Wearing a mask with glasses cause the lenses to fog up when exhaling. Why does inhaling quickly de-fog them?

Question

Wearing a mask with glasses cause the lenses to fog up when exhaling. Why does inhaling quickly de-fog them?

In: Other

Answer 1

The fogging is caused by moisture condensing out of the air, and warm air has more capacity to carry moisture. So as you exhale, your warm, moist breath is mixing with the cooler ambient air, AND blowing across the cooler surface of the glasses. So the exhaled air cools, and can not hold the same amount of moisture, and this condenses on your lenses.

So… when you breath in – you are pulling cooler air across the lenses, this air is now not near it’s limit for moisture and can relatively quickly absorb the moisture back into the air.

You should notice the effect varies based on the ambient conditions – outside with cooler but moist air this will be slower, inside with heated and lower relative moisture air, this will happen more quickly.