Name: Christopher N.
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
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.
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.
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
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
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!
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.
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.
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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Update: June 2012