Hygrometer
The forecast said clear. You checked it.
At six the valley is white, the headlights are doing nothing useful, and the drive that takes twenty minutes is going to take fifty. You are going to be late, and you had the information to know that last night.
"Clear" was never reassurance
This is the trap. A clear forecast describes cloud cover, and radiation fog forms at ground level under a cloudless sky.
Cloud would have prevented it. Cloud radiates heat back down and stops surfaces cooling below the air. A clear night removes that blanket, so the ground cools freely, chills the air above it, and that air can reach saturation.
Clear skies are a condition for fog, not a defence against it.
The number that was in the forecast
Fog needs the air to reach saturation, which means the temperature has to fall to the dew point. So the question is simply: how far apart are they, and how far will the temperature drop?
| Gap | What you will drive into |
|---|---|
| Under 2°F | Fog. Plan for it. |
| 2–5°F | Patchy fog in low ground, mist elsewhere, heavy dew. |
| 5–9°F | Clear roads. Wet car. |
| Over 9°F | Nothing. |
Both figures are in an ordinary forecast. The problem is that most people read one of them.
Why it is patchy, and where
Cold air is denser than warm air, so after sunset it drains downhill and pools in valleys, hollows and river bottoms.
That pool keeps cooling, reaches saturation first, and fills with fog while the slope a hundred feet above stays perfectly clear. Rivers and lakes make it worse by keeping the air above them humid.
Which is why a rural commute is not uniformly foggy but a sequence: clear, then suddenly white at the bottom of the hill, then clear again on the far side. The bad sections are the same ones every time, and you probably already know where they are.
The other two conditions
Light wind, but not dead calm. A very light breeze of a few miles per hour is ideal, because it mixes the chilled surface layer upward into a deeper layer of air. Perfectly still air tends to give dew and a shallow ground mist instead; strong wind prevents fog entirely.
Time. Radiation fog needs most of a night to develop, which is why it appears in the hours before dawn — the coldest moment — rather than at midnight.
Burning off
Usually one to three hours after sunrise. The sun warms the ground, the ground warms the air, the air's capacity rises past what it is carrying, and the droplets evaporate.
In winter, with a weak sun and a long night behind it, it may not clear at all. A December fog that is still there at eleven is likely there for the day.
What to do with the knowledge
Not much, and that is the point — it is a five-minute decision made the night before rather than a fifty-minute problem discovered at six.
Check the two lines on the forecast before bed. If they converge, leave earlier, expect the valley sections to be worst, and know that the car will be soaked and fogging before you even get moving.