Why did my winshield fog up so terribly and incorrigably?

June 30, 2024 9:30 AM

Last night my windshield fogged up terribly and nothing I did and no setting I changed seemed to fix it and it got worse and worse. I could not see. I was able to pull over but wasn't even initially sure if I was pulled over safely. I shudder to think what might have happened if there had been pedestrians or cyclists about. What happened and how do I make sure it never happens again?

My car was parked outdoors in Toronto. I went to drive at around 10pm. The very bottom of the windshield was a little fogged up but as I turned the front defrost on I didn't think anything of it and assumed that little bit of fog would shortly be gone. Looking at Google maps it was around 1.5 km later that I started to notice that it wasn't getting better and was starting to get worse and started trying fiddle with the settings. I tried turning the fan up. The temperature down. The temperature up. The AC on. The ac off. Open the window. Close the window. Turning the whole thing heat/cooling off all together. Nothing seemed to have any effect and it was getting worse and worse. By the time I was about 3km from where I started I couldn't see at all and it was dangerous and I was a little panicked. I turned on my emergency blinkers and pulled over onto a side street and pulled over and called a friend who is better at science stuff than me. He didn't know anymore than I did, but I just kept fiddling. eventually I realized that washing the window would clear it for a few seconds, but really just like 10 seconds and then it would like completely fog up again. But at least with that I was able to pull over a little more safely instead of just stopped in the middle of this side street.

I did, while pulled over, undo my seatbelt and get up to make sure there actually was air coming out of the vents along the windshield (there was). Eventually I disovered that if I turned the tempreature insanely high, like to 30, the fog cleared. So I had to drive home for like an hour in a sauna, but I could see. When I was almost home I tried turning down the temperature because I was cooking, but the fog came back.

WTF. I think there's no going back on having a car, but I think I'll never be a car person.

I have a 2021 Nissan Versa. According to a website of past weather I just found it was 25 degrees with 100% humidity at 10pm last night, but I know 100% humidity means it's raining and it was not raining during any part of my trip.

posted by If only I had a penguin... to Grab Bag (26 answers total)

Best answer: Your windshield was cold and/or being hit with cold air from your A/C, and it was very warm and humid outside. Moisture is condensing on the outside of the glass, not the inside. Running the front defogger blasted it with more cold air, which made the problem worse.

My typical solution is to run slow and warm (not necessarily uncomfortably hot) air on the normal front window setting while running the windshield wipers a few times a minute.
posted by JoeZydeco at 9:34 AM on June 30 [6 favorites]

Best answer: 100% humidity means outside it was at the dewpoint, the temperature at which (or below) moisture will just condense out of the air onto any available surface. Cooling the windshield further just encouraged more condensation on the outside. Echoing Joe Zydeco that your only solution in that situation is to get your windshield hotter than the dewpoint by blowing warm air from your defroster. That will also increase the temp/reduce the humidity inside your car to prevent internal condensation as well.
posted by hydropsyche at 9:48 AM on June 30 [3 favorites]

Often times, too, defrost mode will run the air conditioner as a dehumidifier, even if it's also heating the air. Which is why you'll sometimes see the AC light on with the heat if the defrost is on high.

Fog happens when the humidity is high and the window is lower temperature than the surrounding air, so the water condenses out of that surrounding air on to the cold surface. Heating dries out the air a bit and heats the windshield, so heating plus dehumidification is the best way to take care of it.

Other things that can contribute to high humidity inside the car are your breath, especially if you've been exercising a lot.
posted by straw at 10:01 AM on June 30

Usually rolling down the windows balances the internal and external temperatures and mitigates the fogging. (Not always practical though - a few weeks ago I was out running errands in the pouring rain and my windows fogged up to the point that it was unsafe).

Wiping down glasses and bathroom mirrors with shaving cream helps with fogging and would probably also help with car windows.
posted by bunderful at 10:06 AM on June 30 [1 favorite]

Your heater core is blown.
posted by Max Power at 10:06 AM on June 30 [5 favorites]

If the fog remains persistent, and especially if it seems to include a slick substance (something more than just condensed water) on the inside of the window, or if it is accompanied by a sweet odor, your car's heater core may have a crack that's allowing coolant to vaporize and condense on the inside of your windows.

Edit: What Max Power said.
posted by pullayup at 10:09 AM on June 30 [6 favorites]

Many cars will turn on the AC when the defroster runs, because the AC usually does some dehumifidifying. This can be effective when it's very humid. Lots of good answers here.
posted by theora55 at 10:10 AM on June 30

Response by poster: OK, I did at one very brief point think I noticed an odour of some sort, however, doesn't the fact that washing the windshield cleared the fog completely and instantly every time suggest that it was on the outside? Also, remember that the fog was there when the car had been parked for about 10 hours, so presumably the heater was not running and nothing leaking?
posted by If only I had a penguin... at 10:12 AM on June 30

Response by poster: For wallet related reasons I strongly prefer an explanation that invokes the weather and not my car. LOL. Though of course I realize that the truth does not depend on my preferences.
posted by If only I had a penguin... at 10:14 AM on June 30 [5 favorites]

Forty-some years ago I had an even worse experience like this—I was driving at night in a cool Wisconsin summer and (apparently) drove into a warm humid area. My windshield instantly did what yours did, and driving at 60 MPH on a curving two-lane highway is *very* exciting when you suddenly can’t see at all. Luckily my car had wing windows and I was able to pop one open and see the left edge of the road well enough to come to a stop outside the ditches. Of course, being young and poor, my windshield wipers were rudimentary and ineffective even briefly…

Your discovery that you can toast the windshield to keep it clear is the only workable solution I’ve ever found when you hit a sudden warm wet patch of air.
posted by Gilgamesh's Chauffeur at 10:14 AM on June 30 [2 favorites]

Did activating your windshield wipers clear it, even temporarily? (Was this what you meant by "washing?") If the condensation was on the exterior, which is not impossible if the RH was truly 100%, this should still have instantly wiped it off.
posted by pullayup at 10:19 AM on June 30 [1 favorite]

Response by poster: Did activating your windshield wipers clear it, even temporarily? (Was this what you meant by "washing?") If the condensation was on the exterior, which is not impossible if the RH was truly 100%, this should still have instantly wiped it off.

Yes, I sprayed it with water/anti-freeze or whatever is in there and then wiped it down and that cleared it though it then fogged up again within a few seconds. So if that cleared it does that mean it wasn't the heater core? I don't think I ever tried just running the wipers without spraying it first.
posted by If only I had a penguin... at 10:21 AM on June 30

Yeah there are several possible reasons for it to fog on either the inside or outside, but from your explanation it sounds like JoeZydeco got this one in the very first comment. You were at 100% humidity outside and your big piece of glass was colder than the dew point. Just running with the wipers on as if it were raining probably would have worked.

OR heating the piece of glass up above the dew point, which you must have managed to achieve by turning your car into a sauna.
posted by ctmf at 10:29 AM on June 30

Is there a reason that you didn't try the wipers immediately and leave them on low/intermittent when the fog first became too heavy to see through? (I ask because I'm still not entirely clear on whether the fog was on the inside or outside. Outside fog will be easily cleared by the wipers even if it returns, and washer fluid shouldn't even really be necessary if its just condensed water fog.)
posted by Reverend John at 10:34 AM on June 30 [1 favorite]

Yes, I sprayed it with water/anti-freeze or whatever is in there and then wiped it down and that cleared it though it then fogged up again within a few seconds

Sorry, I'm still still a little bit confused by your description. When you say you "wiped it down," was this with the windshield wipers, after spraying windsheld washer fluid? Or did you wipe down the inside, e.g. with a cloth or napkin or something?
posted by pullayup at 10:35 AM on June 30 [1 favorite]

I apologize, I promose I'm not trying to be intentionally obtuse! I think you're just using different (Canadian? Or maybe just non-car familiar) way of describing what happened.

Basically:

1. If the fog was on the inside, it may have been vaporized antifreeze coming from a crack in your heater core. If it reocurrs or gets worse, especially under different weather conditions, think about taking your car to the shop to get it checked out.

2. If the fog was on the outside, there's no way it could be related to the heater core, and it was definitely due to high humidity and a windshield surface below the dew point.
posted by pullayup at 10:42 AM on June 30 [1 favorite]

Response by poster: I wiped it with the wipers. So I used I pulled tye little stick next to my steering wheel and that made water shoot out on the outside of my windshield. Then I twisted the stick and taht made the wipers wipe the outside of the windshield. That removed the fog thoigh it immediately came back.

I strongly suspect my language is not about me being Canadian and more about me being a car-dolt.
posted by If only I had a penguin... at 10:54 AM on June 30

Response by poster: Is there a reason that you didn't try the wipers immediately and leave them on low/intermittent when the fog first became too heavy to see through?

It never crossed my mind. See car dolt.
posted by If only I had a penguin... at 10:56 AM on June 30 [1 favorite]

Outside fogging happens to my car on spring mornings. Intermittent wipers works for me. I don't live in a high humidity area, but that specific time of year seems to bring the damp air/cold windscreen combo.
posted by How much is that froggie in the window at 11:13 AM on June 30 [2 favorites]

Best answer: This is classic 'cold windshield in a warm, humid environment' behaviour from everything you have described. The humidity in Toronto last night was nearly 100% and it was still pretty warm. It would be made worse by the car moving and with having the air conditioning on (which it might be if it's automatic since, see above re: pretty warm outside).

The solution is defroster on, fan on maximum, heat to maximum, with the engine warm. And even then it can take many minutes for it to clear, so unless you waited a long time between trying different things, it would be hard to see that anything was working better than anything else.

This happens occasionally to me when I've parked in my garage overnight, and the outdoor environment has gotten considerably warmer and is very humid.
posted by FishBike at 12:41 PM on June 30 [2 favorites]

Best answer: For future reference what is reported in the weather report is the Relative Humidity. It is the amount of water vapor in the air relative to what the air can theoretically hold.

At 100% the water that would normally evaporate has no where to go, so it just sticks to anything cooler than it. That is why heating up the windshield worked — the closer it got to the ambient temperature the less water stuck to it.
posted by Tell Me No Lies at 1:05 PM on June 30

Best answer: It's okay, you're just not used to driving cars. All this will become second nature if you keep driving long enough.

Next time your windshield fogs up on the outside, turn the wipers on and leave them on at whatever speed keeps the windshield clear until the windshield stops fogging up on the outside. When to turn wipers on and off is obvious when it's raining but it's less obvious when it's just misting. Your cue to turn them ON is: you can't see. Your cue that the windshield has stopped fogging up and that it's time to turn the wipers OFF is the same as the cue for when it's stopped raining. You'll notice it has stopped misting and it's time to turn the wipers off when the wipers start making a noise like a wrung-out cat because the rubber blades are no longer gliding on water but dragging across dry glass.

These things all become subconscious once you've been driving long enough, and then, I know it's hard to believe it, but driving becomes actually relaxing. You're finding it all a hideous fever dream right now because you're still in the stage where every aspect of driving is conscious and you're in a low-grade panic all the time. If it's any comfort, you're probably a much safer driver right now than everybody for whom this stuff is muscle memory.
posted by Don Pepino at 1:06 PM on June 30 [3 favorites]

Response by poster: Thank you all. Yes, in fact, I was quite panicked when this happened not just because I intensely dislike driving and find it stressful but because travellingthyme's terrible accident has been on my mind whenever I drive over the past few days. Carrying people's lives in my hands is far more responsibility than I want and having to carry that responsibility when I can't even see felt like far more than anybody should be able to handle.
posted by If only I had a penguin... at 1:22 PM on June 30 [4 favorites]

There is a product called Rain-X that works pretty slick. (bad pun) Exterior Rain-X works as a moisture repellent and really makes a difference in fog and rain. I've not use Interior Rain-X, but my brother in California uses it and keeps it in his call always. Bunderful is right that the poor man's version is shaving cream quite well, but it seems to streak for me and washes off pretty fast if you use your wipers much.
posted by BlueHorse at 1:51 PM on June 30 [3 favorites]

Best answer: Make sure your setting is set so that the air blows only into the cabin, not onto the windshield. Letting the AC vents blow onto the windshield will cause water to condense on the outside of the window glass if it's hot and humid outside.
posted by SageTrail at 4:56 PM on June 30 [2 favorites]

In those conditions I was always taught to just drive with the windows down, no heat or cooling inside. It can take a few minutes to even everything out, but at some point the windshield will be the same temperature as the air and you should be fine.
posted by gideonfrog at 5:57 PM on July 1 [1 favorite]