Can someone help me understand the thermodynamics behind this?
My naive understanding would be that if the temperature > human body temperature, we can only gain heat from the environment, leading to death. Obviously sweating is involved. But how?
Evaporative cooling is one of physics's great magic tricks.
Basically, when we talk about a substance's temperature, that temperature is the average temperature of the system.
Temperature is just "how quickly are the atoms/molecules here jiggling?" When the atoms/molecules collide, some of them speed up, some slow down. That means that some atoms are "hotter" than the average temperature, and some are cooler.
Liquids have a certain energy above which a molecule is ejected from the puddle. This is evaporation.
Even if the average temperature (read: energy) is below that threshold energy, some molecules are moving faster, because they have collided with other molecules in just the right way. Those fast molecules get ejected. Evaporation!
But now that the puddle lost the fast (read: hot, read: high energy) molecules, the average energy of the system is lower. The whole thing cools down!
In dry environments, it's possible for sweat to evaporte (cool) at a rate that is faster than the rate at which your body heats up from hot colliding air.
In humid environments two things happen: evaporation slows down, and you heat up faster.
So there is a combination of temperature & humidity at which, if either of those parameters is exceeded, you gain heat faster than you lose it.
The goal of human metabolism is to maintain a consistent body temperature. Metabolism creates heat (as a byproduct of doing useful work) that needs to be shed, this is in addition to the natural heat flux.
Heat flux is proportional to temperature difference, and is solely in the direction of hot to cold.
Let us suppose that at 27 C, the natural heat generation is shed, and let us also suppose that the core body temperature is 37 C. (The actual values don't matter, but this feels approximately correct and gives nice round numbers). In this case, if the temperature is below 27 C, then we are losing too much heat and our body temperature will decrease. If it is above 27 C but below 37 C, we're not shedding our body heat fast enough, so body temperature will increase. If it is above 37 C, then the environment is actually adding excess heat into our body instead of cooling it--and we still have natural metabolic heat to shed.
So we have three main mechanisms we use to regulate body temperature.
The first mechanism is to change the heat flux: by changing our clothing, we can control heat loss. This is obviously more effective for cold temperatures than warm temperatures.
The second mechanism is to change our internal heat generation. For example, staying more active in cold conditions or being less active in hot conditions. The response of shivering is in fact an attempt to generate extra heat to mitigate heat loss.
The final mechanism is to use evaporative cooling. Converting liquid water to gas requires energy. So if you coat yourself in water that gets evaporated, it actively sucks heat away from you in the process. The wet-bulb temperature is a measurement of the coolest you can make something in this method... and that's why wet-bulb temperatures near body temperature are lethal: it's the point at which you just cannot shed any heat whatsoever.
4) Increase/decrease the blood flow towards our skin and arms/legs.
This is why in hot conditions, blood vessels may widen, the heart pumps harder to increase core <-> skin heat exchange, and you see people blushing.
In turn, this "heart pumps harder" (or failing to do so) may worsen other conditions (especially in older people), and eg. contribute to kidney failure (and that in turn, to blood poisoning).
Evaporation (specifically phase change of liquid water to water vapor) draws away a LOT of heat.
Without sweating, we’d be dead even if the air temp was near body temp because of the delta in temperatures required to transfer excess heat from our bodies.
thermokinetic energy is required for water to transition from liquid to vapor.
when heat [thermokinetic motion] spreads from the body to the liquid water, this is a net loss loss of heat from the body as water transitions to vapor phase and diffuses away from the body. if surrounding air is already saturated with vapor [relative humidity 100%] no evaporation can occur at a temperature, that humans will survive
My naive understanding would be that if the temperature > human body temperature, we can only gain heat from the environment, leading to death. Obviously sweating is involved. But how?