Thirty seconds. That is roughly what you have between sitting down and needing to be moving, and the windscreen has gone from clear to completely opaque in about that time.

Nothing has broken. You are the humidifier, the glass has been outside all night, and the two of them have just met.

Why a car is the worst case

Every fogged window is the same comparison — is the glass colder than the dew point of the air touching it — but a car stacks the odds badly.

The volume is tiny. A car interior is two to three cubic metres. A small bedroom is fifteen times that. Whatever water you add, you are adding it to almost nothing.

You are a continuous water source. A person exhales roughly 20–30 grams of water an hour at rest, in air that is nearly saturated at body temperature. Wet coats, damp shoes and a wet footwell add more.

The glass is at last night's temperature. It has been radiating heat to the sky all night, so it may be several degrees below even the outdoor air.

So the dew point inside climbs fast while the glass sits cold, and they cross within a minute or two.

The fix, in order of effect

1. Air conditioning, on, with heat. This is the one that actually works and the one people skip because it feels wrong in winter.

The compressor cools air below its dew point against the evaporator, water condenses out and drains onto the road, and what comes through the vents is genuinely drier air — not just warmer air. That pulls both levers at once: it raises the glass temperature and lowers the cabin dew point.

Heat alone only does the first, which is why it takes so much longer.

2. Recirculation off. Recirculation re-breathes the air you have already filled with water. On demist you want outside air, which — even on a damp morning — is almost always drier than the air in an occupied cabin.

Most cars select both of these automatically when you press the windscreen demist button. That button is not just "fan at the glass"; it is a preset that does the right thing.

3. Direct the airflow at the glass, and give it a moment. The boundary layer against the windscreen is the coldest, wettest air in the car, and it needs replacing.

4. Crack a window if you have nothing else. Exchanging cabin air for outside air lowers the dew point even without any equipment.

Why it comes straight back

Because nothing about the conditions changed. Turn the fan down and the glass is still cold, you are still breathing, and the boundary layer re-saturates within seconds.

It stops for good only when the glass finally warms above the cabin's dew point — which on a cold morning takes several minutes of sustained airflow, not thirty seconds.

The outside-fog version

Occasionally the fog is on the outside, and everything above makes it worse.

On a warm, humid day with the air conditioning running hard, the glass can drop below the outdoor dew point, and water condenses on the outer surface — the same effect as condensation on the outside of a building's windows.

Wipers clear it. The demist setting does not, because the problem is on the wrong side.

Seeing it coming

The dew point spread overnight tells you what kind of morning you are getting. A narrow spread means a car soaked with dew, cold glass, and a slow start — and it usually means fog on the road as well.

Worth knowing the night before, when it is still a five-minute decision about what time to leave.