Hygrometer
Every METAR carries the temperature and the dew point side by side, and one of the first things anyone flying in the morning does is subtract one from the other.
That single number — the spread — is close to the entire fog forecast, and it is available hours before the fog is.
Why the spread is the forecast
Fog is saturated air. Saturation happens when the temperature falls to the dew point.
Since the dew point barely moves overnight, the question reduces to whether the temperature will fall far enough — and the spread tells you exactly how far that is.
| Spread | Expect |
|---|---|
| Under 1°C | Fog likely, or already forming. |
| 1–2°C | The standard warning band. Mist, patchy fog in low ground. |
| 2–4°C | Marginal overnight. Watch it closing. |
| Over 5°C | Unlikely to reach saturation in one night. |
The useful discipline is not the snapshot but the trend. A spread of 4°C at midnight that is 2°C by three in the morning is closing at a rate that will reach zero before dawn — and that is visible two hours before anything appears.
The other two conditions
Spread alone is not sufficient, which is why the thresholds above carry a caveat.
Clear sky. The ground has to radiate heat away freely. Cloud radiates it back down and largely prevents the cooling. A "clear" forecast is a condition for fog, not against it — the point that catches drivers as well as pilots.
Light wind. Counterintuitively, dead calm is not ideal for fog. A few knots mixes the chilled surface layer upward through a deeper layer of air, producing proper fog rather than dew and shallow ground mist. Stronger wind prevents it entirely.
Where it forms first
Cold air is denser and drains downhill after sunset, pooling in valleys, hollows and river bottoms. Those pools cool further and saturate first.
Which is why a field can be clear while the valley two miles away is not, and why river-adjacent sites have a reputation. Local knowledge about which hollows go first is genuinely worth more than the regional forecast.
The cloud base rule of thumb
A related use of the same figure: cloud base in feet ≈ spread in °C × 400.
It applies to convective cloud in a well-mixed air mass and is an estimate rather than a forecast, but it is a quick sanity check on what a soaring day will offer.
Beyond aviation
The same number answers a surprising number of ordinary questions, because it is really just "how close is this air to giving up its water":
- Whether the drive will take fifty minutes instead of twenty.
- Whether dew will land on a fresh coat.
- Whether a tent will rain inside.
- Whether optics will fog at dawn.
Pilots simply have the longest tradition of checking it, and the clearest reason to.
The condensation calculator computes the spread from any temperature and humidity, and the forecast shows it closing or widening hour by hour.