Why Do Glasses Get Foggy in Cold Weather?

This week’s episode comes from none other than Grammar Girl herself! She asks:

“I’ve been wondering why my glasses fog up when I ski and, more important, what can I do to keep it from happening? I know it must have something to do with the warmth of my body and the cold of the air, but it seems to happen at random times.”

That’s a great question Grammar Girl! The answer has to do with a topic we mentioned briefly during our states of matter episode, namely how matter changes from one state to another; a process called phase transition..

Is it Hot in Here?

Most of us are familiar with phase transitions, though we might not call them that in our day-to-day conversations. Most phase transitions come about through a change of temperature or pressure (or both), which result in a change to the substance’s internal energy.

For example, if you take a piece of ice out of your freezer and set it on the counter in your kitchen, it will start to melt (unless you have a very, very cold kitchen). The reason the ice begins to melt is because the thermal energy from the air flows into the ice cube. The reason this happens is because heat always flows from things with a higher temperature to things with a lower temperature, until they are the same temperature, a state known as “equilibrium.”

As you hopefully remember from last week, one of the differences between the solid state of matter and the liquid state of matter is that the particles in a liquid are more energetic, meaning they have more internal energy.

As the ice cube continues to absorb thermal energy from the air, that heat energy causes it to undergo a phase transition from solid to liquid, a process which scientists have given the clever name of “melting.”

Out of the Frying Pan

If we do the same experiment, but instead of placing the ice cube on the kitchen counter, we put it in a frying pan on the stove and turn on the burner, we’ll see that after the ice transitions from solid to liquid phase, it will change yet again. Thermal energy from the hot frying pan will flow into the water, causing its internal energy to increase further, which allows the water to make the phase transition to the gas state, a process called “vaporization.”

If you’re doing this experiment in your own kitchen, you’ll notice water droplets forming on the surface above your stove, or perhaps on the inside of your kitchen windows. That’s because once the steam leaves the frying pan, and hits a surface that has less thermal energy than the steam does, some of the steam’s energy goes into that surface. If it loses enough energy (and the colder the surface is, the more energy the steam will lose), then the steam will find that it doesn’t have enough energy to maintain itself in a gas state so it transitions back to liquid, a process called “condensation.”

If you were to scrape all of that water off the window and put it back into the freezer, it would suddenly find itself in an environment where it had a higher temperature than its surrounding air. So once again thermal energy would leave the water, causing it to lose internal energy until it could no longer maintain its liquid state and had to transition to a solid. You probably won’t be surprised to learn that this type of transition is called “freezing.”

Um, About My Glasses?

That’s all lovely of course, but what does it all have to do with Grammar Girl’s glasses getting foggy? When you’re outside skiing, your glasses are most likely pretty cold, since much of their thermal energy will have been sapped away by the very cold air. Now, you may have noticed that when you breathe, your breath typically has a fair amount of moisture in it. When this moist air hits your cold glasses, the moisture condenses into a foggy film.

Another cause of moisture hitting your cold glasses can come from sweat, especially if you’re the kind of person that sets your glasses on your forehead occasionally. As sweat evaporates from your skin, the water vapor can also condense against your glasses, causing them to fog up.

The most common remedy for foggy glasses is to apply some type of anti-fog material, which generally consists of a transparent, water-repellent material, which allows you to still see through your glasses, but discourages the water from condensing on it.

Conclusion

So now you know how matter changes states, why that makes your glasses get foggy, and what you can do to prevent it.

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Melting Ice Cube and Eye Glasses images from Shutterstock