This question popped into my head while studying simple thermodynamics. Let's say the air temperature outside is 30 degrees Celsius/ 86 fahrenheit, which is relatively high to the average, even though it's lower than our body's normal temperature, we feel that the air is hot, what is going on?
Note: I asked my physics teacher, he told me that the body got used to a certain amount of temperature that's getting lost to the air, and when that changes we feel hotter.
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1$\begingroup$ Define "our body's normal temperature". The core temperature is, on average 98.6 deg F. Humans generate a lot of "excess" heat, which must be transferred to the environment. This means that skin temperature is substantially lower than core temperature (e.g., 10-20 deg F lower), because heat transfer from "core to skin" necessarily requires a temperature difference. Likewise, heat transfer from "skin to room" requires a temperature difference. 86 deg F air does not give you the temperature difference that is required. $\endgroup$– David WhiteCommented Jan 5, 2018 at 2:49
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1 Answer
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We have to constantly dissipate all the heat we generate. Our body is constantly doing work, and the second law of thermodynamics tell us that we cannot do work with $\%100$ efficiency. That is some of the energy that we spends while doing work gets converted to heat as a byproduct. This heat needs to be dissipated to keep the body temperature from rising.
Our body needs to keep its temperature constant. That means it needs to dissipate heat at the same rate it generates it. But the rate of heat dissipation is determined by the outside temperature and the heat transfer coefficient (this is determined by wind, blood circulation pattern, sweating, clothing, ...). So our body has to modify its processes in response to the outside temperature.
We feel cold, when our body needs to produce more heat to keep its temperature constant. We feel warm when our body needs to either produce less heat, or dissipate it more efficiently (by sweating or changing the blood circulation pattern).
So we feel warm at lower than body temperature, when the natural metabolic rate of our body produces more heat that can be dissipated without extra effort such as sweating.
What we call feeling cold or warm is not a measure of the temperature, it is a signal for our body to modify behavior to keep its temperature constant.
This fact has an interesting consequence: consider three rooms at $20^\circ$C, $25^\circ$C, and $30^\circ$C. If you stay in the first room for a long time until your body gets used to this temperature then move to the second room, you feel warm. But if you stay in the third room for a long time and then move to the second room you feel cold. When your body gets used to a temperature, that means it has made all the necessary changes (either in metabolic rate or adjusting the heat transfer coefficient) to keep its temperature constant given the outside temperature and when moved to a new environment it has to make some changes. In our examples body needs to make opposite changes in the two scenarios and therefore feel different at the same temperature.
