Why do I feel hot when outside temperature is still below body temperature?

When the human body gets too warm, it puts into effect all sorts of measures to cool itself down. Your skin secretes sweat for evaporation; at night, you sleep with your arms and legs spread apart to increase the surface area of the body; and all sorts of other things. In my case, it starts to happen in the upper twenties centigrade, i.e. in the eighties Fahrenheit, even though it may be a bit more or less for other people.

Still, this threshold when I start feeling hot is still noticeably below body temperature, which is roughly 37 °C / 99 °F, and I would think it is the same for most people. Why is this the case? You would think that as long as the outside temperature is below body temperature (which it is for most of the inhabited world pretty much all of the time), the body would still be busy heating, rather than cooling; as temperatures rise, the body would have less heating work to do, but it would still need to heat, not cool.

Your body produces more heat than it needs. So, it needs to shed excess heat to maintain a constant internal temperature. When it’s close to body temperature outside, it feels “hot” because the body needs to perspire to remove excess heat.

Just to complement this post, I venture a guess: Maybe the biological functions of the body generate more heat than is necessary to maintain body temperature, so the body is constantly in need of cooling, and this cooling becomes more difficult as it gets warmer outside the body because the difference in temperature gets smaller. But this is really just a layman’s guess.

Edit: Just saw beowulff’s post, which seems to confirm this.

Cecil: https://www.straightdope.com/columns/read/658/if-body-temperature-is-98-6-f-why-do-we-feel-uncomfortable-when-its-90/

Yes. The homeostasis of warm blooded animals is not the most significant thing going on here, i.e. we don’t just generate heat because maintaining a certain body temperature is desirable.

The Second Law of Thermodynamics says that total entropy always tends to increase, i.e. (loosely) things become more disorganized. But the definition of being alive is pretty much resisting the tendency to deteriorate into disorder, i.e. maintaining the organization within our bodies (notably maintaining cell membrane potentials in non-equilibrium states). So the reactions taking place to maintain our bodies must necessarily increase the disorder of the external environment in some way.

The interaction with our environment is that we take in usefully “organized” energy sources in food and oxygen. The ultimate source of this “free energy”, in the sense of available useful energy that can do stuff, was the photons of sunlight. In addition to poop, the notable output from our bodies as a result of all the reactions taking place to keep us chugging along is diffuse & disorganized heat energy. So, in order to drive all the reactions in our bodies that are required to stop us turning into lumps of rotting meat, we (with help from other organisms in the food chain) convert the “concentrated” useful energy of the high-energy photons in sunlight into relatively useless diffuse low-grade heat.

Strictly speaking, this isn’t necessary–the waste heat is a function of inefficient chemical processes in our bodies, not a general condition of a mechanism that takes in high-grade energy and converts it to other types of energy.

For instance, suppose we “ate” compressed springs, using their mechanical forces to power our bodies before pooping out uncompressed springs. This mechanical conversion can be arbitrarily efficient; there’s nothing in the laws of physics that demands production of waste heat (it does sound painful, though).

All that said, the chemical reactions our meatbodies depend on do inevitably produce waste heat, and as said, there’s no way to get rid of that other than moving it to the environment.

You can’t get around the laws of thermodynamics. The theoretical limits on the efficiency of getting energy out of stressed springs are very, very high, so high that they’re almost never relevant, but they’re still there. Remember, computation inherently generates entropy, so any system that’s capable of computing or depends on computing will have waste heat.

Computation only (needs to) generate heat when it’s irreversible–that is, when you erase bits. If you can arrange the computation to be reversible (that is, you can work backwards from the output to get your input again), there is no upper limit to efficiency.

The same is true of the spring. Although real-world springs will experience some degree of crystal dislocations and the like that dissipate heat, these things aren’t fundamental. One can, in principle, have a completely reversible spring. Two charged electrodes being pushed together will get pretty close.

All this is unlike heat engines or exothermic reactions, where the inefficiency is completely unavoidable. For heat engines, the Carnot cycle assumes reversibility (since that represents the upper limit of efficiency), and from there proves that you can only extract so much energy from the heat–less than what went into the heat to start with. The rest–a pretty large proportion, depending on the relative temperatures–has to be sent to the environment.

An interesting factoid - humans generate more energy per unit volume than the Sun!

We would feel comfortable at 98.6ºF - if we were nudists. Since we wear these things called ‘clothes’, which trap body heat, we feel more comfortable at lower temperatures.

It depends on the ambient relative humidity.
I can guarantee that you would not feel comfortable at 98.6°F and 90% RH.

The flowing robes worn by desert dwelling cultures seem to be contrary evidence.

This is nonsense. We are constantly generating waste heat that we need to dispose of, for reasons explained above.

When there is little or no temperature gradient, the only way to dispose of heat is by evaporative cooling from sweat. So, as beowulff says, at high temperature our comfort level is highly dependent on RH. But even at low humidity, nobody would remotely describe 98F as “comfortable”. And you will quickly die without water and electrolyte replacement.

*raises hand

That’s my favorite kind of weather. It’s awesome!

Which one? The 90% relative humidity or the low humidity (I’m assuming 90% is not considered ‘low’ by any reasonable human being)?

Living on the Gulf Coast, being outside anything over 95 degrees and 70% relative humidity is like trying to breath through a sponge, aka “powder your nards” weather as some local radio DJs put it.

Low humidity and 98.6 degrees is infinitely preferable but not comfortable by any stretch, since that’s high enough for actual A/C to be necessary for many people lest we see a spate of heat related emergencies and deaths.