Why Do Glasses Fog Up When You Go Indoors in Winter?
On a snowy evening you open the restaurant door and the world turns white. You can smell the broth, but you cannot see the menu or the faces of the people you came with. Take your glasses off, wait a moment, and they clear as if nothing had happened. They were fine outside, so why do they fog over the moment you step somewhere warm?
The chilled lens cools the indoor air right next to it below the dew point, and water vapour in that air settles on the lens as tiny droplets. Your glasses have not become dirty; water has changed from gas to liquid on the lens surface.
This article centres on the principles of dew point and condensation (water forming on a cold surface). Fogging with a mask, anti-fog coatings and how to clean lenses are added briefly as general information.
There is a limit to the water air can hold
Air contains water in an invisible gas form: water vapour. How much vapour air can hold depends on temperature, with more when warm and less when cold. So even if the amount of vapour stays the same, air that keeps cooling reaches its limit at some point.
The temperature at that point is the dew point. The US National Weather Service's explanation defines it as the temperature at which air, when cooled, reaches 100% relative humidity. Cool it further and the vapour the air can no longer hold turns into liquid droplets.
The same explanation gives the example of air at 80°F (about 27°C) with a dew point of 60°F (about 16°C), which has a relative humidity of 50%. In air like that, water can form on the surface of anything colder than about 16°C. The whole room does not need to be 16°C.
What happens beside the lens as the door opens
While you walk outside on a cold day, your glasses are cooled by the outdoor air too. Step indoors in that state and warm air meets the cold lens. It is not that all the air in the room cools; only the thin layer of air touching the lens suddenly turns cold.
When that thin layer falls below the dew point, droplets begin to form. It is the same principle as the outside of a glass of cold drink getting wet, or dew forming on blades of grass at dawn.
Going the other way, from a warm room into the cold, this usually does not happen. The lens is the warm side, and the dew point of cold outdoor air is far below the temperature of the lens.
Why does clear water look cloudy?
Water is transparent, so why does the lens look white? The answer lies in its shape more than its amount. A 2026 review of anti-fog surfaces for medical goggles explains that separate droplets scatter light strongly, because air and water bend light to different degrees.
It helps to think of each droplet as a tiny convex lens, sending light off in its own direction. When a lens is densely covered with these droplets, light from the scene beyond cannot travel straight to your eye.
The same amount of water behaves differently when it spreads evenly into a thin film. That is why splashing water on a steamed-up bathroom mirror actually lets you see your face.
When is it worse, and why does it clear by itself?
The colder your glasses have become and the higher the dew point of the place you enter, the more easily droplets form. A high dew point means there is a lot of water vapour in the air. So on the same day, glasses may fog more in a steamy restaurant than in a dry building lobby.
The same principle explains why they clear after a moment. Once room air has warmed the lens above the dew point, the water that formed evaporates again. If the lens is still cold, droplets may form again even after you wipe it. If you are not in a hurry, one option is to take your glasses off for a moment and wait.
Why do masks make glasses fog up?
With a mask, the raw material for fog is breath, not room air. A 2011 article in the Annals of the Royal College of Surgeons of England explains that when a mask pushes exhaled air upwards, warm vapour meets the cooler lens and forms tiny droplets. Breath contains a lot of water vapour, so it condenses more easily than room air.
A 2021 article in the Indian Journal of Ophthalmology points to the gap between mask and nose as the cause. So the first step is to press the nose section to fit your face and reduce the gap. If there is a set way of wearing a particular type of mask or in a particular place, follow that guidance first.
Fogging from sweat and temperature changes during exercise is touched on briefly in our article on protective eyewear.
What do anti-fog coatings and soap change?
Anti-fog treatments usually change the shape of the water that forms, rather than stopping it forming in the first place. The review mentioned above explains that making a surface compatible with water causes condensed water to spread quickly into a continuous film. It adds, however, that anti-fog agents weakly attached to a surface come off easily, and that water-repelling microstructures may be vulnerable to everyday friction and washing. Since this summarizes research on goggles, it cannot be applied directly to glasses lenses.
Washing with soap is explained by a similar principle. The Annals article wrote that washing with soapy water leaves a thin detergent film that lowers surface tension, so water spreads evenly. The Indian Journal of Ophthalmology article, on the other hand, points out that soapy water can damage the coating on glasses.
The sources disagree on this point, so it is hard to give one definite answer. Many lenses have treatments such as an anti-reflective coating, so it is best to confirm how to care for your own lenses with your optician or the manufacturer's guidance.
When wiping fogged lenses
Optometry Australia's guide to caring for glasses explains that wiping with a T-shirt, old rag or paper towel may be convenient but scratches lenses over time. It also advises avoiding hot water and abrasive materials.
When lenses are very dirty, it recommends this order: wash them using a small amount of hand soap or gentle detergent on your fingertips, rinse with lukewarm water, pat dry with a soft tissue and finish with a microfibre cloth. As noted above, care can differ with the coating, so here too it is good to check the guidance for your own lenses.
Next time your glasses turn white, watch for a moment before reaching for the hem of your shirt. As the cloudy lens slowly clears, the same thing that puts dew on grass at dawn has appeared and is vanishing in front of your face.