Drive blind

August 9, 2005 12:06 PM

How do I de-fog my car windows on a humid summer night without using my air conditioner?

Frankly, I'm not even sure the science behind the summer fog-up. Is it the cool yet humid air hitting the warm windshield? Or is the windshield cool and the air warm and humid? I've tried blasting cold air on the windshield, blasting hot air on it, and running with all windows down. Nothing seems to help, so I end up running the a/c, which I'd rather not do (and which itself leads to small areas of condensation on the windshield -- I can't win).

posted by schoolgirl report to Travel & Transportation (8 answers total)

A/C on a warmer setting should do it fastest. Heat alone also should do it, but the A/C works best as it actually condenses some of the moisture out of the air.

Warm air can hold more moisture than cold air. When warm, moist air hits a cooler windshield you can get condensation. Using warm air will warm the windshield to revaporize the fog on the windshield. Warm, dry air from using both the heater and A/C allows the air to carry away even more moisture hastening the defogging.
posted by caddis at 12:27 PM on August 9, 2005

Response by poster: Excellent, thanks, I'll try a/c on something other than the coldest setting. I'd rather not blast straight hot air on a steamy summer night. Any thoughts as to whether I should engage the recirculation button as well?
posted by schoolgirl report at 1:07 PM on August 9, 2005

Recirc is actually the best way to defog. In fact, some domestic cars have something called 'MAX AC'- this is just recirc+AC.

Recirculation in cars means passing through the filter, and using the AC to dehumidify. Plus, the air that's being sucked in is already colder because it's been AC'd first, so recirc has an additive effect. It's a good thing!
posted by id at 1:34 PM on August 9, 2005

The fog-up happens because warm moist air is hitting a comparatively cooler windshield. I used to have this problem a lot when I drove a honda accord with a leaky trunk. When it would rain water would pool in the trunk, I'd park it with the windows closed, the water in the trunk would evaporate and saturate the air in the car. Even with a low outside dew point the slightest cooling of the windshield would cause it to fog.

The solution to this problem was simple, park the car in the sun after a rain storm with the windows open so it could dry out. If you store water, have a wet floor mat or a leak you are unaware of this solution might work for you.
posted by 517 at 1:38 PM on August 9, 2005

Recirc is actually the best way to defog.
Um - maybe if there are no people in the car, or if they are not breathing. My experience is that recirculating the air results in slower defogging, because the humidity produced by the breathing humans exceeds the humidity in the ouside air.

I don't live in the tropics, so your humidity may vary.
posted by Kirth Gerson at 4:52 PM on August 9, 2005

I agree with Kirth: Recirc makes the fog problem worse for exactly the reason stated: moister given off by the occupants of the car. If you run the a/c, this usually more than overcomes this, but then you get the condensation on the outside like you said.

If you don't want to use a/c, go with Vent (as opposed to Recirc) if you want to stand even a chance of defogging.
posted by Doohickie at 11:11 PM on August 9, 2005

Heat on all the way, air conditioning on all the way, use the windshield and side vents only, and make sure to switch to bringing in outside air, and not re-circulating inside air, just like Kirth and Doohickie said.

Side comment. If your car is starting to overheat on a hot day (that temperature gauge is there for a reason), turn off the air conditioning and then turn on both the heat and the fan all the way. Open the windows, because you will roast, but your engine will cool off quickly.
posted by marsha56 at 10:51 AM on August 10, 2005

The most effective way to get rid of fog is to open a window. That quickly pulls the humid air out and lets (comparatively) dry air in. If you're not using the A/C, then it's obviously not warm enough to need it.

If you're going too fast to open a window, or decide not to, put the Recirc/Vent lever on Vent. That brings outside air through and pulls the humid air out.

And why don't you want to use the AC? It uses only a little extra gas, and paying for that is a lot better than paying hospital bills when you hit something you can't see.

In any case, put the vent control on Defrost and turn the blower to High.
posted by KRS at 12:51 PM on August 10, 2005