Why windows fog and frost
Fog on the inside of the glass is moisture from the air condensing on a surface colder than the air in the cabin. People breathe out a lot of moisture, and wet coats, floor mats and umbrellas add more. In cold weather the glass is cold, so the moisture settles on it as a fine film of droplets.
Frost forms when moisture freezes on the outside of the glass, usually overnight. Sometimes it forms on the inside too, if the cabin was very damp when the car was parked.
The two problems need different fixes, which is why it helps to know which side of the glass you are dealing with.
Clearing frost on the outside
Safe clearing starts before you drive, and clearing only a small patch is a common and dangerous shortcut. You need clear glass on the windshield, side windows, rear window and mirrors, as well as lights and sensors.
- Start the engine and set the heater and defroster to clear the windshield, so the glass warms while you work.
- Use a plastic ice scraper, working from the center outward. Metal tools can scratch glass.
- Brush loose snow off the roof, hood and trunk, so it does not slide onto the windshield or blow into other drivers’ view.
- Clear the lights, mirrors, any camera lenses and sensors, and the license plate where it is required to be visible.
- Wait until the glass is clear all the way to the edges before moving off.
A de-icing spray designed for car glass can speed things up. Check the label and use it as directed.
What to avoid
Do not pour hot or boiling water on frozen glass. A sudden change in temperature can crack a windshield, particularly if it already has a chip. Lukewarm water is better than hot, but it can refreeze quickly in cold weather, so it is not a reliable method. Wiper blades frozen to the glass should not be forced loose; let the defroster work first, since tearing the rubber damages the blades.
It is also unwise to leave a car running unattended with the keys inside. Depending on where you live, this may be illegal, and it invites theft. Check local rules, and plan a few extra minutes in the morning instead.
Clearing fog on the inside
Fog needs two things: drier air and warmer glass. The typical approach:
- Turn the fan up and direct airflow to the windshield.
- Select fresh air rather than recirculation, since outside air is usually drier in cold weather than the damp air inside the cabin.
- Turn on the air conditioning if your car has it. The system dries the air, and in many cars it is engaged automatically when you choose the defrost setting. This works even in cold weather, although it uses a bit of extra fuel or energy.
- Raise the temperature gradually. Warm air holds more moisture, so heat helps once the air is dry.
- Use the rear window heater button if the car has one, which warms the glass with thin heating wires. Be careful when scraping near those lines on the inside.
In many cars, there is a single button with a windshield symbol that sets all of this up. Your owner’s manual will show how your system behaves. Opening a window slightly for the first minute can also help clear damp air in a hurry.
For electric vehicles, heating can use a noticeable share of the battery, and some cars let you pre-condition the cabin while plugged in. That is covered in more detail in what affects electric range.
Stopping it from coming back
A few habits cut down on repeated fogging:
- Keep the inside of the glass clean. Dirt and the film left by smoking or plastics give moisture something to cling to. Clean glass with a microfiber cloth and a product made for car windows.
- Shake snow off boots and umbrellas before getting in, and do not leave wet items in the cabin overnight.
- Check floor mats and carpets for dampness, since a persistent damp patch can point to a leak, and a musty smell is worth investigating.
- Replace the cabin filter on schedule. A clogged filter reduces airflow, which makes defogging slower. The role of the air filter and cabin filter is simple, but it is often forgotten.
- Park facing the direction where morning sun will reach the glass, if you have a choice.
Some drivers use an anti-fog product on the inside glass. If you try one, follow its directions and test on a small area first.
Driving while the glass is still clearing
Do not start driving until you can see properly in every direction. If you must move while the windshield is partly misted, wipe it with a clean dry cloth, not your sleeve or hand, because oils from skin cause streaks that scatter light, especially at night. Night driving makes smears much more distracting, so a clean windshield matters even more in the dark.
Make sure your mirror adjustment is correct after clearing the glass, and check blind spots carefully, as fogged side windows hide more than you think.
When it does not work
If the windshield will not clear, or the glass fogs very quickly even after a good clearing, there may be a fault. Common reasons include a weak blower, a failed cabin filter, a heater that is not warming up, a damp interior from a leak, or a heater core problem that can show as a sweet smell or a greasy film on the glass. Coolant loss is also worth checking. A qualified mechanic can diagnose these.