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Name: Christopher N.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 12/10/2002

Dear Recipient,

My students want to know why their glasses fog up when they come inside on a cold day. I could not explain this with any sense of clarity. Help.

The eyeglasses have been cooled outside to a temperature that is below the dew point (the temperature at which water vapor in the atmosphere will condense). When brought inside, the moisture/humidity in the inside air will come in contact with the cooled eyeglasses and condensate or fog. This fogging also occurs inside your car, on the windows in the house, etc.

In the summer, drink glasses with cool beverages, chilled cans, etc. will "sweat" in much the same way - the moisture in the air comes in contact with the cooled surface of the can or drinking glass which is at the dew point and condensates.

Steven Miller


When water vapor contacts cold glass, heat energy flows from the vapor to the glass. When that happens, the water molecules in the vapor lose some of their thermal energy and become able to cling to each other and to the glass surface -- thus the fogging to which you refer. When the glass warms up, the film of water will evaporate and the glass will clear up.

ProfHoff 560


The simple explanation is that water vapor present in the room air in the vicinity of the cold glass condenses and this is seen as fog on the glasses. Once the glasses warm back up to room temperature, it is likely the "fog" will re-evaporate into the room air.

If you try an experiment where a room has a dehumidifier running in it and has removed a good deal of the moisture, and someone again enters from a cold area with glasses (or removes a pair from the freezer to simplify things) you will note less fog, or perhaps none, in this situation. Note that by chilling the glasses you create a temperature effectively below the 'dew point' of the room. Water vapor in the room then condenses from the air onto the cold surface. Once the glasses warm up above the 'dew point' again, the liquid re-evaporates.

Speaking of dehumidifiers, this is the process in which they dry out a room. Air from the room is passed over a chilled surface (inside the dehumidifier) and the water condenses from the air leaving a drier air which then exits the dehumidifier.

Thanks for using NEWTON!

Ric Rupnik

The glasses are cold compared to the inside temperature. The amount of water vapor in the inside air exceeds the vapor pressure of water at the cold eye glass surface, so water vapor condenses to small droplets on the glass that look like "fogging". If the inside air were to very dry so that the amount of water in the air did not exceed the vapor pressure of water at the cold surface of the glasses, no "fogging" would occur, that it, no microscopic liquid droplets would form. If it is REALLY cold, like it gets in northern Wisconsin, Minnesota, or North Dakota, frost (ice) can form on the glasses rather than liquid mini-droplets.

Vince Calder

The students glasses are below the dew point temperature of the surrounding air. At that point, the water vapor in the air condenses on the glasses causing fog. The same thing happens in the summer when the students are in a cold building and they walk out into a hot humid day. Have your students look into psycometrics/HVAC to better understand dew point, dry temperature, wet bulb temperature, etc.

Good luck,

Chris Murphy

Their glasses fog up for the same reason that your bathroom mirror fogs up when you shower. Warm moist air, in this case from the students' face, hits a cooler surface, the students cold glasses, and the water vapor in the air condenses on the cool surface.

Paul Mahoney, PhD

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