Car in fog Stays Dry

My 6 year old daughter asked me why the car doesn't get wet when we are driving through fog. I suspect it's a combination of water surface tension that makes the molecules less likely to adhere to the surface of the vehicle, and evaporation due to air speeding by. Do you know? Thanks!
- Anastasia (age 40)
Berkeley, CA

Your daughter has asked an excellent question!  When she applies to physics grad school in a few years*, I hope she considers Illinois. Seriously, asking that sort of thing seems to me like a better indicator of a future scientist than is obeying instructions in school.

Unfortunately, I'm not sure of the answer, but I think you're right. The materials used in car paint and especially waxes don't stick well to water, so that makes a car's surface a bad place for water to condense. When it does condense, like you often see in the morning, that makes the water bead up into drops that blow off easily, instead of spreading out into a thin layer. That's another way to make the same point you made about surface tension. Also, if the car is a bit warmer than the surroundings that keeps water from easily condensing. Still, I wonder if driving a cold car through fog on a warm day would leave some drops of water on the car, particularly in spots where the air flow won't carry them off. Let us know with a follow-up if you and your daughter notice that.

Mike W.

*In talking about the future etc., I'm trying to take an optimistic tone.

(published on 11/13/2016)