Hygrometer
The whole forecast in one number
Fog is what happens when air reaches saturation — not just a surface, but the air itself. So the question "will it be foggy" reduces to: will the temperature fall to the dew point?
Since the dew point barely moves overnight, that is really a question about how far the temperature will drop. And the gap between them tells you how far it has to go.
What each gap predicts
| Spread | What to expect |
|---|---|
| 0–2°F | Fog. The air is at or effectively at saturation. |
| 2–5°F | Mist, patchy fog in low ground, heavy dew everywhere. |
| 5–9°F | Dew on exposed surfaces. Air stays clear. |
| 9–18°F | Dew only on the coldest things — car roofs, open lawn. |
| Over 18°F | Nothing. The air is a long way from giving anything up. |
The condensation calculator computes the spread from any temperature and humidity, and the humidity forecast shows it closing or widening hour by hour.
The three conditions
Fog needs the same things dew does, for the same reasons — it is the same process taken one step further.
- A clear sky. The ground has to radiate heat away, and cloud prevents it.
- Light wind. A dead calm actually works against fog, oddly — a very light breeze of 2–5 mph is ideal, because it mixes the chilled surface layer upward into a deeper layer of air. Dead still air produces dew and a shallow ground mist instead.
- A small spread. The air has to be close enough to saturation that a night's cooling gets there.
Why valleys go first
After sunset, air chilled by contact with the ground becomes denser and drains downhill. It pools in valleys, hollows and river bottoms, where it keeps cooling.
So a valley floor can be several degrees colder than the slope above it, reach saturation first, and fill with fog while the hillside stays clear. Anyone who has driven down into a white valley on an otherwise clear morning has watched this happen — and it is why the drive that takes twenty minutes takes fifty.
Water also helps: rivers and lakes keep the air above them humid, which is why fog so often follows a valley's watercourse.
Radiation fog and advection fog
The kind described above is radiation fog — formed in place by ground cooling, most common on clear autumn and winter nights, and usually gone by mid-morning.
The other common kind is advection fog, formed when warm humid air moves over a cold surface — sea fog rolling inland, or fog over snow. It behaves differently: it can arrive suddenly, persist all day, and it does not need a clear sky at all. If fog appears with a breeze from the sea rather than forming in place at dawn, that is what it is.
Checking it the night before
The gap is in every ordinary forecast, if you know to look for both numbers rather than one. The humidity forecast draws them together for the hours ahead.
It is worth the thirty seconds for anything with an early start — a morning flight, an airport run, a dawn shoot with optics that fog at exactly the wrong moment.