It is the same reason why beads of moisture appear on your cool glass of lemonade in the summer. Condensation occurs when water vapor cools and changes from a vapor to a liquid. Where does this water vapor come from? If we're talking about your eyeglasses or sunglasses, it comes from your own breath. You could also be sweating on your glasses, but that is a slightly different reason.
Your lenses will be relatively cool compared to your breath, especially when the outside air is cold. When vapor hits a surface that has less thermal energy, some of its energy goes into that surface. Condensation occurs when the gas, or vapor, doesn’t have enough energy to maintain itself in a gas state so it transitions back to liquid. Particles in liquid have less internal energy than gas but more internal energy than particles in a solid.
There are three phases in which matter can exist; a solid, liquid, or gas. The change from vapor to liquid is and example of a phase transition. Other examples of phase changes are melting (changing from a solid to a liquid), freezing (changing from a liquid to a solid), and evaporation (changing from a liquid to a gas.) Changes in external conditions such as temperature or pressure can cause matter to shift from one form to another.
So now you know why your glasses are fogging up, but what can you do about it?
Try buying glasses, sunglasses, or goggles with anti-fog properties included in the lenses. Wiley X and Liberty Sport are known for having really great anti-fog lenses. Anti-fog is especially important for snow sports. Some other options to explore: Anti-fog wipes Choosing glasses that sit farther from your face Contact lenses
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Why Do My Glasses Fog Up? (2015, March). Retrieved from coldavenger.com: https://coldavenger.com/blogs/news/17573392-why-do-my-glasses-fog-up
Why Do Glasses Get Foggy in Cold Weather (2013, March). Retrieved from quickanddirtytips.com: http://www.quickanddirtytips.com/education/science/why-do-glasses-get-foggy-in-cold-weather
When it's cold outside and I walk into a heated building, why do my glasses fog up? (2013, October). Retrieved from quora.com: https://www.quora.com/When-its-cold-outside-and-I-walk-into-a-heated-building-why-do-my-glasses-fog-up