How to defog your windshield

Should you be using hot or cold air?

To answer that, we have to understand why your windshield is fogging in the first place. 

To understand that, we need to know why condensation forms, and to understand THAT, we need to know what water vapour is. Stick with me. 

Water vapour: the invisible gaseous phase of water. Think steam, but it doesn’t have to be boiling hot. The air you breathe out has some water vapour in it, because lungs are a bit wet.

Water droplets: water in its usual liquid form.

Now for condensation: Warm air can hold more water vapour than cold air. We won’t delve into why that is here, it all gets a bit too molecular. But, when there is water vapour in the air, and that air cools down, at a certain point it can’t hold that water vapour anymore. At this point we’ve reached 100% humidity, also called saturation. If the air continues to cool, the water vapour will ‘condense’, turning into water droplets. If there’s a surface nearby, the droplets will collect on that surface. A lot of very small droplets make surfaces look misty or foggy, whereas larger droplets are easier to see through. 

Oftentimes in winter, we are sitting in our warm cars, happily breathing out our moist, water-vapour-laden air and increasing the humidity in the car. Meanwhile, the windshield is exposed to that wintery air outside, and it cools down. The warm, moist air inside the car touching the windshield cools, and the water vapour within it condenses out and forms tiny water droplets on the inside of our windshield. Now we’re going 100km/hr on the motorway and can’t see a thing. 

Don’t panic! It’s time to blast warm air (that up until this point we’ve been directing at ourselves) onto the windshield. This warms the windshield and the surrounding air, and reverses the condensation process, allowing the water droplets to evaporate! This may take a second if your air isn’t already warm, or you don’t have the A/C on (to dehumidify the air that comes out). Pro tip, some cars have an ‘air recirculation’ mode that pulls air from inside the car and sends it back through the air vents – turn that off! We want it to take dry air from outside the car, to help clear out the moisture inside. 

The hard folk option: Roll down your windows. If you roll down your windows, you’ll let in the dry air faster and clear out that moisture (especially quickly if we’re going 100km/hr down the motorway). 

Now…. What about those humid summer days when you actually get moisture on the outside of your windshield? In this situation, our A/C is cooling down the windshield, letting the warm, moist outside air cool and condense onto it. Luckily, we’ve got windscreen wipers, so turning them on is option one. Option two is to stop blasting the A/C and let your windshield warm up a little.