Fog watch

Clear night + calm air + temperature within ~2.5 °C of the dew point = expect fog.

Why it works

Radiative cooling drops the surface air to its dew point and water condenses out. The smaller the spread, the less cooling is needed.

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

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More in Environmental

• Lightning distanceCount the seconds from flash to thunder, divide by 3 for kilometres (by 5 for miles).
• Air cools as you climbTemperature drops about 6.5 °C for every 1,000 m of altitude.
• Pressure halves every ~5.5 kmAtmospheric pressure falls by half roughly every 5,500 m you climb.
• Ten metres of water = one atmosphereEvery 10 m of seawater adds about 1 atmosphere of pressure.

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