Why and how is it that my car can be covered in frost in the morning, but it never got to or below freezing? 35 degrees F the other morning and the windshield was covered!
In: Chemistry
Heat can pass by convection (moving fluids), conduction (heat movement through an object or between touching objects), and radiation (objects radiate “light” (electromagnetic energy) at a frequency proportional to their temperature).
A clear night sky is bloody cold. If it weren’t for the warm blanket of greenhouse gases it’d be about three degrees absolute. Even with, the sky is well below freezing.
The air around your car can’t pump enough heat into it to make up for the heat that the car is radiating into the cold night sky. The car will be colder than the air around it. If the car ends up warmer than freezing you get dew, if it ends up below freezing you get frost.
If it was 35 in the morning, how sure are you that it didn’t dip to 32 at some point during the night?
Air temperature doesn’t equal the temp of surfaces. Even if your average air temp is 35, the surface of the car can easily be cold enough to cause ice to form. That’s why you need to be mindful about black ice is any time the air temp starts dipping below 40.
Also, when you look up the air temperature in your area, you’re seeing the average of a few points taken over a pretty large area, so the air temp in the immediate vicinity of your car could easily be at or below freezing when you’re already that close.
Edit: forgot a couple words.
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