Defogger and Demisting: Keep Your Windows Clear

A misted-up windscreen can blind you in seconds. It happens on cold mornings, in the monsoon, on hill roads in winter and whenever a car full of people sits in humid air with the windows closed. Knowing which buttons to press, and in what order, clears the glass in under a minute. Never drive while you are peering through a small cleared patch.

Why windows mist up

Mist is tiny drops of water that form when warm, moist air touches cold glass. There are two kinds, and they need different treatment:

• mist on the inside is the usual problem. The air inside the car is warmer and wetter than the glass, from people's breath, wet clothes and umbrellas. It is common in cold weather, in rain and when the windows are shut; and
• mist on the outside happens in hot, humid weather when the air-conditioning has made the glass colder than the air outside, for example early on a monsoon morning or after a rain shower. Inside heat will not clear it: use the wipers, and make the air on the glass a little warmer or the fan slower.

Wipe the glass with your hand: if the mist comes off, it is on the inside.

Clearing the windscreen (front demister)

1. select the windscreen setting on the air-direction control (a symbol of a curved windscreen with wavy arrows). In many cars this automatically switches on the air-conditioning and fresh air;
2. turn the fan to high;
3. switch the air-conditioning (A/C) on. The A/C dries the air, which is what clears the mist; it works even with the temperature set to warm;
4. select fresh air, not recirculation. Recirculation keeps the moist air inside the car and makes misting worse;
5. in cold weather, set the temperature to warm once the engine has warmed up; dry, warm air clears mist fastest; and
6. open a window slightly if the mist is heavy and you have no A/C.

Clearing the rear window and mirrors (rear defogger)

• the rear defogger (or demister) button shows a rectangle with wavy arrows. It heats the thin lines printed on the rear glass and clears mist, frost and light rain in a few minutes;
• in many cars the same button also heats the door mirrors;
• it usually switches itself off after 10 to 15 minutes. Press it again if needed; and
• do not scratch the heating lines with stickers, scrapers or rough cloths; a broken line leaves a misty strip.

Before you set off

• clear all the windows you need: the windscreen, the front side windows (for your mirrors and blind spots) and the rear window. The UK Highway Code (rule 229) says drivers must be able to see and should demist the windows thoroughly before setting off;
• clean the glass inside regularly. Dust and the film from plastic dashboards hold moisture and make misting worse; and
• keep the wiper blades and washer fluid in good order before the monsoon (Rule 21). Rule 101 of the Central Motor Vehicles Rules requires windscreen wipers, and they must work when you need them.

Keep the glass clear: the legal side

• the windscreen and rear window must let through at least 70% of light, and the side windows at least 50% (Rule 100 of the Central Motor Vehicles Rules);
• in 2012 the Supreme Court ruled that films of any kind may not be pasted on car glass. Curtains and sunshades on moving cars are also enforced against in many cities. Dark glass makes it harder to see at night and in the rain, and harder for others to see you; and
• do not hang objects or stick large stickers where they block your view of the road.

When it mists up while driving

1. do not wipe the windscreen with your hand or a cloth while moving; it smears and takes your eyes off the road;
2. switch on the windscreen setting, fan high, A/C on, fresh air; and
3. if you still cannot see clearly, slow down, signal, and stop in a safe place until the glass is clear (Rule 11). On a highway, leave at the next safe place, not on the shoulder unless it is an emergency.

Common mistakes

Remember