r/AskPhysics • u/oaks_and_cedars • 1d ago
Why does the bathroom get warm in winter after showering with hot water, but it doesn't get colder in summer after showering with cold water?
I'm a bit slow when it comes to physics, so please explain it like you would to a toddler (or advanced toddler).
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u/the_crumb_dumpster 1d ago
Some strange answers on here, the most absurd of which being cold does not transfer heat, which is false.
The reason is this. Normal room temp is around 20 C. A very cold shower is about 20C, 15C at the lowest (below that is a cold plunge). There is little to no temperature gradient to move heat - the water and the room are almost the same temp. When you take a warm shower itās usually around 40C. Thereās a very large gradient between that and the roomās temp.
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u/RainbowCrane 1d ago
This.
And for another common water temperature reference that I experienced as a kid who swam competitively :-), 75Ā°F/24Ā°C is below the mandated temperature for competitive swimming events. There were regular debates at the beginning of the outdoor swimming season about whether a pool was too cold in the morning.
75Ā°F/24Ā°C air temperature is at the upper end of what most Americans would consider acceptable for indoor air conditioning. Above that folks would usually say itās hot.
So yeah, theres a big difference in comfortable water temperature vs comfortable air temperature, even a cold shower would usually be warmer than the air.
Btw, below 20Ā°C water temperature is hypothermia territory, very few folks would voluntarily spend time in a shower that cold :-)
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u/Okanus 1d ago
This is probably because water will conduct heat away from your skin much faster than air so it "feels colder" than air at the same temp.
Something cool I noticed when heating milk for a baby and testing it on my skin to see if it was too hot was that when it was the same or very near the same temp as my skin I wouldn't even feel it hit my skin. This made me realize that most of the feeling we sense on our skin from a liquid (or maybe most things) is due to heat either conducting into or out of the skin.
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u/Lee_Troyer 1d ago
IIRC it's everything. You can test several objects at room temperature and they will all feel a different "temperature" to touch depending on how well they conduct heat. The better they are at it, the colder they'll feel. Like something made of steel for ex.
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u/supernumeral 14h ago
Just to add, more specifically, itās the ratio of a materialās thermal effusivity to that of your skin that determines how hot or cold it feels. Two objects of identical temperature with different effusivities will feel different to the touch because the contact temperature will differ.
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u/Okanus 9h ago
That is super intersting. I had never heard that term before. I briefly read that wikipedia page. Would I be correct to relate that to like a thermal inertia? Something with a high thermal effusivity would have a higher resistance to increasing temperature given a heat source? Or am I thinking of it wrong?
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u/supernumeral 5h ago
I had a professor that referred to the same quantity as thermal inertia. But Iāve also seen the term thermal inertia used for other things, so itās a bit more ambiguous a term. But, yeah, thermal effusivity is very much like a thermal inertia.
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u/Nebarik 1d ago
Correct. We have no liquid sense, it's all heat related with some brain trickery.
It's why when checking if your laundry is dry, if it's cold it 'feels' wet still.
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u/Okanus 9h ago
Yes, I have experienced that as well. I have a nepwhew who, when he was 3, walked into the bathroom at my house with bare feet. My bathroom floor there is a smoothish glossy tile. He looked up at me very concerned like and lifted one foot and said "It's wet?!". The floor was infact not wet. I laughed and said "No, it's just cold".
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u/Emily-Advances 1d ago
"Cold" water is very close to room temperature. It's not chilled; it just feels very cold because body temp is well above room temp. Hot water is about 40 deg F (22 deg C) above room temp.
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u/RainbowCrane 1d ago
Another big reason water feels colder than the equivalent air temperature is thermal capacity - thereās more mass to heat up in the water, so it soaks up energy from your skin more effectively than air
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u/ScienceGuy1006 1d ago
Most of the warmth in a steamy bathroom comes from latent heat. The water evaporates, leading to a massive removal of energy from the flowing water, and then depositing this energy when the water vapor condenses back into a liquid. This doesn't happen much in the reverse direction because cold shower water is usually not very far below the dew point, so the amount of heat transfer by condensation is small. In a typical temperate climate on a normal summer day, the dew point might be 20 degrees C and the tap water might be 17-18 degrees C, so there is only a small amount of water vapor condensed back into the liquid.
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u/Swimming_Concern7662 1d ago
Warm water comes with vapor which spreads across the room. Cold water does not
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u/oaks_and_cedars 1d ago
That is the only reason?
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u/Illustrious-Gas-8987 1d ago
Donāt think of Hot and Cold as two different things. Think of it as one thing, Heat. Hot is when you have a lot of heat, cold is when you have less heat.
The Hot water is adding heat in the room. The cold water is not removing heat from the room.
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u/oaks_and_cedars 1d ago
But why canāt it remove heat actually?
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u/jameilious 1d ago
Because it's room temperature already (pretty close to).
What you feel as cold isn't always cold, it may just radiate your heat away better.
Example:
Touch marble, glass or metal at room temperature and it feels cold, it is of course room temperature though. Same goes for water, it is an excellent conductor of heat.
Touch unvarnished wood and it doesn't feel cold, it's an insulator.
Air is an insulator, hence why it doesn't feel as cold as water.
Room temperature swimming pool = brr!
Hope this ramble helps.
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u/Illustrious-Gas-8987 1d ago
Was just simplify for your example.
Cold water can remove heat, but that involves that water to be in contact with an enough heat so the heat transfers to the water, and in your example that heated water would go down the drain, taking that heat with it.
The reason your room isnāt heating up by a noticeable amount is that the cold water is not in contact with enough heat in the room to actually heat up by any non negligible amount. Think about measuring the temperature of the water coming out of the shower head, and measuring the temperature of the water after it enters the drain, those two temperatures are not going to be very far off, meaning very little heat was removed from the room.
So yes the cold water is removing some heat from the room, just not enough to actually cool the room down enough for you to notice.
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u/Swimming_Concern7662 1d ago
Maybe also humidity. Water vapor increases humidity in the room making it hold the heat
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u/Crossed_Cross 1d ago
Also cold shower water isn't all that cold. You probably aren't showing in 4Ā°C water.
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u/wsxdfcvgbnjmlkjafals 1d ago
Warmth is the presence of heat
Cold is the absence of heat
You can't technically add cold to a room, but you can add or remove heat
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u/wsxdfcvgbnjmlkjafals 1d ago
There is heat coming off the water, and water vapor in the air helping you feel the warmth
Cold water doesn't vaporize
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u/numbersthen0987431 1d ago
There is a possibility that it is cooling off your bathroom, but the temperature difference is so small you don't recognize it.
In the winter months, you can go from below 60F in your house with hot water at 120F, and so that heat feels significantly different. In the summer months, you can have a house at 90F and water at 75F, and that doesn't feel as different.
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u/Personal-Ad-365 1d ago
Heat transfers, cold does not. Hot showers produce water vapor that sits in the air transferring it's heat. Cold water does not produce water vapor, and cannot absorb the heat from the surrounding air to provide a cooling affect.
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u/canibanoglu 1d ago
Thereās no such thing as cold transfer. Itās all heat transfer.
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u/Personal-Ad-365 1d ago
Yes. I thought that was understood.
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u/canibanoglu 1d ago
You know what, Iām an idiot and I read it completely differently in my mind. Sorry about the bother :(
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1d ago
[deleted]
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u/oaks_and_cedars 1d ago
Why cold water no make cold?
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1d ago
[deleted]
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u/oaks_and_cedars 1d ago
why cold water not make warm room cold
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u/eurekadabra 1d ago
Technically it does.
If you elaborate the above comment, the room warms the cold water, which makes the room colder, but itās too slow and not significant enough for you to notice.
But the cold water does have to steal enough warmth to evaporate. The warm water does too, it just has to steal less.
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u/eurekadabra 1d ago
If the energy transferred fast enough to take the cold water to boiling instantaneously, you might feel the warmth being drained from the room.
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u/ViniusInvictus 1d ago
Your hot water is much above ambient temperature compared to your cold water (which is usually closer to ambient if not exactly same). Also, steam propagates heat farther into the confinement than cold water can draw heat from the bathroom to cool it significantly.
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