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This chart shows how much grams of water per cubic meter air can hold at 100% relative humidity.

View attachment 177889

At 104*F a cubit meter of air can hold, 51.1 grams of water, but only 4.89 grams of water at the freezing point of water.

So what happens to the moisture when the temperature cools down? It condenses to form water droplets, is invisible when in the form of moisture. This is what fogs up your windows. If warm moist air hits a cool or cold front, you get fog. Occurs at the so-called dewpoint the temperature where the moisture in the air condenses. Varies considerably dependent on the relative humidity of the warmer air.

Major culprits for forming humidity in a vehicle is us humans, every time we exhale, putting more moisture into the air, then all the moisture in our skin is evaporating as well. Ha, even worse is carrying four German Shepherds in your vehicle, they don't sweat like we do, but put tons of moisture into the air when they exhale. Ha, regardless of vehicle, when I was carrying this many dogs, only alternative was to crack open the rear windows, far enough to exhaust that humid air, but not far enough so one or two would decide to jump out.

Contrary to popular belief, moist air or on high humidity days, the air is actually lighter than dry air. Have to take this into consideration when landing or taking off in an aircraft, lift is greatly reduced, and you may run out of runway.

What I find to be totally ridiculous, and practically true on all vehicles, is switching on the AC compressor when you switch to defrost.

First off, there ain't that moisture in the air to begin with, second the differential pressure between the high and low side is so small, the evaporator doesn't get that much colder than the incoming ambient air. Thirdly, the compressor seals are rock hard, and the refrigerant oil is very stiff. So all you are doing is wearing out your seals causing refrigerant leaks and wearing out the mechanism of the compressor. But they can get more business from you by doing it this way.

For this reason, either disconnect the compressor clutch coil, or don't use defrost at all, just crack open the driver's window, that clears fog in a big worry. And my AC systems last very very long.
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