Viewing a single comment thread. View all comments

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

602

[deleted] t1_j0kvqoh wrote

[deleted]

115

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.

46

[deleted] t1_j0l6ffl wrote

[removed]

28

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.

28

[deleted] t1_j0lhpn1 wrote

[deleted]

4

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.

3

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.

3

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.

14

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.

3

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.

1

s0rce t1_j0lgqxm wrote

Then the humidity would make you warmer by reducing evaporative cooling

1

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.

2

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.

11

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.

7

[deleted] t1_j0lccuw wrote

[removed]

8

[deleted] t1_j0li6so wrote

[removed]

7

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.

5

[deleted] t1_j0lrrm0 wrote

[deleted]

2

Dr_Vesuvius t1_j0ltj95 wrote

Ah, gotcha.

So I think there are three issues wrt. sensory sentiment.

  1. 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.

  2. 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.

  3. 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!

5

[deleted] t1_j0lv0kd wrote

[deleted]

1

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.

1

[deleted] t1_j0naeh5 wrote

[deleted]

1

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.

1

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.

4

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.

1

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.

0

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.

2

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.

1

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.

1

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.

  1. 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.

  2. 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.

32

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

10

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.

7

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.

5

[deleted] t1_j0l7ckl wrote

[removed]

15

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.

12

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

5

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

3

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.

2

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.

1

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.

2

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.

2

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.

https://physics.stackexchange.com/questions/521843/why-most-distribution-curves-are-bell-shaped-is-there-any-physical-law-that-lea

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?

1

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.

2

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.

2

Step845 t1_j0l6hg1 wrote

Wouldn't it be exothermic if energy (heat) exits your body?

1

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.

3

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!

1

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.

1

_Kartoffel t1_j0ld2hu wrote

This is the correct awnser, but I have one isse: Evaporation isn't a reaction. It's a physical process

−2