One comparison, every case

Is the glass colder than the dew point of the air touching it? Yes → it fogs. No → it stays clear. There is nothing else to it.

The reason this feels unsatisfying at first is that it involves three temperatures, not two — the air, the dew point, and the surface — and everyday language only has words for one of them. Once you separate them, every case on this page is the same case.

The three ways to get there

Fogging happens when that comparison flips, and there are only three ways it can flip.

1. The surface gets colder. A window on a clear night radiates heat to the sky and falls past the dew point of the air that has not changed at all. This is house windows at dawn and dew on grass.

2. The air gets more humid. The surface has not moved, but water is being added to the air around it and its dew point climbs past the glass. This is a car windscreen, a bathroom, or a kitchen with a pan boiling.

3. The object moves. A cold object carried into warm humid air is instantly below its new surroundings' dew point. This is glasses coming indoors, a camera lens leaving air conditioning, and a laptop brought in from the car.

Why cars are the fastest

A car is the extreme version of case 2, and it is worth understanding because it is the one people meet most often under time pressure.

The interior is perhaps 3 m³ of air — about a fourteenth of a small room. A person breathing out puts water into it continuously, wet coats and shoes add more, and there is no ventilation until you turn something on. So the dew point inside climbs quickly, while the glass is still sitting at the outdoor temperature it soaked up overnight.

The gap closes in a minute or two, which is why the windscreen goes opaque just as you need to be moving. Air conditioning clears it faster than heat alone, because it removes water rather than just warming the glass — it pulls both levers at once.

Which side tells you which case

On a house window, the side the water forms on identifies the cause immediately:

In a car it is nearly always the inside, for the reasons above. On a hot humid day with the air conditioning running hard, it can be the outside — the same effect as a cold drink sweating.

Working out whether it will

If you have the air temperature, the humidity and some idea of the surface temperature, the condensation calculator does the comparison and tells you the exact temperature the surface has to stay above.

For anything outdoors, the dew point is the number to know, and it barely changes through the day — so checking it once in the morning tells you what every cold surface will do until the air mass changes.