The windows in my car get foggy too, sometimes. It depends a lot on the
weather and also the people inside and whether the air
conditioning/heater is running.
Car windows fog up when water condenses on them. This can happen
both on the inside of the window and on the outside. You can check to
see in your mom's car where the condensation is because it will be wet
to the touch.
It sounds like outside humidity might be a problem (especially on
the East coast). It is here in Illinois, particularly on hot, stormy
days in the summer. Water will condense on a surface if the temperature
of the surface is below the dew point of the air next to that surface.
So you need warm, humid air next to a cooler surface to fog up.
If it is very humid outside and you're running the air conditioner
inside the car, water can condense on the outside of the windows. Using
the windshield wipers and the rear defroster can get rid of
condensation there by wiping it off or making it evaporate.
Often, the air in the car will be warmer than the air outside the
car (this happens in winter here). Moisture will condense on the inside
of the windows if the outside air is cooler than the dew point inside
the car. This happened to me a lot when driving through a thunderstorm
caused by an incoming cold front. In this case, running both the air
conditioner, the heater, and the vents for defrosting the front window
worked very effectively. The air conditioner dehumidifies air because
water condenses inside it when the air goes past the cold tubes with
the refrigerant inside. Heating it up way past its dew point makes it
feel dry, and it is dry -- it'll help evaporate any condensation that's
on the windshield -- and even more quickly the hotter it is because
heat is needed to cause the liquid water to change phase to a gas.
(republished on 07/24/06)