How to defrost a windshield fast (and clear a misted one)

A frozen or fogged-up windshield (windscreen in the UK) usually clears in a few minutes if you use the car's own systems and a scraper together. The one rule that matters most: do not drive until you can see out of every window. Peering through a small cleared patch is how people miss a cyclist or a child crossing the road.

Frost or fog: know which one you have

• Ice or frost on the outside. The glass is white or crunchy on the outside. You need heat on the glass plus a scraper and de-icer.
• Mist on the inside. The glass looks foggy and wiping it with a finger leaves a clear streak. This is moisture from the cabin air condensing on cold glass. You need dry air, which means the A/C.

On a cold, damp morning you often get both at once. The steps below deal with both.

Step by step

1. Check the wipers are switched off before you start the engine. If they are frozen to the glass, trying to move them can tear the rubber or strain the wiper motor.
2. Start the engine and press the windshield defrost button (the curved windshield symbol with wavy arrows).
3. Set the temperature to hot and the fan to full. The system is designed to warm the glass without harming it, so you do not need to start on cold.
4. Turn the A/C on. It works in winter too, and it takes moisture out of the air so the inside of the glass stops misting. On many cars the defrost button turns the A/C on for you.
5. Make sure recirculation is off, so the car draws in fresh outside air. Recirculating keeps the damp air from your breath inside the car and makes fogging worse. Many cars switch to fresh air automatically on the defrost setting.
6. Switch on the rear window demister and heated mirrors if your car has them. Some cars also have a heated windshield with fine wires in the glass, which clears ice very quickly.
7. Brush off any snow first, from the roof as well as the glass, so it does not slide down over the windshield when you brake.
8. Spray de-icer on the outside glass and give it a few moments to work, then scrape with a proper plastic ice scraper. Do the windshield, side windows, rear window, mirrors, lights and number plate.
9. Stay with the car while it warms up. Do not leave it running and go back indoors.
10. Free the wipers last. Once the glass is warm and the ice has softened, lift the wiper blades gently off the glass. Check the washer jets work and the reservoir has winter-strength washer fluid.
11. Wipe any leftover mist inside with a clean, lint-free cloth (not your hand or sleeve, which leaves greasy smears that fog again).
12. Only drive off when every window is fully clear, not just a patch in front of you.

What not to do

• Do not pour hot or boiling water on the glass. The sudden change in temperature can crack the windshield, especially if it already has a chip. Warm water can also refreeze into a fresh sheet of ice and freeze the wipers in place.
• Do not scrape with a credit card, a CD case, a dustpan or anything metal. They break, clear ice badly and sharp edges can scratch the glass. A proper scraper costs little and works much better.
• Do not drive "porthole" style, looking through a small hole you cleared on the driver's side. You cannot see pedestrians, cyclists or cars at junctions. In the UK, Highway Code rule 229 says you must be able to see, so you must clear all snow and ice from all your windows, keep lights and number plates clean and visible, and demist the windows thoroughly. The AA warns that driving with an obscured view can mean a £100 fine and 3 penalty points.
• Do not leave the engine running unattended. Thieves look for cars left defrosting on driveways, and your insurer may refuse to pay if the car is stolen that way. In the UK, Highway Code rule 123 says you must not leave a parked vehicle unattended with the engine running, or leave it running unnecessarily while stationary on a public road. Check your local rules elsewhere: many places have anti-idling laws.
• Do not force frozen wipers or use them to clear ice. It can tear the rubber blade or damage the wiper motor.
• Do not set the defrost to cold air on a damp day. Mazda's manual warns that this can make the outside of the windshield fog up.

If the inside keeps fogging up

Constant misting means there is a lot of moisture in the cabin. Common causes:

• Wet coats, umbrellas or floor mats. Take them out and let them dry.
• Dirty glass. A film of grime and grease on the inside of the windshield gives the moisture something to cling to. Clean the inside with glass cleaner and a lint-free cloth.
• Recirculation left on, or the A/C switched off.
• Water getting in. If the carpets are damp or you see standing water in the footwells, a blocked drain or worn seal may be letting rain in. See car leaking water for what to check.

If the windshield fogs with a sweet smell or an oily film, have a garage check the heater, as that can point to a coolant leak.

How to prevent frost

• Use a windshield cover, or a piece of cardboard held under the wipers, on nights when frost is forecast. Peel it off in the morning and the glass underneath is clear. Note that a towel works less well in very cold weather.
• Park in a garage or under cover if you can. Close to the house is better than an exposed spot.
• Park where the morning sun will reach the windshield, if you have the choice.
• Keep the inside of the glass clean and the cabin dry to cut down on fogging.
• Keep winter-strength washer fluid in the reservoir so it does not freeze.
• Get the car ready for winter in general. Check your tire pressures, as they drop in cold weather, think about winter or all-season tires, and read what to do if your car won't start in the cold.