Defoggers

[jyakulis]

Okay, I think I finally figured my defoggers out today. I wanna know if anyone can clarify if I'm right though. Heh this may seem obvious to some but it's been a mystery to me for years.
Is fog on windows caused by a difference in concentration of water on the inside and outisde of the car. I arrived at this because my car has two circulation buttons. One that circulates air internally in the car and one that takes air from the outside. I noticed the internal circulation button doesn't work when I have my defoggers on. This led me to believe that the water concentration has to reach an equlibrium. I've had some transport so I would think this would be right. The differences in the pressure would have to be equal inside an out. The diffrence in concentration of water would cause a difference in pressure causing the water to try to diffuse through the window, thus condensation forming. I also noticed that in the summer if my car gets fogged up and I open the window it seems to increase the rate the windows defog as well. Anyway my questions would then be....
Is this logic right?
Does the temperature matter.....my guess would be yes but I can't seem to remember why this is so.

[Edmond Zedo]

Okay, I think I finally figured my defoggers out today. I wanna know if anyone can clarify if I'm right though. Heh this may seem obvious to some but it's been a mystery to me for years.
Is fog on windows caused by a difference in concentration of water on the inside and outisde of the car. I arrived at this because my car has two circulation buttons. One that circulates air internally in the car and one that takes air from the outside. I noticed the internal circulation button doesn't work when I have my defoggers on. This led me to believe that the water concentration has to reach an equlibrium. I've had some transport so I would think this would be right. The differences in the pressure would have to be equal inside an out. The diffrence in concentration of water would cause a difference in pressure causing the water to try to diffuse through the window, thus condensation forming. I also noticed that in the summer if my car gets fogged up and I open the window it seems to increase the rate the windows defog as well. Anyway my questions would then be....
Is this logic right?
Does the temperature matter.....my guess would be yes but I can't seem to remember why this is so.
Temperature difference, in any practical sense, is most important. But extremely high humidity can cause condensation without the old delta T. Your windows will defog faster with A/C, because 'conditioned' air is dehumidified, and will take on more water.

[jyakulis]

Temperature difference, in any practical sense, is most important. But extremely high humidity can cause condensation without the old delta T. Your windows will defog faster with A/C, because 'conditioned' air is dehumidified, and will take on more water.

Well, in winter heat always seems to work. But in summer it seems to behave differently. I would assume that cold air should technically work. But there have been times I noticed my cold defoggers causing condensation on the inside of my car. Ahh well it doesn't matter that much. Little things irriatate me. For instance why are wal mart entrances and exits opposite, it baffles me.

[Robespierre]

Humidity differences between the inside and outside of the car are not at all the issue.

As stated, it is the difference in temperature. It's all about the dew point, that temperature at which the air can't hold any more moisture, and it begins to condense out onto cold surfaces. There is a dew point for the outside atmosphere and the inside atmosphere.

[jyakulis]

Why does opening the window help? If it's all the temperature of the surface of the window then opening the window should not really interupt the system at all right?

[SheepDog]

You are right. Opening the vent will generally help.

You are breathing out moist air when you're inside the car (unless you're dead, in which case the rotting flesh would probably still give off moisture, but I digress). Venting the air will lower the moisture inside the vehicle (unless it's really really humid outside). This will help reduce the effect of the temperature differential, which is the cause.

The defroster in my vehicles works by running the compressor, which also reduces moisture in the inside air. The evaporator coil causes the moisture in the air to condense there for the same reason that it was condensing on the inside of the windows previously, that it's cold.

[SheepDog]

You could also say that reducing the moisture lowers the dew point.
[edit: logic bug]