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How-To  Prevent   Fog   &   Frost   On   Windows

fog and frost on windowsInside a car there are several competing effects. Firstly, you have at least one person breathing. The air that you exhale is very moist. In cool weather your breath will often supersaturate as it cools to the temperature of the surrounding air, hence you see a fog as you exhale. Every breathing person in the car is adding water to the air. Eventually the air will saturate. You will get fog or frost. So to prevent fog and frost on windows, follow these 5 simple things.

Keep a good flow of air coming in from the outside

In fact, the colder it is outside the drier that air is, so the more beneficial it is to compensate for your breathing. Typically a car fan will have settings from "off" (obviously not good) through various levels up to "recirculate". The most common mistake is to drive with the fan set at "recirculate" in order to have a high interior temperature to avoid fogging the windows. This may work in a car, it is less likely in a van, but in either case if you park on the street be prepared for a huge amount of frost on the inside windows when you return. "Recirculate" means reuse the air inside the car - so yes it is faster to get to higher temperatures, but the relative humidity of the air in the car will keep rising. I use recirculate only to attain a more comfortable state quickly, and possibly to melt existing frost more quickly, but to keep it on once the car temperature has started to rise is asking for trouble.

Keep the car cool

Even a few degrees can affect the maximum amount of water in the air quite significantly. The less water in the air, the slower the windows will frost up. The dilemma here is that if the windows are frosty it seems best to heat up the car. Its best to scrape that frost off, but if you want to do it with heat take a long enough drive that you can turn down the temperature and drive until the air is cool and dry inside (or else you'll have the same start with frosty windows problem next time.

Use the temperature setting to control the temperature

That might seem obvious, but I often see people turn down the airflow as the car gets too hot. It might be more pleasant due to less wind and reducing noise in the car, but if frosting windows is your concern don't lose sight of #1 above. There is a temperature control that affects how the air is heated as it enters the car - use that to control the car temperature.

Warm the windows

Your windows will be colder than the air inside the car. This is especially true if cold rain or snow is hitting them, but with a constant flow of cold air against them as you drive they will naturally cool down. You need to direct the warm air from the vents against the windows to compensate for this. The bigger the difference between the air and the window temperatures, the lower the relative humidity that will still fog the windows up. This can be illustrated with the doubling every 10°C rule. For example, suppose the air is 20°C (68°F), and a window is at 10°C (50°F). If the air in the car is over 50% relative humidity the window will fog up. If the window is at 0°C (32°F) it will fog up if the air is above 25% relative humidity. Hence, warming the window by even a few degrees can make all the difference between a window fogging up or not under given conditions.

Keep outside air flowing over the windows

Even if the air isn't warm, the fact is that outside air - having been colder - will probably be drier than the air inside the car. The drier the air against the window the faster the water can evaporate off because there is less condensing back on from the air. You can demonstrate this by putting a little water in two saucers. Leave one standing, set a fan blowing over the other. The one with the fan will dry out faster (both will take a long time, depending on how much you put in). The one without the fan gets moist air over the water surface from evaporation, and without that being pushed away it is soon condensing back in about as fast as it evaporates.

(article originally published by DC Physics)

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