Breakdown: Why your glasses can get foggy

Updated: Aug. 19, 2019 at 7:43 PM CDT
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MEMPHIS, Tenn. (WMC) -Why do your glasses fog up when you go from your cooled car to the hot and steamy temperatures outside?

Those people who wear glasses probably have had this happen quite often.

You are driving in your nice air-conditioned car, only to get out into the hot and steamy conditions outside and your glasses are fogged up, obstructing your view.

Your glasses will fog up whenever you go from one environment to another where the dew point is reached. The dew point is the varying temperature at which air becomes saturated with water vapor.

Why does this nuisance happen?

When the temperature falls below the dew point moisture in the air condenses, change from vapor to a liquid, and causes fogging.

If your glasses have adequate time, they will cool to the temperature of the surrounding air, so if your car is a comfortable 70 degrees, your glasses will be too.

Then you step out of the car to a steamy and hot day and the moisture content of the air or dew point is in the upper 70s. The surrounding air that is touching the glass surface will condense. This will continue to occur until your glasses warm to a temperature higher than the dew point.

The higher the dew point the longer it will take your glasses to defog.

To prevent fogging you would have to set your temperature in your car higher than the dew point. Sure you may get out sweaty but there would be no fog. Ha!

The same thing can occur when it is cold outside, only it would be opposite. Inside it would be nice and toasty but outside it is frigid. The only difference is winter air is a lot drier so if the dew points are in the 30s inside, it would have to be even colder outside.

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