Riddle me this. The air temperature is 75 degrees, I get into the pool and feel chilly. After 5 minutes or so I feel comfortable in the water but those first 5 minutes are pretty bracing. The water temperature is 85 degrees. Why do I feel cold getting into water that is warmer than the air?
Your body is 10° warmer than the water.
OK, sure. But how come I’m not uncomfortable in the air which is 20 degrees cooler than my body? Is it the shock of entering the water that has a different texture than the air? That’s what I don’t get.
Basically because water has very high specific heat compared to air, so it can drain heat away from your body more effectively than air even if the water is warmer than the air, as long as it’s cooler than your body temperature.
It’s the same reason why car radiators use water and not air.
IME the water will not feel noticeably warm until it is at least 32 degrees, or nearly human skin temperature. A swimming pool should be between 25 and 28, for comparison. On the other hand, room temperature for air would be something like 20–25.
Ferdinand Porsche does not like this post.
Or VW.
But- no radiator, just fins.
By the same token air at the temperature of 140 F (58 C) will feel uncomfortably warm, water at the same temperature can give you a serious burn, because the energy content of water is greater than that of air.
while this is true, it’s not the complete answer. Water is also a much, much better thermal conductor than air, so it is much more efficient at transferring the heat from your body to itself, and coupled with the higher heat capacity, it takes a lot of it out very quickly. It’s the same reason (in reverse) you can briefly reach into a 350 degree oven to take out a cake you are baking but putting your hand even very briefly in a (relatively mild) 200 degree pot of simmering water will almost instantly burn you very badly.
Really, no difference.
That’s a good point, thanks. Yes, both factors are important, both the high specific heat of water compared to air (at atmospheric pressure and nominal temperature) and the higher thermal conductivity.
One reason is that your nerves are detecting the rate of heat transfer, not necessarily the temperature. The water can pull the heat from your body much quicker than the air can. Part of that is because the water is so much denser than the air. The water molecules are completely covering your skin and are all absorbing heat from your body. The air has a lot of gaps in it and there aren’t as many air molecules touching your skin. Also, the air is bouncing around a lot. The air molecules don’t spend a lot of time on your skin to absorb the heat away.
Another example of the rate of heat transfer is if you have a hot pan and pot holder at the same temperature. The pan will feel burning hot, but the pot holder will just feel warm. That’s because the metal in the pan is able to dump lots of heat into your hand quickly, but the fabric in the pot holder is very slow about moving heat. A thermometer may register them at the same temperature, but your body will feel the pan’s heat more. And that’s a good thing. The rapid transfer of heat is what causes injuries. When your body can’t dissipate the incoming heat, the skin overheats and gets burnt. Or when the heat is pulled out quickly, the skin freezes. If the heat is transferring slowly, your body is able to deal with the influx or outflow of heat. The nerves don’t need to register pain in that case.
Water has higher specific heat, but there’s more. It’s also got higher thermal conductivity, and it’s about 800 times as dense as air, so you can get a much greater mass of water (and therefore a much greater total heat-absorbing capacity) in very close proximity to your skin. All of these factors combine to make water much more effective than air at pulling heat from your body.
For those unfamiliar with the terms:
Specific heat is a material property that describes how much thermal energy is required to raise the temperature of the material by a certain amount. It has units of energy per unit mass per degree. So if water has a specific heat capacity of 4.18 kj/(kg·C), it means that a 1-kg mass of water will have its temperature increased by 1 degree C if you apply 4.18 kilojoules of energy. A smaller number means a given mass of the material experiences a greater temperature increase with a smaller energy input. Air’s specific heat capacity is only 1 kj/(kg·C), so air next to your skin heats up more quickly than water and stops stealing heat from your skin sooner.
Thermal conductivity is a material property that describes how large of a temperature difference is required to achieve a given heat flux through a material of a given thickness. It has units of power times distance per degree. So water has a specific heat capacity of 0.6 W/(m·C), meaning that if you have a layer of water 10 mm thick next to your skin and a temperature difference of 15C from your skin to the outer surface of that layer of water, you’ll develop a heat flow of (0.6 ·15/0.1) = 900 watts per square meter of skin. For comparison, air has a thermal conductivity of 0.026 W/(m·C), about 1/23 that of water. Convection (movement of the air or water) also plays a role, but over short distances and in stagnant fluid, conductivity does matter .
Car radiators use air and (liquid) coolant. But there’s not nearly as much surface area wetted with coolant as there is in contact with the air - just the insides of the tubes going from one side of the radiator to the other, versus all of the thin corrugated fins that are in contact with the air. The other difference is in the flow rates of the two fluids: you have to move a lot of air through the radiator to take the heat out of the engine coolant.
For longer exposures to hot air, sweating will also help keep your skin cool. Sweating into hot water doesn’t help at all.
This. You can grab very warm things for a short time without a problem, but you can’t handle hot things for very long without getting a burn injury. I typically warm breakfast plates in the oven, and after plating the food, I’ll grab the plate for a couple of seconds with my bare fingers to see if I think I can get it to the table (another ten seconds) without cooking my fingertips. Sometimes it’s too to get it to the table without a towel, but the two-second test doesn’t cause an injury.
From my water rescue training, you lose body heat 25x faster in water than in air.
And your body is constantly producing heat that you absolutely need to transfer away, or you’ll overheat and die. That’s why the most comfortable air temperature is considerably less than body temperature: At that temperature, you’re easily shedding excess heat at about the right rate. Since water transfers heat much more effectively than air, at any given temperature, the most comfortable temperature for water is much higher (very close to body temperature) than it is for air.
This also means that if the ambient temperature is actually hotter than body temperature, and you have a choice between air at that temperature or water at that temperature, the air is better (though neither is very good), because when your surroundings are hotter than your body temperature, they’ll transfer heat to you, and just make your problems even worse.
The ocean around here tops out at about 88F/31C. Sounds pretty comfy. And is.
You can “freeze to death” = die of hypothermia within a few hours if trapped floating in those frigid conditions.
Very useful thread, increasingly so as more answers were supplied. Thank you. Gives me something to think about as I immerse myself in the pool.
This may explain why in the summer when the air temperature is high, I find a colder shower comfortable but in the winter, I need really hot shower water to be comfortable.