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Old 07-27-2016, 04:04 PM
engineer_comp_geek engineer_comp_geek is offline
Robot Mod in Beta Testing
Join Date: Mar 2001
Location: Pennsylvania
Posts: 20,458
The air coming out of your car's AC is not colder than 32 degrees.

I'm not an expert on automobile AC systems, but it's my understanding that they generally aim for an outlet temperature of somewhere around 40 degrees. I've also heard that as long as the outlet temperature is 40 degrees below the ambient temperature that the AC is considered to be operating ok, but I think some folks disagree with that. In other words, if it's 100 degrees out, the AC isn't going to be able to maintain a 40 degree outlet temperature, but as long as it maintains 60 degrees or cooler, it's not considered to be a problem with the AC.

Maybe one of our resident car experts can give some better info on that.

Anyway, the answer to whether it is better to have ice cream in 70 degree stagnant air or 40 degree moving air depends on how fast the air is moving. If the 40 degree air were still, then obviously the ice cream would melt less quickly in it than the 70 degree air. The faster the 40 degree air moves, the more heat it can draw out of the ice cream, so at some point it breaks even, where the ice cream would melt equally at 40 degree moving air as 70 degree stagnant air. Past that break-even point, the 40 degree air will always draw more heat out than the 70 degree stagnant air.

I'm bad enough at thermodynamics that I am not even going to attempt to calculate exactly where that break-even point is.

My gut feeling is that you don't need a whole lot of moving air to draw a lot of heat out of the ice cream, so most likely you are better off not putting the ice cream in front of the vent.