Foggy windows: why most drivers press the wrong button

An October morning, a cold car — and half a minute later the glass goes white on the inside. What follows is almost always the same: a palm-sized porthole wiped with a sleeve, into gear, off we go. That porthole is the problem. Not the misting — the climate control clears that in ninety seconds — but the habit of pulling away while the only clear glass is in front of your face.

Where all that water in the cabin comes from

In autumn a car is a small damp room, and the people who get into it bring the damp in with them:

• Breathing and body heat. Every breath carries water vapour; four people saturate the air in a closed cabin within minutes.
• Wet clothes and shoes. A jacket soaked in the rain and soles that carried water in both dry out inside the car.
• Water sitting in the mats. Fabric ones hold it for days and release it again every morning.
• Damp air outside. Autumn air is humid to begin with, and with it comes the dark season — rain, early dusk, wet asphalt.

In summer the glass is warm and that water stays as vapour. Autumn adds the second half of the problem: cold glass.

Why the glass mists on the inside, not the outside

Warm air holds more water vapour than cold air does. Any air has a temperature below which it can no longer hold all its moisture — the dew point. Let it touch a surface colder than that, and it gives up the surplus as droplets. In a car the coldest surface is the glass: the street cools it from outside, while the cabin and its occupants warm and dampen the air on the other side.

So there are two things to work on: take the moisture out of the air, and raise the temperature of the glass. The heating and ventilation system does both — but it is usually set up to do precisely the opposite.

Recirculation: the button that ruins everything

Recirculation is the mode in which the system stops drawing air from outside and cycles the air already inside the car. People press it by reflex in autumn, assuming the cabin will warm faster. It will — but the warmth arrives with all the moisture already in there, and the air keeps picking up water until the glass is completely white.

The mode that works is fresh air from outside. It feels counter-intuitive: outside it is four degrees and raining, so how can that air be dry? It is. Cold air carries very little water, and once the heater warms it up it turns thirsty for moisture and lifts it off the glass. Recirculation earns its place only in short bursts — driving behind a smoking lorry.

In winter, the air conditioning is a dehumidifier

Most drivers think of the A/C button as a summer button. In reality the air conditioning dries the air more than it cools it: air passes over the cold evaporator, the water condenses there and drains away beneath the car — that is the puddle you find under a parked car in summer. The air, now dry, goes on through the heater and reaches the glass warm. Which is why many cars switch the compressor on by themselves in the windscreen demist setting.

The combination is simple: A/C on, fresh air rather than recirculation, blower at the windscreen, temperature turned up. None of the four works as quickly on its own as all four together. In a hard frost the compressor may refuse to engage — system protection, not a fault; you are left with the same recipe minus the A/C, and freezing air is already dry.

Why wiping the glass with your hand is the worst option

There is always a thin film on the inside of a windscreen: dust, vapour from the cabin plastics, fingerprints, tar if anyone smokes. While the glass is dry you never notice it.

Drag a hand or a sleeve across misted glass and you are not removing the water — you are smearing it around with that film. What you get is an uneven, greasy layer. In daylight it looks tolerable. In the evening, facing oncoming headlights, every light source blooms into a halo, and you see less than you would through the untouched, even mist. Glare itself is a separate conversation — it is in the post on night driving.

And the core point: a hand rubbing the glass is a hand off the wheel. If the glass mists over once you are moving, turn the blower up — and if you still cannot see, pull over somewhere safe.

The two-minute routine

I see this every autumn in lessons: the student gets in, wipes a porthole, pulls away. They are looking ahead through a palm-sized patch of clear glass, and at that moment a pedestrian steps off the kerb on the right — someone they physically cannot see. That habit belongs with the other common beginner mistakes: on the list of things you drop straight away.

The Latvian road traffic rules (CSN, Ceļu satiksmes noteikumi) require you to choose your speed with road and weather conditions in mind, visibility in the direction of travel above all — that is paragraph 99. Separately, they oblige the driver to check the car before setting off, not on the move — paragraph 25.1.

• Knock the snow and water off your shoes and clothes before you get in: it beats any button.
• Start the engine and set the mode straight away: blower onto the windscreen, A/C on, fresh air, the fan up high.
• Crack a window open a couple of centimetres while the engine warms up, so the damp air escapes.
• Wait for the whole screen to clear, not just a porthole — the same time your seatbelt and mirrors take anyway.
• Once moving, leave the blower on for a few more minutes: the glass needs to warm through, or it mists over again at the first set of lights.

In an autumn lesson this routine builds itself: we do not pull away until the glass is clear.

If it mists up constantly, the buttons are not the problem

Sometimes everything is done right and the glass still goes white every day. That means the car has a water source of its own, and that is what you need to find.

The usual culprit is a clogged cabin air filter: the flow from the vents is weak, so there is nothing to dry the glass with. Second is a wet floor — a leaking door seal, a blocked air-conditioning drain, water under a mat. Lift the mats and feel the floor; if the carpet is damp, the car is drying itself through your windscreen. Third and nastiest is a leaking heater matrix: a greasy film on the glass, a faintly sweet smell, a coolant level that keeps creeping down. That one is a job for a workshop.

Moisture-absorbing bags on the dashboard do soak up water, but that is pain relief, not treatment.

The wording of the rules can be amended — check the current text on likumi.lv (verified September 2026).

A misted windscreen is neither a fault nor bad luck. It is physics, and it repeats itself in the same way every autumn. Learn it once: the two minutes before you set off almost always cost less than thirty seconds driving blind.