Cold air can not hold as much moisture as warm air. Why does the car a/c clear a foggy window?

Answer 1

it is true that hot air can hold more moisture than hot one. But the fact that cold air can clear a foggy window can be explained in the following manner.
Cold tends to precipitate moisture, now you would have noticed that during Winters when you turn ON your car heater the windows tend to get foggy from the inside this is because in side the car the temperature rises, but the windows which are at the same temperature as the outside air tend to precipitate moisture onto themselves, (this is the same concept as the formation of clouds) now turning on your A/c would lower the air temperature in side the car to almost the same or even below the outside temperature, (at least near the windows) when this happens the temperature gradient is no longer present (gradient is a fancy name for difference) when this happens the fog or precipitated moisture evaporates into the air.
The phenomena descried above only occurs when the moisture levels are not abnormally high, if they are then turning on the A/c would turn the entire air inside the car foggy, but this fog would soon be cleared as this moisture would be collected and precipitated at the cooling plant of the A/c. (this is the water that drips from the A/c)

Answer 2

At higher temperatures, liquids vaporize into gases. For this reason when the air in the car/house is warmed up, the water moisture tends to be in a gaseous state and when coming in contact with a cold glass windshield/window, the water vapor essentially condenses back into its liquid state as tiny droplets. These tiny droplets cause the window to be foggy.

When you cool the area around the window and partly warm the glass, the condensing effect is lessened until the condensation vaporizes back into the air. That is why sometimes when blast the window defogger against the windshield to clear the window and turn back down the fan, the windows slowly begin to fog up again.

By running the a/c you are equalizing the inside temperature and outside temperature making the fog seem to disappear when in fact you reduced the amount of vapor in the entire car and less condenses on the glass.

By running the heat on to the glass, you are heating the glass so that when the vapor particles come in contact with the glass they do not condense.

Answer 3

It's just moving air over the windshield so you get some drying .If it's a lot colder outside your car than inside then the a/c itself is actually helping of course- but so would opening the windows and just using the fan . Under many conditions using the a/c will Not work. Depends on a number of factors. Most situations call for using some heat to clear the windshield quickest.

Answer 4

Well its not clearing the air, whats happening is the moisture in or around the car is sticking to the window because the tempature outside is different enough inside so that condensation occures, causing fog, by turning on the defrost you are taking air from outside and blowing directly onto the glass inside which counter acts the condensation.

Answer 5

Lets think about this when you have you AC on at home it takes the humidity (water) out of the air right? So it is the same concept in the car. If the amount of water is decreased inside the car there isnt any water to condense on the windshield.

Answer 6

so many long winded answers, But simply the A/C removes the moisture from the air in your car. what you were eluding to is relative humidity compared to the dew point. so if there is 80% less water vapour present in your car with the A/C on, then the air would have to get so much colder than present in order for fog to form.

Answer 7

AC air is very dry so it will take water.
The hot air is external air heated by the engine so it contains water.
So, although the cold air can hold less water you do not start from the same point in terms of "available capacity" for extra water to be taken into.

Answer 8

Air Conditioners remove moisted from the air with their evaportators, atleast in home systems. ACs in cars are diffrent however, they are built diffrent and follow slightly diffrent rules. Howeve,r they still remove moister from the air like any other AC system.

If you want proof of this, turn on your car and run the AC for about 10 minutes. Then turn it onn and look under your car. You will see condensated water dripping from the engine in the aproximate area of the AC system under your hood.

Answer 9

The A/C temperature is still higher than the temp of the glass. Thus it will clear the condensation on the glass.

Answer 10

Yup.