Last night while sitting with friends, I watched as several people came in from the minus seventeen degrees outside to the very warm atmosphere of the café. Every one of them wearing glasses had to stop at the top of the stairs, blinded by their own eyewear.
People who have never fully experienced the kind of climate we usually can expect in January and February in my part of Canada may need an explanation of a phenomenon that happens to everyone who wears eyeglasses.
There is always water in the air landing on and evaporating from your glasses. under normal conditions, this is almost (if not outright) invisible.
When you’ve been outside in cold weather for sufficiently long, your glasses cool down, and the water that condenses on your glasses will not be warmed as much, hence the rate of evaporation decreases. Upon entering a warm building, the water vapour coming into contact with your glasses immediately condenses on the cold surface, but cannot evaporate quickly enough so the glasses fog up.
Hilarious watching patrons of the café trying to flag down the newcomers who we all know are standing there baffled and disoriented.
Fortunately the glasses will eventually assume room temperature and the condensation evaporates. This can be aided by wiping the lenses, or just removing the glasses until they become clear again.