De fog, boss, de fog!

March 24, 2005 9:45 AM

Hot or cold defog?

Living in California, I do not often need the defogger in my car, but when I do, I am in a quandary. Does hot air or cold work better, and why?

posted by Kafkaesque to travel & transportation (11 answers total)

Hot. It makes the tiny water droplets that make up the fog evaporate faster.

Also, make sure that you are venting air in from outside, as this will be less humid than the recycled air.
posted by sauril at 9:50 AM on March 24, 2005

If your car will allow it, use hot air with the A/C on. Your A/C actually dehumidifies the air, making for quicker defogging.
posted by split atom at 9:52 AM on March 24, 2005

If you have AC, that will work best of all since it's removing moisture from the air. In Vermont, my drill is like this. Get in car, turn on hot recirculated air to get the ice off. Start driving. Switch to outside air to keep the moisture content down. If I've waited too long for this step, I turn on the AC and it sucks all the water right off the windows. For maintaining, unless it's truly muggy, I use hot air + outside air.
posted by jessamyn at 9:53 AM on March 24, 2005

Hot air with the AC on works best to remove inside condensation.

Two reasons: warmer air can hold more moisture so that will cause some of the water to evaporate. You have the AC on to reduce the total amount of moisture in the air. In my experience, the AC by itself is the single most powerful factor in removing condensation inside the car. If, however, it's fogging on the outside, you need to be sure you're using warm air since that is caused by the glass being below the dew point of the air outside. So the AC doesn't help and can only hurt.
posted by skynxnex at 9:53 AM on March 24, 2005

Since you live in the bay area, where the air is often moist but not especially cold, here's the routine I suggest: Turn on the AC, recirculated air, and crank the blower. The windshield should defog within a 60 seconds. Temperature setting is a matter of your personal comfort, it's the dry AC air that defogs the windows. Chances are the windshield will defog well before your heater gets up to temp in any case.
posted by majick at 10:03 AM on March 24, 2005

Saw a trick on some site last week that said your windshield will defog faster if you lower your sun visors, makes the hot air blow back on itself.
posted by furtive at 11:57 AM on March 24, 2005

It doesn't really matter, using the AC is the single most important factor in defogging your windshield. You can set the heater to whatever temp you like.
posted by exhilaration at 2:41 PM on March 24, 2005

I used to do the AC + hot trick, but it's a balancing act. If you crank up the hot air on the windshield, it goes away a little slower, but it stays that way for the rest of the drive.
posted by knave at 4:19 PM on March 24, 2005

Many, if not most cars run the AC automatically when you turn the heater control to 'defrost'. Warmer temperature settings defog quicker, and the AC still runs as long as you're in defrost mode.
posted by Kirth Gerson at 6:09 PM on March 24, 2005

I'm gonna have to go with hot (although, perversely, it seems rolling the windows down helps more sometimes). You might consider trying this
posted by Pressed Rat at 6:31 PM on March 24, 2005

Many, if not most cars run the AC automatically when you turn the heater control to 'defrost'. Warmer temperature settings defog quicker, and the AC still runs as long as you're in defrost mode.

Argh, I hate this! Damnit car, I control you, not the other way around. An old car of mine did that, and I had to manually turn the A/C off every time I chose to defog.
posted by knave at 9:25 PM on March 24, 2005