I understand that goggles fog up because of a rapid temperature change. In other words, goggles that have been sitting on a table outside (i.e., are very cold), when you put them on your face which emits warm air, can start to fog. Dangerous for skiing, so the conventional wisdom is to try to always not take them on/off constantly.
However, I just read another piece of advice: when you take them off, don't keep them on your head, because your head emits warmth.
But that seems contradictory… if your head emits warmth, then wouldn't keeping the goggles warmer/closer to body temperature mitigate the large temperature difference when you do put them on your face again, meaning it's less likely to fog??
Or, is it that your head is much hotter than your face, so there still ends up being a temperature difference, but in the other direction?
Best Answer
When you do sports, your body needs to cool down. This is done by sweating: Your body vaporises water to cool itself down. Since your brain need a lot of blood, a "huge" percentage (something like 50%) of the heat is emitted by your head. If you wear the googles on your head, you put them into a "humid environment". Therefore, as soon as the temperature drops, they can not maintain the vaporised water. The water condenses.