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Why Do Glasses Get Foggy in Cold Weather?

Everyday Einstein answers Grammar Girl’s question about why her glasses get foggy in the cold.

By
Lee Falin, PhD,
March 29, 2013
Episode #048

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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.

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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.”

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