Yesterday I was driving my 97 Accord down the highway with the air conditioning turned on. It started raining heavily for a little over 5 minutes, and then the rain stopped. Suddenly, what appeared to be whitish smoke started coming out of the 2 center vents. I quickly turned off the air conditioning, and the smoke stopped coming out. After around 10 seconds, I turned the air conditioning back on and no smoke came out. I drove for another 30 minutes and didn't see any smoke again.

I have never seen this happen in any car in my life, prior to yesterday. Also, it may not have been smoke, because I did not smell anything - so maybe it was water vapor? None of the warning lights on the dashboard lit up. Also, I had the ventilation system set to "recirculate" and was not using the defroster. Any idea what might have happened?

Edit: On 2 occasions in the past week, on sunny days, I have seen similar white smoke with no odor (although not quite as thick as what I described originally) coming out of the vents for about 10-15 seconds with the AC turned on.