Few things slow down a winter morning like a windshield covered in frost, or glass that fogs up the moment you get in and start breathing. Clearing it properly matters: driving while peering through a small hole scraped in the ice is dangerous, and in many places it can earn you a ticket. This guide covers how to defrost the outside of the glass, how to defog the inside, and what not to do.
Frost outside, fog inside: two different problems
It helps to know which problem you’re dealing with, because the fix is different.
- Frost and ice on the outside form when the glass gets colder than freezing overnight and moisture in the air freezes onto it. You need heat and a scraper.
- Fog on the inside is condensation. Warm, damp air inside the car (from your breath, wet coats, snowy boots) hits cold glass and turns into tiny droplets. You need dry air more than hot air.
Many winter mornings bring both at once, which is why the defroster setting on your dashboard is built to deal with each.
How to use your defroster settings
Front defrost
The front defrost button (usually a curved windshield symbol with wavy arrows) sends air to the vents at the base of the windshield. On most cars it also turns on the air conditioning compressor automatically. That isn’t a mistake: the A/C dries the air, and dry air clears fog much faster than warm, damp air.
Rear defrost
The rear defrost button (a rectangle with wavy arrows) powers thin heating lines in the rear window. Many cars also heat the side mirrors on the same switch. It usually switches itself off after a set time, so press it again if the glass isn’t clear yet.
Fresh air vs recirculation
For fog, use fresh outside air, not recirculation. Recirculating keeps the damp air from your breath inside the car, so the glass keeps fogging.
Step by step: clearing a frosted windshield
- Start the engine and make sure the exhaust pipe isn’t blocked by snow, especially if the car is parked against a snowbank.
- Set the heat to defrost, with the temperature on warm, the fan on high, the A/C on and the air on fresh, not recirculate.
- Turn on the rear defrost and any heated mirrors.
- Clear snow off the whole car while the glass warms up: roof, hood, lights, mirrors and the cowl below the windshield, where snow can block the air intake.
- Scrape the windshield once the ice starts to soften. Push the scraper in short strokes and work from the edges toward the middle. A soft-bristle snow brush handles loose snow without scratching.
- Clear the side windows and mirrors, not just the windshield. You need them for lane changes and parking.
- Wait until the glass is fully clear before driving. A small porthole isn’t enough.
How long this takes depends on the cold, the car and how thick the ice is. Don’t leave the car running unattended while you go back inside; besides the theft risk, some towns have rules against unattended idling.
Mistakes to avoid
- Never pour hot or boiling water on frozen glass. The sudden temperature change can crack the windshield, especially if it already has a chip.
- Don’t use a credit card, spatula or anything metal as a scraper. They can scratch the glass or break.
- Don’t run the wipers on ice. Frozen wipers can tear their rubber edges or strain the wiper motor. Free them gently once the glass has warmed.
- Don’t wipe fog with your hand or glove. It leaves greasy smears that make glare and future fogging worse. Keep a clean microfiber cloth in the glovebox.
How to keep your windshield clearer
Keep the inside of the glass clean
Fog clings to dirt. The inside of the windshield slowly collects a hazy film from the dashboard plastics and from people touching it. Clean it with a glass cleaner and a microfiber cloth every few weeks in winter, and you’ll notice it fogs less and clears faster. Our interior cleaning hacks include a few tricks for awkward corners.
Cut down on moisture in the cabin
Knock snow off your boots before getting in, shake out wet coats, and dry soaked floor mats indoors when you can.
Cover the windshield overnight
A windshield cover put on in the evening keeps most of the frost off the glass, so you peel it off in the morning instead of scraping. Our guide to car accessories for winter covers covers, scrapers, de-icers and what to keep in a winter emergency kit.
Look after wipers and washer fluid
Use a winter-rated washer fluid that won’t freeze in the reservoir or on the glass, and replace wiper blades that streak or chatter. Lifting the wipers off the glass overnight when frost or freezing rain is forecast can stop them from freezing to the windshield, though check your owner’s manual first, since some designs shouldn’t be left up.
Help the side mirrors
If your mirrors aren’t heated, rain and wet snow can leave them blurry. Small visors that sit over the top of the mirror, like these side-mirror rain guards, are meant to keep some of the rain off the glass. Check the fit on your mirror shape before buying.
Make cold mornings a little easier
A frozen steering wheel is miserable for the first few minutes. A wrap-on cover such as this microfiber leather steering wheel cover is less icy to hold than bare plastic and gives a better grip with gloves on.
When fog won’t go away
If the windows fog up constantly, even with the defroster on fresh air, something may be adding moisture. A damp carpet from a leaking door seal or windshield seal, a blocked cabin air filter, or a sweet smell and greasy film on the glass (which can point to a heater core leak) are all worth having a mechanic look at. A cabin filter is a simple part to check during a routine tune-up.
FAQ
Should I use hot or cold air to defog the windshield?
Use the defrost setting with the A/C on. Warm, dry air clears fog fastest. On a very cold morning, start with the heat on so the glass doesn’t stay icy outside while you wait.
Why does my windshield fog up when I turn on the heat?
Warm air holds moisture, and when it hits cold glass it condenses. Switch to fresh air, turn the A/C on and the fog should clear.
Is it OK to use de-icer spray?
Yes. A de-icer made for windshields softens ice so it scrapes off easier. Follow the label and keep it away from paint where the label says to.
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Top image: Daderot, CC0 1.0, via Wikimedia Commons.