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Q & A: car defrost fan fogging window

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Most recent answer: 03/17/2015
Q:
I had the windows on my 2003 suburban completely fogged over instantly and I man instantly at 2:10 am one morning. I was maneuvering through a parking lot and the windows started to fog, I turned on defroster fan on high and if you blinked you missed it. The windows went complete fog! It was cold, very damp, had rained during night. It happened so fast that by the time I could hit the brakes I had an accident where I side swiped parked car. I have never seen anything like this happen that fast.
- Steven (age 61)
Fitchburg, ma, usa
A:

Someone else experienced a less dramatic version of this, discussed in follow-up #3 here: https://van.physics.illinois.edu/qa/listing.php?id=1511.

The problem was most likely that the window was cold enough to make water condense out from moist air. When you first turn on the defroster fan, you're supplying a big stream of moist air, so you get a lot of condensation quickly. Once the window warms up, the problem should go away, but in your case that was too late.

Mike W.


(published on 03/17/2015)

Follow-up on this answer.