Posted on April 13th, 2011 by george.
I posted this on Facebook yesterday and got such a response I decided to crosspost it here. The following is my response to a question Patrick asked me yesterday.
Patrick: If my body temp is 98.7, then why does 82 degrees feel hot? Wouldn’t it have to be hotter than me (which is hard, I know) for it to feel hot to me?
George: We were just discussing this on Sunday. Your *core* body temperature is 98.6 degrees F on average…that’s why you have to measure it on the inside of your body: mouth, ear, rectum, etc. Put a good thermometer to the surface of your skin, though, and you won’t measure 98.6. Your skin surface temperature is lower than 98.6. It can be anywhere from 70 to 90 degrees F, depending on the person. And it varies all over your body. For instance, extremities like fingertips, which are long, skinny, and surrounded by air, are naturally going to be much colder than large, flat surfaces like your chest, or warm crevices like your armpit. But I’m getting ahead of myself.
The nerves in your skin that tell you if you’re feeling hot or cold do so by measuring heat flux. Flux is just a fancy word for transfer. Heat is not something you can hold; you can hold temperature, that is, the random motion of your atoms. But heat is by definition only a transfer of energy. Your body obeys this law of physics. You touch a hot stove, and heat is transferred TO your body, which is much colder. Your nerves measure the influx of heat energy, send a signal spike to your brain, and your reflexes pull your hand back before you know what happened. When you hold onto an ice cube, your hand transfers heat from your body TO the ice. Your nerves measure the outflux of heat, realize it points outward instead of inward, and tell you that the ice feels COLD. Your nerves can only measure change in temperature, not absolute temperature.
This is why you can become accustomed to a particular temperature. It’s like the examples of the frog in a pot of water on the stove…turn the temperature up slowly, and he’ll never notice when it finally boils. Sit in a hot tub long enough, your blood vessels dilate in order to increase surface area which allows larger heat flux from the water to the regulated blood temperature, and you “get used to it.” Jump in the pool, though, and the heat flux is very large for the first few seconds. The pool may be 85 degrees, but it FEELS cold, because your now-hotter body is rapidly rejecting heat to the water. Give it a few minutes, though, your blood vessels constrict to reduce heat transfer, your brain saturates with the “cold” signal from your nerves and starts ignoring them, and now you’re “used to” the temperature of the pool water.
Add to this fact that your body is always using the chemicals in the food you eat to power your cells, and a byproduct of this power generation is heat, which your body uses partially to heat itself, but must reject the excess to the air via convection, or to whatever you touch via conduction. It’s difficult to get rid of much energy at all through radiation, the least effective method of heat transfer. We have this problem on the space shuttle, which is why there are giant radiators on the inside of the payload bay doors. Nonetheless, your body does radiate a little. This is why you show up on infrared night vision goggles as a bright green blur: you produce much more radiative energy than a tree. Remember: you’re a warm-blooded mammal.
Conduction, or bringing into contact two solids, surface to surface, is by far the most efficient method of heat transfer. The second best is convection, in which you transfer heat between a solid and a fluid, or between two fluids. The least efficient is, as mentioned before, radiation. This is why you get hypothermia in water much more quickly than you do in air, even if they’re at the same temperature! Think about it. Would you rather stand in 72 degree air for an hour without moving, or 72 degree water? Obviously air is a better insulator, and allows you to retain more body heat. The REASON it’s a better insulator is microscopic: the distance between the molecules, called “mean free path” (just the average distance they travel before they bump into each other), is MUCH larger for air molecules than for water. Water molecules are, in fact, so tightly packed together that they’re actually *further apart* in solid water. But that’s another story. Because water molecules are so close together, there are more of them available per square inch of your skin to rob your body of heat. So the amount of heat transfer from your body to 72-degree water is much higher than from your body to 72-degree air. This is why you can get hypothermia in water that is well above freezing. It’s also why you’re so hungry after going swimming: your body has been burning extra calories just to keep you warm in the water.
Your metabolic rate controls how much heat you need to reject. It varies from moment to moment and person to person. This is why two people can be in the same room and one feels hot while the other feels cold. One person needs to reject more heat, but the air isn’t cold enough relative to their skin temperature for efficient heat transfer to take place. This person feels hot. The other person’s metabolism is such that their body wants to retain heat to keep their core temperature in a stable operating range, but that same air is cool enough relative to THEIR skin surface temperature that it absorbs more heat than their body wants to give up. This person feels cold. If the rate of heat transfer available with a given air-to-skin temperature gradient balances with the amount of heat your body wants to reject, you feel like Goldilocks: just right, neither hot nor cold.
Topics related to this discussion are exercise, which increases body temperature; sweat, which through evaporation absorbs large amounts of body heat to provide the latent heat for the phase transition (liquid water to water vapor); circulation, which is of paramount importance in regulating localized body temperature (see also cold feet, frostbite, hypothermia); and blankets which insulate your body, i.e. reduce the rate of outward heat flux, and allow you to stay all toasty when the air is cold by not allowing your body to lose heat to the cold air. As every school kid knows, whales have a lot of blubber because they spend so much time in frigid water. Blubber is just fat. Fat is a great insulator. We all have it, to varying degrees, and it’s a very good thing (not just for heat retention but also quite necessary for cushioning internal organs and protecting muscle as well as vital energy storage). Some people have more fat, some less. A person with low body fat is usually going to feel cold at room temperature, but quite comfortable at the beach, in the sun, or in the tropics. A person with high body fat feels hot at room temp because they’re walking around with several blankets wrapped around them, and their body has to work extremely hard to push heat through all that insulation in order to regulate their core temperature.
So, to sum up, even though your core temperature might be 98.7, you felt hot when the air was 82 degrees because the temperature of the surface of your skin was less than 82 degrees, so heat was flowing from the air into your body, and your nerves told you as much.
Patrick: You just blew my mind. And Paolo’s. And soon, Johanna’s.
George: Hahaha ok good. Always happy to help.
Comment on April 14th, 2011.
This was beautifully explained. I wanted to fave this but realized your blog doesn’t have that function. I will remember it’s here so I can send your explanation to the next person that asks me this question (happens often).
Comment on April 14th, 2011.
Thanks Atoosa! It’s good to get this sort of thing out into the public for the purpose of quick reference.
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