Can Outside Humidity Affect Vapour Coming from AC Vents in a Car?

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Visible vapor or mist from car AC vents can occur under normal circumstances, particularly in humid weather. When warm, humid air from outside enters the AC system and is chilled, the relative humidity can exceed 100%, causing condensation that appears as fog. This phenomenon is more likely when the vehicle is in motion, as airflow can enhance the effect. Concerns about AC performance, such as unusual noises or past leaks, can lead to anxiety about potential failures, but occasional misting is typically not indicative of a malfunction.
DaveC426913
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Are there any normal* circumstances under which one might see vapour coming from the AC vents of one's car?

*non-broken-thing


I'm wondering if it had something to do with the outside humidity and the internal temp or something. It was about 28C out, and it's a sunny, muggy day. I had the internal temp at 22. It seemed to happen only when the car was in motion; the moment I pulled over it seemed to stop but if I started back on the road, it would start up again.



On the other hand, when I shut off the car today, the AC made that long, rattly, breathy sigh before going quiet. Has my AC finally given up the ghost?
 
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Oh, my AC on my old SAAB 9000 Turbo did that right before it started spitting little chunks of ice at me. $600 later I pick it back up from the repair shop and about 5 minutes from the place it starts spitting out little ice pellets and dies. I drove it back to the shop, "OH, well, what we did needing fixing, but there must be another problem." Quote to fix 2nd problem $700, because they have to take everything apart again. I told them where they could stick their quote.
 
DaveC426913 said:
Are there any normal* circumstances under which one might see vapour coming from the AC vents of one's car?

*non-broken-thing

Yes, I experienced visible vapour (misty haze) coming from my air vents just a couple months ago. I have an 04 Accord with no problems. I don't remember the circumstances though, but I'm sure weather conditions played a part.
 
It might not be mist coming out of the vent so much as the cold air from the vents forming mist when it hits the warm wet air inside the car?
 
Evo said:
...

(giggle giggle)
 
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It's completely normal. The AC takes in warm, humid air from outside and chills it. The relative humidity goes up as the air is chilled, and goes over 100%. The water condenses into a fog, and comes out the vent.

- Warren
 
K, thanks. I'm a bit paranoid because I know there is - or was - a tiny leak. It drained once but when I had it repaired they found nothing. I'm driving on eggshells, waiting for it to go kerflooey.
 

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