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How To Deal With Foggy Sunglasses In The Cold

It's annoying to get into your car on a cold morning, take out your sunglasses, and have them instantly fog up to the point that they are useless to see through. But taking them off and letting them get cold again just makes the problem return. So what is the best way to deal with foggy sunglasses in the cold?


Foggy Sunglasses

First, let's understand why the lenses fog up in the first place.


Fogging is just water condensing from the air, and it happens because the lens is colder than the air around it. If you could zoom in on the fog, you would see tiny droplets of water. This is the same effect a cold can of soda just out of the fridge has, or a mirror next to a shower. Mark Rober has a good video on how to defog your car windshield as quickly as possible, where he shows how water condenses with a few analogies:



For sunglasses, the main issue is the heat coming off our faces. The air near our faces is warm and humid. When we exhale, the air coming out of our mouth and nose is even warmer and even more humid.


When that warm, moist air hits a cold surface, the air cools down fast. Cooler air can’t hold as much water vapor, so the extra moisture has to go somewhere. The water settles on the lens as tiny droplets. That’s the fog you see.


(Smudges make it worse. If your lenses have oils or dirt, those tiny droplets stick and spread unevenly, which makes the fog look thicker.)


There are a lot of different ways to help this, from vented sunglasses (not common), anti-fog coating or spray (which don't last very long), or warming your sunglasses in advance (who has time for that in the morning?). Realistically, most people are not going to be using other products to solve this problem.


But there is one solution that works for any pair of sunglasses, and it is what I do.


What is the solution? Letting them warm up more slowly by wearing them down your nose for two minutes.


Yes, for two minutes, I look a little funny, like the girls who wear the Ray-Ban Round sunglasses halfway down their noses on a bright day. I wear it even further down than they do. Silly, but it works.



It works because it creates a huge venting area between the lenses and my face, and because it isn't warm enough at that distance to fog the lenses, but it is warm enough to start heating them. I also try to breathe out with a bit of force so the air moves away from my face and not straight up.


After around two minutes, the lenses are warm enough to push up my nose and wear normally.


Another aspect that is useful to know is that different materials and coatings make a difference here. Some materials fog less naturally. Glass lenses with certain hydrophobic coatings clear faster than polycarbonate without coatings.


Glass has a higher thermal mass and better thermal conductivity than polycarbonate or TAC.


Thermal mass is basically a material’s ability to absorb, store, and hold heat.


When we say glass has a higher thermal mass than polycarbonate, we mean that it takes more energy to change its temperature, and if you put both materials in a cold environment, glass cools down more slowly, while plastic cools down very fast.


If you bring them into warm air, glass warms up faster at the surface because it can absorb more heat without its temperature swinging wildly. Plastic stays cold longer because it doesn’t hold heat well.


(This is also one of the main reasons why plastic lenses and their coatings get ruined and delaminate more than glass lenses in high heat or very cold.)


Because glass doesn’t get as cold as quickly when warm air hits it, the surface warms up faster.


A warmer surface reduces condensation because the air doesn’t cool down as sharply on contact. Less temperature shock = less fog.


Polycarbonate and TAC are light and low-mass, so they equalize with the outdoor temperature almost instantly. When warm air hits them, they stay cold longer and fog up right away.


Hydrophobic (water repellent) coatings also help by making water bead up and roll off instead of spreading across the lens.


This helps in two ways:

Serengeti Sedona
Serengeti Sedona

1) Drops stay smaller, so the fog layer is thinner.

2) They clear faster as the drops move or evaporate.


In contrast, uncoated polycarbonate is slightly hydrophilic. Water spreads out more, turning into a cloudy film rather than discrete droplets.


For what it's worth, I wear the Serengeti Sedona lens the most in the winter (review here). It is glass and has hydrophobic and oleophobic coatings.


Here are some brands that have glass lenses with hydrophobic coatings (make sure it's glass, most have non-glass as well), and not just anti-reflective coating:



I review many of these lenses here on Sunglass Science, and talk about the best lenses to wear in the winter here.


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