on to defrost your windows.
mr47 t1_j0mro23 wrote
> As you add heat energy to water, its temperature increases linearly until it gets to about 100 degrees C. From here it takes in additional energy, but its temperature doesn't rise - this energy is instead used to break its bonds to complete the phase change from liquid into gas. Once the phase change is complete and it has become a gas, you see a linear relationship between heat and temperature once again. > > So what's happening is you're transferring your body heat into the sweat, which is evaporating and carrying that energy away.
While technically correct (if you read the two passages separately), it seems as if you imply that sweat reaches 100 degrees C prior to evaporation. It does not.
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TonyJPRoss t1_j0mwe8v wrote
What I always thought was that a single molecule of water might have enough energy to change phase, even if the temperature of "the water" isn't that high. The energy of the water molecules follows a bell curve with a long tail in the high energy end - the evaporating particles are in that tail.
But wind has such a strong effect on the evaporation rate that I think this explanation must be lacking something. Do you have any insight?
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tatodlp97 t1_j0mzgnv wrote
What we see as humidity in weather forecasts is more specifically relative humidity (RH).
It ranges from 0% to 100%. 0% humidity means there’s practically no water vapor in the air. 100% humidity implies that the air is saturated with water, meaning it has the highest concentration of water vapor that can be held at that specific temperature. Any addition of water vapor will lead to it condensing into a liquid creating a cloud or fog. This can also occur when the water concentration is held constant but the air temperature drops. Colder air can hold less water valor which results in the excess concentration of water condensing again into fog. That’s how most clouds form where humid warm air from the surface rises and it expands due to lower pressure, causing it to cool (through adiabatic cooling) and the now excess moisture crashing out of the gaseous phase.
TWeaKoR t1_j0n0iu3 wrote
It's done automatically in cars when you set it to the window clear settings, for the most part, and more and more cars have proper climate controls, but that isn't always the case.
Regardless, the reason it automatically turns AC on is because it clears the mist. If you have AC off and find you're getting misting on the windows, putting AC on will probably clear it, even when it isn't blowing on the front windshield it should lower the relative humidity in the cabin. Yes, it will use a tiny bit more electrical power from the engine, but it serves a purpose.
doghouse2001 t1_j0n1auw wrote
Swimming can help as long as water is cooler than your normal body temperature.
If it's 100 degress F and the water is 90 degrees F you'll still overheat and die.
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squirlol t1_j0n7ejk wrote
Mate, if you're bothered by the cold at the moment, get yourself some merino thermals/base layers (if you can afford them, smartwool, icebreaker, or similar brands are the best, but there are much cheaper options too). I'm from New Zealand where the weather is even milder than here in the UK (rarely goes below 0), but our houses are even more useless, often completely uninsulated and very draughty. So we're used to indoor temps of below 10 in the winter. When I was a student sometimes it was below 5 degrees, just the same temperature inside and out. Lived in thermals all winter as a matter of course. Now I've just brought that habit along with me to the UK and I'm finding it extremely comfortable here this winter.
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Dani_924 t1_j0nany9 wrote
Good point. I was thinking of a regular pool or natural body of water, which for the most part are usually cooler than a persons body temperature. I would definitely not recommend swimming in a hot tub during a heat event.
squirlol t1_j0nha8q wrote
Yeah, they don't overheat easily, if it's less than about 20 inside they're very comfortable for me, tolerable up to maybe 23. But they still make a huge difference outdoors or when it's cold inside. I wear mine to/at work, we have the office at 19 I believe.
They don't require a lot of washing, they still smell fine after 4-5 wears if you don't, like, exercise in them. Downside is they do need to be washed with wool detergent rather than normal laundry powder/liquid, but that's not a huge deal.
ZLVe96 t1_j0nj0wg wrote
For heat - humidity makes your body's cool mechanism suck. In Arizona (low humidity), it's hot. You sweat, it evaporates, and you feel cooler. You do the same in say Thailand (humid), it's hot, you sweat, and it can't evaporate as much because of the humid air. Your body doesn't cool as well. It sweats more. You feel wet uncomfortable and hot.
I have hiked a mountain in Arizona in 105, and ended with a dry shirt/hair. It was hot, but not terrible. I've run 1 mile in 85 degrees and 90 percent humidity and sweat through my shirt and been muh "hotter" and more uncomfortable.
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TheRobotFromSpace t1_j0nrs8a wrote
This is a great explanation. In NZ our temp is 0-30°C on average, but it is an island with very high humidity. Bliss is between 15-24°C. Everything over 24°C is hot, muggy, sweaty and oppressively hot, the temperature in the shade is the same as the sun because of the humidity, you can not hide from the heat. In the winter 10°C and below chills you to the bone, you can feel the water in the air and the same thing, you can't really hide from it because the air is wet everywhere.
I never really realised how it was different in the rest of the world till I lived through the seasons in places with larger land mass and drier air/less humidity through the seasons. In inland Canada and Midwest USA for example. I loved winter because it may be -30°C but I wasn't cold because the air wasn't wet. It just felt like I was standing in front of cold air conditioning, it didn't chill me to the bone or sap my energy like it did in NZ above 0°C because the air was dry. Summer the same 30-40°C was hot but not oppressive, you could get out of the heat because it was actually cooler in the shade, and you could cool yourself sweating.
I'll take extreme temps in a dry climate any day over the high humidity here in NZ. I suppose this is why we have two temperature readings for the weather here, especially for the tourists. The actual temp in °C and then another "feels like °C" after taking the humidity into account.
wedividebyzero t1_j0nzrrx wrote
I'd recommend you check out the ASHRAE 55 standard. It details two metrics, PMV and PPD, which you can use to calculate an estimated level of thermal comfort.
If you're into Python, there is a good library for this called pythermalcomfort.
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iowamechanic30 t1_j0ojm98 wrote
I don't think we're on the same page. There are people that will read this post and turn their heat off and turn the ac on to defrost their windshield. I am trying to prevent that, when you turn the defroster on the car automatically engages the ac to dry the air out but also blows hot air. Just let the car do its thing.
Bladabistok t1_j0olzyh wrote
What is 100% humidity? Wouldn't that just be... underwater?
Olbaidon t1_j0oonay wrote
100% humidity is the tipping point at which water vapor in the air would begin to condensate (dew).
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Alternative_Yam5242 t1_j0plfog wrote
Ah, this is why we feel hotter in the Southeast countries, when their weather is similar to the upper lands.
It is good to know for my Glasshouse, because I may save energy by spraying the water from time to time and help the temperature stay high long, which makes less consumption of gas.
Chemomechanics t1_j0qky4o wrote
At an interface, it’s very likely for just-detached molecules to be deflected right back to the surface by the surrounding atmosphere, where they reattach. This is less likely to happen if convection is carrying them away, so the evaporation rate increases with wind.
Gastronomicus t1_j0sec3v wrote
> Some people are sharing anecdotes that they feel much colder in more humid weather - I think that's more to do with moisture on your body than moisture in the air.
It's not as straight-foward as this. Moisture in the air isn't just present as a dissolved gas. It's dynamic and often shifting back and forth between gaseous and water phases, forming microscopic droplets that can remain aloft and form an important component of energy flux in the atmosphere.
Furthermore, warmer air can become super-saturated with moisture if it cools rapidly, like the kind of inversions that frequently occur near maritime regions in the winter when warmer air from the ocean comes inland and cools off. This can form fogs or various less opaque mists as it precipitates.
Combine both situations with winds and the synergy between moisture and wind can leave you feeling much colder than a dry windy area alone.
TonyJPRoss t1_j0ksrt5 wrote
In cold weather our bodies lose heat mainly by conducting heat directly to the air. Moist air is a better conductor than dry air, so moist cold air feels colder.In warm weather, we mainly cool ourselves by sweating. The sweat evaporates, which is an endothermic reaction which takes heat out of your body. When the air is already saturated there's little evaporation, so this process stops working. So moist hot air feels hotter.
So humidity makes extremes of temperature more extreme - hot gets hotter, cold gets colder.(edit) People have pointed out a mistake. The humidity of cold air is always low. The difference in conductivity between cold air of high relative humidity vs cold air of low relative humidity is negligible, as displayed in this graph. https://www.electronics-cooling.com/2003/11/the-thermal-conductivity-of-moist-air/
Some people are sharing anecdotes that they feel much colder in more humid weather - I think that's more to do with moisture on your body than moisture in the air.
(edit) Some people want to note the detail that evaporation is a phase change, not a chemical reaction.
As you add heat energy to water, its temperature increases linearly until it gets to about 100 degrees C. From here it takes in additional energy, but its temperature doesn't rise - this energy is instead used to break its bonds to complete the phase change from liquid into gas. Once the phase change is complete and it has become a gas, you see a linear relationship between heat and temperature once again.
So what's happening is you're transferring your body heat into the sweat, which is evaporating and carrying that energy away.
https://socratic.org/questions/how-can-i-calculate-thermochemistry-equations-for-phase-changes