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 four 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; past about 3°C the night rarely has time.
• Wind under ~8 km/h. Calm air lets a cold, saturated layer sit still and thicken. Anything past ~15 km/h mixes it away.
• A clear sky overnight. Radiation fog is driven by the ground radiating its heat into space, so it needs a sky that can lose heat. A low cloud deck acts like a blanket and the surface never cools to the dew point. Thin high cirrus barely matters; low and mid cloud is what kills the morning.
• 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: cold air drains downhill and pools, so river valleys, lakes and wet meadows fog first and hold it longest. A hill above a valley is the classic vantage — you want to be looking down onto the layer, not standing in it.

Why it didn't fog, even though the forecast looked perfect

Most disappointing mornings come down to one of three things, and all three are visible in the forecast the evening before if you know to look:

• Cloud moved in. Radiative cooling stopped, the temperature levelled off a couple of degrees above the dew point, and the spread never closed. This is the most common failure and the easiest to miss, because the humidity numbers still look excellent.
• The wind picked up. Even a light breeze after midnight will mix the saturated surface layer into drier air above it.
• The ground was dry. Radiation fog needs moisture to work with. After a long dry spell the same numbers produce much less fog than they do two days after rain.

Radiation fog vs advection fog

Not all fog is forecastable the same way. Advection fog forms when warm, moist air moves over a colder surface — it needs wind rather than calm, doesn't need a clear sky, and is what rolls in off cold water onto a coast. Steam fog is the opposite pairing: cold air over warmer water, which is why lakes smoke on the first hard morning of autumn.

ShootWindow scores radiation fog. It's the kind that's genuinely predictable from a forecast, and the kind that fills valleys. If you're on a coast waiting for a sea fog to roll in, the four numbers above will not help you — that's an honest limitation, not an oversight.

When is fog season?

At mid-latitudes the best mornings cluster in autumn and early spring. You need two things at once: nights long enough for the surface to cool several degrees, and ground that still holds moisture. Autumn gives you both — lengthening nights over soil still wet from the season's rain. Deep winter often has the long nights without the moisture, and midsummer nights are too short to cool far enough. The mornings after the first cold snap following a wet week are usually the best of the year.

What ShootWindow does with this

It scores every forecast hour on the dew-point spread, wind, cloud cover and sun position, and flags the mornings worth setting an alarm for — including the marginal ones a plain weather app renders as just “clear, 9°C”. Weather comes from the Norwegian Meteorological Institute. It's a forecast, not a promise: the point is to cut wasted 5 a.m. alarms, not to guarantee a shot.