Biology and Physics
Why Does 90 ºF Feel Hot?
We've talked a lot about heat and temperature, but we still haven't really explained why 90 °F feels hot and 50 °F does not. Sure, at 90 °F objects have more internal energy than at 50 °F. Molecules are zipping about a lot faster, but what does the internal energy of the air have to do with how we feel? And how do zippy air molecules make us sweat buckets?
The answers to all of these questions lie in the Second Law of Thermodynamics. Heat always flows from warm objects to cold. If a warm object is next to a cold one, the heat will shimmy about between them until both objects are at the same temperature. The universe, essentially, strives for everything in it to be at the same temperature. Obviously, it has a long way to go to achieve that goal.
Our human body temperatures average a comfortable 98.6 °F. If it gets too much higher than that, processes within the body start going terribly, terribly wrong. Proteins denature and cells die. It's kind of like being cooked from the inside out. So, not our idea of a good time.
Being feverish is a problem even on a molecular level. See, all of the normal chemical reactions that keep us alive raise our body temperature, so our bodies works very hard at kicking out all of that extra energy—even when we're not home sick in bed.
On a 50 °F day, there is a large heat gradient between our internal body temperature and that of the outside. All that heat generated by our body can quickly escape down that steep heat gradient (as if the heat were "rolling down" a hill) through our skin by radiation and conduction. On a 50 °F day our bodies are like a hot reservoir surrounded by much cooler air.
On a sticky summer day, when the air temperature approaches the value of our body temperatures, it's harder for heat to escape our bodies. On a 100 °F day, heat is actually flowing into the body. Now we can't dump our extra heat through radiation and conduction. The only way to keep cool is to sweat. That sweat can then evaporate from the skin, taking with it some unwanted heat. In the Southern U.S. or in the tropics, even sweating isn't enough (because of the high humidity reducing evaporation), and that's why fans were invented.
How does evaporating sweat cool us down? Is it any surprise that the answer is "thermodynamics"? Think about it: what does it take to turn liquid sweat into a gas? It's a phase change, and those things need energy. We have to get those sweat molecules moving faster and faster, and the energy for this comes in the form of body heat. Body heat is used up in evaporating the sweat. With every bit of sweat we evaporate, our bodies are able to lose some unwanted heat.
That's why 90 °F feels hot and uncomfortable: because our bodies have to work harder (read: sweat) to maintain normal temperature. On a 50 °F day, dumping heat is a cinch, with no need for sweating, fans, or distress.