Question: Why does cold air hold less moisture than warm air? Why does cool air clear a foggy windshield better than warm air? (Asked by a foggy column reader via internet.)

Reply: Warm air provides a better environment for gaseous water than does cold air. The reason water is found in gaseous form is because molecules have sufficient energy. At lower temperatures, they will be in liquids and solids. Warm air transfers energy to solid and liquid molecules, helping them to achieve a gaseous state.

A somewhat simplistic way of looking at it is that the molecules in cold air are more “tightly packed” and there is no “vacant” space for water molecules to collect. When you have warm air, the voids between the molecules are “spread out” and water vapor can easily form. This is basically expressed in the ideal gas law, pV=nkT. Increase the temperature (T) of a given amount of gas (n), and the volume (V) will increase, along with pressure (p).

Remember that if the air temperature is lowered to the dew point, then the air is saturated (100% humidity). The air cannot hold any more moisture. To add moisture, you have to raise the temperature or warm the air.

With regard to the windshield fogging, let’s first look at the cause, namely the condensation of moisture or humidity. If the air inside the car is hot and humid, moisture may condense on the relatively cool surface of the windshield, causing it to fog up. Water vapor condenses on cold surfaces. If it’s hot and humid outside the car and cool inside, there may be condensing on the outside of the windshield. Use the windshield wipers to clear and turn down the air conditioning.

In the first case, rolling down the windows and getting some fresh, less humid air may help. If not, turn on the defroster (defogger?). The car’s air conditioning unit removes moisture from the air (as evidenced by a puddle when you park). Then, the cooler, dryer air blowing across the windshield promotes evaporation and defogging.

The main thing to keep in mind is the humidity inside the car. In addition to normal air humidity, more is produced by you and your passengers in breathing — moisture from air inside the lungs. Air conditioning can help remove this. Some people keep their car’s air conditioning/heating on “recycle,” thinking it cools or heats up faster using recycled air. This may be true, but you are recycling a lot of humidity.

Oh, about the “defroster.” Cars have a warm air heating mode for the winter. This may help not only to melt ice or frost on the windshield, but to defog on the inside. The warm air can help promote evaporation to the inside cooler air.

C.P.S. (Curious Postscript): “Things turn out best for the people who make the best out of the way things turn out.” —Art Linkletter

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