Foggy Car Windows

Welcome to “My Two Sense Worth,”  a little corner of my blog that I just came up with the other day. As I traverse life, I often run across things that I see other people struggle with, and I have often wondered how I could share my solutions to these problems. Then it hit me (Blinding Flash of the Obvious!) Share it in the blog! So here goes the first installment!

Foggy Car Window

Foggy Car Window

As the cold weather has set in, just about daily I will see someone driving down the freeway in their car with nearly completely fogged up windows, save for the spots where they have rubbed the fog off so that they can drive. Just to clarify, I’m speaking of fog on the inside of the window, not on the outside, nor am I speaking about the tan haze that accumulates on the inside of front windows over time in the summer when the sun releases chemicals from the dash.

Nope, I mean fog. The kind that get’s worse once you get in the car and begin breathing, or when you turn on the heat. This is caused purely by moisture, and in a car with a properly functioning climate control (read: most cars) the fix is easy.

Avoiding a scientific explanation (which I could not explain anyways) the fog forms because the windows are cold, and the air (especially with you breathing) is warm. And, there’s one more important part, there is enough moisture in the air inside the car to cause fog to form on the inside of the window. The fix has two parts:

recirc_switch_logoFirst, 99% of the time (in my humble experience) the fog is created by cars running with their air “recirculation” or “recirc” switch on. What this does is recycles about 90% of the air in the car back inside the car. Problem: when we exhale, we give off air that is more humid (has more water in it) than the air we take in, thus, making the air inside the car have more moisture in it, which is the recipe for fog. The fix in many cases is simply to just make sure the recirc switch is in the “off” or “fresh air” setting. The recirc function in car climate controls serves essentially two purposes, 1) to allow the Air Conditioning in the summertime to continue to “re-cool” the air inside the car when tempuratures outside approaches triple digits, and 2) to allow you to close off the outside air temporarily when passing through extremely dusty areas and areas with strong odor (or when following that car that is stinky.) It (recirc) should not be used really for more than 30 minutes, as the small interior of a car should receive as much outside “fresh” air as possible.

The other fix seems counter intuitive, but it works fantastically, is not “hard” on the climate control system, and in fact has been the way most automatic climate controls (a topic for another post) work. Eurpoean cars like Mercedes Benz have functioned this way automatically for the last 20+ years. You simply turn on the Air Conditioning, with the heat turned up. Yep, you heard that right. Have you ever noticed the water draining out from the bottom of your car in the summer after a drive in the heat, with the A/C on? That’s because one of the by-products of air conditioning, is condensation. Essentially, A/C lowers humidity (takes moisture out of the air.) By turning on the A/C, and turning the heat up, you will still get plenty of hot air (once the car is warm of course, as your heater’s output in most cases is much stronger than your A/C’s ability to cool) and the A/C will dry the air inside the car, making it less possible for fog to form on the windows.

If you’re still not convined, get this: in most late model cars with A/C, when you press or select “defrost” also turn on the A/C at the same time, for this very same reason.

So next time your windows fog up, make sure that your “recirc” is off (fresh air setting) and if you still have fog, turn on the A/C. Oh, and the fuel economy difference in most cars with the A/C on vs off is neglible, and in the process of clearing your windows, you’ll keep the lubricants in your A/C system circulating during the winter, and  your A/C system will be in better shape come summer.

I hope you find this helpful!

3 thoughts on “Foggy Car Windows

  1. Aditi Bakshi

    thanks so much for this tip. It is really helpful as I often get confused with the controls. So if I understood correctly, the A/C should be on with the heater set on a comfortable level.

  2. bwcole

    Hi Aditi, yes that’s the way to do it. In fact, many cars actually run that way from the factory now. I know for a fact that Mercedes runs the A/C at all times unless you push the “Economy” button. Enjoy !

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