Science Questions

Why does a spray of hot water from the shower feel cold on my skin?

Sun, 18th Dec 2011

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Question

Daniel Spain, Tennessee asked:

Whenever I get in the shower, and I've already got the cap turned all the way up to a full blast that's hot, but when I first get in, the smallest droplets that I feel, the kind of ones that spray off a thing to hit my skin, are not hot but actually rather cold. I was wondering exactly what causes this?

Answer

Chris -   I think what's probably happening in your shower is that although the water leaves the tap, or at least the showerhead, nice and hot, the fine spray is caused by two things happening.  One, some of the droplets will break up as they pass through the air at high speed and two, a lot of the spray is there because the droplets hit something like the wall or the sides of the shower and then they split up into lots of smaller droplets and fly onto you.  But they've had contact with something which was a cold surface and so, they've lost a lot of their energy to the cold surface.  They've also, as they've gone through the air, lost more energy to the air, because they have a high surface area to volume ratio, so they tend to feel cooler.  

I think that's probably the most likely reason why the fine misty spray feels cooler than the big droplets which have got more energy in them because there, because they've got a bigger volume relative to their surface are.  So they're losing less of their energy less quickly.  Would you agree, Dave?

Dave -   Yes and I think the other really big thing is just evaporation.  Water evaporates, and actually, if you have a bowl of water sitting at room temperature, because all the hot water molecules evaporate off the top, what's left actually gets cooler than room temperature.  These tiny particles have got a huge area to evaporate from.  So even just from the spray head down to your body, they've got enough time to cool down below your body temperature.

Dominic -   I think the other thing that will be going on is that when you're standing in the bath and the air is mostly still, you have a warm layer of air next to your skin.  But when you turn the shower on it will start stirring the air around and blowing that warm layer of air off your skin, allowing cold air to come in instead.

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