How do you predict a fog morning the evening before?

The fog that makes photographs — valley fog, mist over water, light shafts in forests — is mostly radiation fog, and it's predictable from three numbers in any hourly forecast:

• Dew-point spread ≤ ~1.5°C (or relative humidity above ~97%). The spread between air temperature and dew point is the distance the night has to cool before the air saturates. A small evening spread on a clear night usually closes by dawn.
• Wind under ~8 km/h. Calm air lets a cold, saturated layer sit still and thicken. Anything past ~15 km/h mixes it away.
• Timing: sunrise ± 2 hours. The coldest moment of the night is just after sunrise — fog is thickest right when the light gets interesting, then burns off within an hour or two. That overlap is the shot.

Terrain multiplies the odds: river valleys, lakes and wet meadows fog first. ShootWindow scores every forecast hour on the humidity, wind and sun-position conditions and flags the mornings worth setting an alarm for — including the marginal ones a plain weather app renders as just “clear, 9°C”.