results of driving around in humid a little foggy weather in UTC San Diego Sun 23 Sep 2012:
- (background) a friend reported that in the past, when it was colder, she managed to defog the windshield by turning on the air conditioner, to a chilly setting (in the 60s?), without directing the air especially to the front windshield
- (background) i recall that in CT when the windshield got foggy, turning the heat all the way up was the way to go
- (background) the car manual warns not to direct chilly air to the front windshield as the difference between the temperature of the inside and the outside of the windshield will cause condensation
- the thing that makes the condensation on the front windshield the worst/build up the quickest is turning up the air flow all the way, setting it to the coldest, and directing it to the front windshield
- the thing that makes the condensation on the front windshield go away the quickest is turning up the air flow all the way, setting it to the hottest, and directing it to the front windshield
- the critical temperature is a little above what the car tells me is the outside temperature. tonight the car said 68 degrees F, and 74 degrees F was sufficient to cause the condensation to go away (lower temperatures were inconclusive, although the lowest (<65) definitely makes the condensation build up, and i think 70 did also tho am not sure -- my guess is that the critical temp was between 68 and 74, and probably between 71 and 73).
- the condensation gets better at the hottest setting faster than it gets worse at the coldest. i tested a procedure where you make it hotter briefly with the air directed to the front, then colder for a long time with the air not directed to the front, in order to minimze the max temperature reached by the passengers while still removing the condensation. This works.
- whatever you do with the climate control system doesn't have much of an effect, at least not rapidly enough for me to notice in this experiment, unless the air is directed to the front windshield
- there is a bunch of momentum in the system and if you switch the climate control from low to hot or vice versa it takes a few minutes for the direction of change on the windshield to switch (probably more than one minute, less than 10)
- it seems that driving around makes the condensation happen faster, although i'm not sure because i often sat still just after switching and then drove after a few minutes, so the driving around effect is confounded with the momentum effect
- opening the windows doesn't have much of an effect, at least not rapidly enough for me to notice in this experiment, but if anything they made it slightly worse
- recirculated or outside air doesn't have much of an effect, at least not rapidly enough for me to notice in this experiment, but if anything they made it slightly worse. when the air is directed it to the front windshield the Prius only allows outside air so in this case i wasn't able to experiment with this setting.
-- conclusion: the key is set the climate control system to something at least several degrees above the reported outside temp, and to direct the air to the front windshield. If this is too hot for some passengers, you can alternate between that and un-directing air to the front while making the temperature cold.