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Avoiding Frost & Fog on Windshields

January 2003

January, that magical time of year when those of us in "Winter Cities" live in a beautiful world of white. The problem is... when that whiteness includes our car windows it can be downright dangerous. I watched my van drive up today after being loaned out for driving to a winter youth camp for the weekend. Some windows were quite white. Admittedly it was a cold day of -20 C (-4 F), and I'm sure the van was full of teenagers who just wouldn't stop breathing! However, it occurred to me that even in Winnipeg where this is a fact of life, there are probably many people who could use some practical advice about how their choices affect the fogging up of a vehicle's windows.

Dew Point, Relative Humidity, and Fog in Air

Before explaining fog and frost on a window, we'll begin with water in the air. After all, that's where the frost is coming from!

At any given air temperature there is a maximum pressure of water that air can hold. The higher the temperature, the higher that pressure is... doubling approximately every 10oC (20oF). So, air at room temperature can hold approximately 4 times as much water as air at freezing (0oC or 32oF). By 100oC (212oF) that pressure is (sea level) atmospheric pressure. Trying to heat water so its vapor pressure is above the pressure of the surrounding air will cause "boiling". Below sea level (higher surrounding air pressure) water can be heated above 100oC. Above sea level (lower surrounding air pressure), water will boil at less than 100oC.

The ratio of the water pressure in the air compared to the maximum possible pressure at that temperature is known as the "relative humidity" of the air. When air has reached 100% relative humidity we say it is saturated. If more water is put in, the air is supersaturated - which means the water will form into droplets. For an example, look carefully at a boiling pot of water. The water which has heated into a gas - "steam" - leaves the pot at 100oC (212oF) and is invisible. However, the air above the pot is at room temperature. As the steam mixes into the surrounding air it cools and there is now too much water gas for the temperature it is at. So, shortly above the pot the air supersaturates, and the water forms into tiny droplets (making a fog which many people mistakenly refer to as the "steam"). Follow that mist further into the room, and it disappears again. Now as the droplets get into air that isn't saturated yet there is room for more water. So the droplets evaporate, and the relative humidity of the whole room rises. Eventually, especially with windows closed, the whole room will supersaturate and fill with a mist, or you will see fogging on colder surface (such as, perhaps, the window if it is cold outside).

We can turn this around the other way then -- given that we have air which has some amount of water in it, there is a certain temperature at which it will supersaturate - this is called the dew point. Typically during the day water evaporates into the air, but not all the way to 100% relative humidity. There is then a temperature where the water will start to condense out of the air - perhaps as a fog, dew, frost... this is called the dew point. If the overnight temperature dips below this, there will be dew for the morning.

Another well known effect is how winter is "static" season due to the dry air in buildings. The most saturated outside winter air contains very little water compared to the saturation level for warm inside air. When the outside air comes into your house and warms up (and if you don't eventually die of suffocation, it means you are exchanging air with the outside) - that small water content at a higher temperature now corresponds to a very low relative humidity.

Molecular Dynamics of Fogging and Unfogging

Let's consider this at a slightly more advanced level. Suppose you have a bowl of water with air above it at the same temperature. There are continually water molecules evaporating from the bowl. The higher the temperature of the water the faster this evaporation will occur. Simultaneously there are water molecules from the air hitting the water surface and "condensing" into the bowl. When the relative humidity is less than 100%, what is happening is that the rate water leaves the bowl is greater than the rate it is condensing back in. At 100% relative humidity the two rates are equal so the amount of water doesn't change. If the air supersaturates, anywhere water starts to form it will keep growing larger and larger. Hence fog (clouds), dew and frost will form as mentioned above. These are all indications of condensation being faster than evaporation.

This partly explains why humid hot weather is so much more upleasant than dry hot weather. Normally in hot weather your body sweats so it can cool itself by evaporation. The more humid the air is the slower the evaporation will be. So, there is a greater build up of sweat - making you feel sticky, and the sweat is less effective at cooling you (causing the body to produce more).

Now, about that car...

Inside 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:

job #1 is 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.

#2: 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).

#3: 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.

#4: 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 10oC rule. For example, suppose the air is 20oC (68oF), and a window is at 10oC (50oF). If the air in the car is over 50% relative humidity the window will fog up. If the window is at 0oC (32oF) 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.

#5: keep outside air flowing over the windows #4 emphasized using warm air flow against 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.

Something curious I've found is that fogging windows isn't exclusively a winter phenomena. We sometimes get the windows fog up on hot humid days and long trips. With the air conditioner running full time in the car the windows can become enough cold compared to the outside air to start fogging up on the outside. This only happens significantly near air vents, so an obvious solution is to change the vent direction. If it happens on the front windshield there are always the wipers to help out. Failing all else... it might be necessary to turn off the air conditioning for a while.

What about anti-fog coating?

These are popular for glasses, I don't know if I've ever seen them advertised for windows. What I've written about above talks about water in the air and on surfaces, but it is possible to slow the buildup of water on surfaces in the first place to reduce the problem. That's what antifog coatings are for. Maybe I'll write about that another time...

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