Solving the Mystery of Car Fog - Explaining the Science

[likephysics]

I don't understand what really is happening.
When there is fog on the (inside)windshield of my car, I turn on the AC and it goes away.
Are the water droplets freezing? Am I changing the dew point?
Why does it go away?
If I leave the temperature at 62F, the fog increases.
Can somebody explain please.

 

[Greg Bernhardt]

Often, the air in the car will be warmer than the air outside the car. Moisture will condense on the inside of the windows if the outside air is cooler than the dew point inside the car. The air conditioner dehumidifies air because water condenses inside it when the air goes past the cold tubes with the refrigerant inside. Heating it up way past its dew point makes it feel dry, and it is dry -- it’ll help evaporate any condensation that’s on the windshield -- and even more quickly the hotter it is because heat is needed to cause the liquid water to change phase to a gas.

 

[likephysics]

Thanks.
You said "water condenses inside it...". Are you saying water condenses inside the AC?
what happens to the moisture on the inside of the car when the AC blows it?

Heating the car would dry up the moisture and there would be no moisture. But to get there, the humidity increases(bcoz of increase in temperature) first. right?

 

[adriank]

Water condenses inside the AC in the cooling part, and (correct me if I'm wrong) this water is not returned to the inside of the car, so the amount of moisture decreases.

Humidity decreases with increasing temperature; hot air has more capacity for moisture than cool air.

 

[rcgldr]

The issue is the temperature of the windows is less than the air inside, so dew forms. That's why rear window defrosters simply heat up the window, although it takes a while. Using the "defroster", uses the AC, which dehumidifies the air and it blows dry air at the front window, air that is low humidity and low water vapor pressure, so the water on the windows evaportates quickly regardless of the window and air temperature (within reason).

Simply blowing hot air at the front window to heat up the front window also works, but it takes longer.

Eventually the air inside the car is dried out by the AC, and/or the windows are heated up and/or the air is cooled down (you could open up the windows to also cool the air, but it wouldn't be the dry air as from the AC), until the windows vesrus air temperature versus air humidity is enough to prevent dew.

 

[russ_watters]

Thanks.
You said "water condenses inside it...". Are you saying water condenses inside the AC?
what happens to the moisture on the inside of the car when the AC blows it?

Yes, water condenses on the coil of the ac unit. That's why you see water dripping from the underside of cars in the summer.

Heating the car would dry up the moisture and there would be no moisture. But to get there, the humidity increases(bcoz of increase in temperature) first. right?

No. Heating the air will decrease the relative humidity because the air's capacity for holding water increases, but it will not decrease the absolute humidity. Heating air does not have any way of removing moisture from the air.

Also, if the air outside is pretty dry (as it is in winter), simply bringing in fresh air will often get rid of the window fog.

 

[likephysics]

Russ, dew on the AC pipes is from the moisture in the air on the outside of the car.
Does the AC somehow absorb the moisture inside the car?

 

[rcgldr]

Russ, dew on the AC pipes is from the moisture in the air on the outside of the car. Does the AC somehow absorb the moisture inside the car?

Yes if the vents are in "recirculate" mode.

 

[pixel01]

That is well similar to the case that you wear a watch which is leaked and some water gets into. If you leave the watch on the table for some time, it looks quite normal, and you wear it, the glass will get opaque because all the metal parts will soon reach the temperature of the body while the glass will not.

 

[nasu]

The AC system remove part of the humidity in the air. Buildings ACs for sure and I suppose that the car ones too. If you don't do that and just cool down the high humidity summer air you will end up with very humid (relative humidity) cold air (at least in areas with very "wet" summers as Michigan or Washington DC).
When this part of the system does not work (as it happened quite often in the building I used to work) you can reach humidity of 89% or more in the summer and the water will condense on the water pipes and drip down.

If the AC works properly it provides cold dry (relatively) air.

 

[likephysics]

Yes if the vents are in "recirculate" mode.

I think so. But got to check that. It does put an end to my doubt.
thanks!

 

[likephysics]

To get rid of the dew, I bought a small bag of silica gel-about 8oz. Doesn't make much difference.
Any other solution. Turning on the AC just makes it really cold.

 

[russ_watters]

Russ, dew on the AC pipes is from the moisture in the air on the outside of the car.
Does the AC somehow absorb the moisture inside the car?

Yes if the vents are in "recirculate" mode.

I don't know if cars pull in 100% outside air when not in recirc mode, but either way, the air conditioned air that enters the cabin of the car through the ac vents must first pass over the air conditioning coils. Air conditioned air has been dehumidified and car air conditioners do a very good job of it (often providing dew points below 45F).

 

[russ_watters]

To get rid of the dew, I bought a small bag of silica gel-about 8oz. Doesn't make much difference.

Yeah, too small to make a difference.

Any other solution. Turning on the AC just makes it really cold.

Most cars today will run the air conditioning when in "defog" mode to lower the humidity . Have you tried running in that mode?

Failing that, you can run the air conditioning and turn the thermostat up. Most cars will keep the air conditioning on, then reheat the air via the engine. You'll simulate what most cars do in their "defog" mode.

I think so [re: recirc mode]. But got to check that. It does put an end to my doubt.
thanks!

Again, if it is relatively cool outside (below about 50F), you cannot do better by having the ac in recirc mode than by having it in outside air mode. That's why many new cars will lock you out of recirc mode when you are in "defog" mode.

What kind of car do you have and how old is it? Does it have manual or automatic climate control modes?

 

[likephysics]

Russ, the car is acura rsx 2003. It has auto climate control.
Thank for the answers.