alt text
Comic strip of a ghost and a person with the American flag pasted on the head. The ghost repeats "Boo!" in the first three panels without getting any reaction, but when it in the fourth panel says "kg, cm, km, °C" the American gets scared and screams "AHHHH!!!".
Edit: fixed alt text
It's based on how humans react to the heat, you need active cooling such as sweat, moving air isn't enough above 100 degrees. 100% hot out is just a silly way of putting it.
I see. But is zero degrees Fahrenheit based on anything?
Supposedly the temperature salt freezes at, but it's off by quite a bit. I'm not sure if it has any implications for staying warm in cold weather.
I found it on Wikipedia. At first, he fixed zero at the stable temperature of a "mixture of ice, water, and salis Armoniaci [transl. ammonium chloride]" and 96 at the human body temperature, but later he would change the lower reference point to water's freezing point at 32 and still later the upper one to the boiling point of water at 212. So it has always been pretty arbitrary.
Edit: But I will agree that the scale of zero to one hundred does correspond more closely to how warm humans feel.