"Temperate is basically the jiggling motion of atoms. The more they jiggle and move about, the hotter the temperature!" So exclaimed the late Richard Feynman in a 1983 interview with the BBC.
Knowing this renowned physicist's words, one might wonder why a pocket of air in motion — i.e. wind — feels cooler than still air. When you sit in front of a fan on a hot day, the blades propel air molecules toward you, speeding them up and smacking them against your skin. Why don't these energetic molecules burn?
The explanation lies in the fact that there are two types of motion on the atomic scale. One feels hot; the other feels cold.
When atoms and molecules are jiggling really rapidly in random directions, they feel hot against our skin. But the collective motion of all the atoms and molecules in a single direction doesn't affect their overall temperature. To the contrary, when air bombards us, it cools us down, because it increases the rate at which heat leaves our bodies. [How Hot Is Hell?]
Heat is removed from the skin by processes of evaporation, convection, radiation and conduction. For any of these to happen, molecules in the air must bump against the hot (i.e. rapidly jiggling) molecules of our skin, so that some of that heat energy can be transferred from the latter to the former molecules. After they've made contact, the air molecules must move away, carrying what used to be our body heat with them in the form of jiggling.
The faster the turnover of air molecules bumping against and then moving off our skin, the cooler we become. That's why, on a 90-degree day, you can find me in front of a fan.
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Natalie Wolchover was a staff writer for Live Science from 2010 to 2012 and is currently a senior physics writer and editor for Quanta Magazine. She holds a bachelor's degree in physics from Tufts University and has studied physics at the University of California, Berkeley. Along with the staff of Quanta, Wolchover won the 2022 Pulitzer Prize for explanatory writing for her work on the building of the James Webb Space Telescope. Her work has also appeared in the The Best American Science and Nature Writing and The Best Writing on Mathematics, Nature, The New Yorker and Popular Science. She was the 2016 winner of the Evert Clark/Seth Payne Award, an annual prize for young science journalists, as well as the winner of the 2017 Science Communication Award for the American Institute of Physics.