A lot of people don’t understand the best way to defog their car windshield in wet, cool weather. About 30 years ago, I was on a group trip on a hired shuttle bus. The outdoor temperature was probably in the 40s Fahrenheit and it was raining. Perfect conditions for fogging up the interior glass, right?
Unfortunately, the shuttle driver didn’t understand psychrometrics, the study of the mixture of dry air and water vapor. She chose setting for the bus’s HVAC system that not only didn’t work well, but made us all uncomfortable for the whole five-hour trip. She did have one setting right, though.
In the decades since then, I’ve figured out the optimal way to defog a car windshield on those days and I’ll share it with you here. I’ve written about all the principles behind the optimal method in this blog before so let’s put them together in a lesson on applied psychrometrics. And at the end, I’ve got a video that shows you exactly what’s going on and why the settings work.
The air in the car is fairly isolated from the outdoors. If people are inside breathing, the humidity can rise and the dew point can easily go above the temperature of the front and back windshields and the side windows, too. That’s when those glass surfaces fog up.
As you know if you’ve been reading this blog for a while, that gives you two pathways to fix the condensation problem:
The amount of water in the vapor phase depends on temperature and availability of water. As the temperature goes higher, more water from liquid and adsorbed phases goes into the vapor phase. Conversely, as the temperature drops, water vapor condenses out of the vapor…