Re: How to defog a car windscreen .

The best thing about answering Mad Scientist questions is that it forces me to find the answers to questions I'd been wondering about for a long time. To answer your question, we need to talk about humidity and condensation. I apologize for the length of this answer: it's a complicated subject!

The air we breathe contains a fair amount of water, mixed in with the air in gaseous form ('water vapor'). It evaporates from oceans, mud puddles and, and your sweat into the air, and if conditions change, it can turn back into a liquid and fall back down again (that's rain). Your breath also contains lots of water vapor, because you're wet inside.

Air that's holding the maximum possible amount of water for its temperature is 'saturated'. The 'relative humidity' of air measures how close to saturation it is. For example, if the air outside has half as much water vapor as saturated air would have, we say the 'relative humidity is 50%'. When the humidity is less than 100%, the air can suck up more water by evaporating water from oceans, puddles, and your sweat. When the humidity is 100%, the air can't hold any more water, so sweat won't evaporate, so you feel hot and sticky in humid weather.

The most interesting thing about water vapor is that warm air can hold more water vapor than cold air. It's the same as dissolving sugar in water: you can dissolve more sugar in hot water than in cold water. Suppose we take a glass of hot water and mix in as much sugar as it will hold. Then we cool it down. The water says "Hey! I can't hold this much sugar anymore! Get out of here, dirtbags!" and kicks some of the sugar out of the water: it turns back into sugar crystals on the bottom of the glass. The same thing happens with water in air: if you take warm, humid air and cool it down, it can't hold as much water vapor as it used to, so it kicks some of it out of the air: it turns into water droplets, which form clouds, fog, dew, frost, snow, or rain. This is called condensation. It works the other way around, too: if you take cool air which can't hold much water and heat it up, it can now hold lots more water vapor, (its relative humidity decreases) so it evaporates any water it comes across. This is how hot-air clothes dryers work.

So, why does your windshield fog up? If you're sitting in a car on a cold day, your body heat will warm the air inside, and evaporation from your skin and breath will quickly raise the relative humidity to nearly 100%. The windshield is touching the cold air outside, so it will be colder than the body-warmed air inside. Air inside the car which is touching the windshield will also become colder. But if it gets colder, it can't hold as much water vapor, so it must get rid of some. It does this by condensing drops of liquid water on the windshield. So, the fog on the windshield is really water from your sweat and spit. Eeeew, yuck!

Air that's been warmed by your car's heater has low relative humidity, because its temperature has been raised. So if you blow it against the car window, it'll suck up the water on the windshield very quickly, just like a clothes dryer. However, once you've done that you've got hot, humid air inside the car and a cold windshield, which was the situation which fogged the windshield in the first place. So it's likely to fog up again. This agrees with my experience: if you defog with hot air, it goes away very quickly, but it all comes back again several minutes later.

If you use cold air, it will be warmed when it touches the windshield. As it warms, can hold more water, and so it will suck it up off the windshield. Even better, once the air is fully saturated, still can't condense on the windshield because the windshield is warmer than the air. The problem is that this way is awfully slow. Why? Suppose the temperature outside is 20 C. If you use cold air at 10 C, you can suck up a certain amount of water vapor as it warms back to 20 C. However, 20 C air warmed to 30 C can suck up hold 8 or 9 times as much water! The formula for water vapor capacity (the Clausius-Claperyon Equation) is non-linear.

So, here's my suggestion: If the windshield's already fogged, use hot air to clear it quickly. To keep it from coming back, switch to cold air. Never use the 'recirculate' setting on the heater/air conditioner, because that will allow vapor from your sweat and breath to build up in the car.