Hacker News new | ask | show | jobs
by alexeldeib 996 days ago
Does it work with lower temps? I’ve always found it odd that human body temp is close to 100F but getting into 90 degree water feels hot. I wonder if it’s the evaporative effect like you said and if it reverses when it’s hot enough. Like if it’s 110 and I get in a 90 degree tub, does it feel good? That’s below the temperature my body naturally wants to be at, why doesn’t it feel “cool”?
1 comments
MaxRegret 996 days ago
The human body is producing about 80 watts of heat just from basic metabolism (unless you're dead) which you need to dissipate to keep your body temperature from going up. The rate of heat transfer out of your body is proportional to the temperature difference, so the environment needs to be cooler than your body.

If your environment was exactly the standard body temperature, your body would actually get hotter until the temperature delta was enough to dissipate 80 watts, which would likely be too hot to survive.

link
alexeldeib 996 days ago
Huh! Does this account for the difference between what we perceive as "room temperature" and our body temperature? I suppose that delta in temperature depends on medium the body is surrounded by and effectiveness of heat transfer (e.g. similar issue as evaporation, but maybe not the same direction?)
link
MaxRegret 996 days ago
Yes, indeed! The actual rate of heat transfer is equal to the temperature delta divided by the thermal resistivity of the body-environment interface. So, to keep your body temperature constant, the temperature delta needs to be your metabolic rate multiplied by the thermal resistivity.

The thermal resistivity depends on, among other things, the material you're surrounded by. That's why if you wear insulating clothes, you're comfortable at a lower environmental temperature (i.e. higher temperature delta).

To this you can add the effect of convection: if the air is moving then you don't accumulate a layer of warmer air around you, so the effective temperature delta is higher. And unless you're in a hot tub or air at 100% relative humidity, then some of the heat you produce goes into evaporating sweat. It takes energy to vaporize water, and this energy is locked into the water vapor until it condenses somewhere else.

link