Radiation fog is a type of fog that forms overnight when the ground loses heat rapidly and chills the air just above the surface until moisture condenses into tiny water droplets.
How It Works#
The ground radiates heat upward into the atmosphere after sunset. On clear, calm nights, this heat escapes quickly because clouds cannot trap it and wind cannot mix it away.
As the ground cools, it draws heat from the thin layer of air directly above it. That air cools to its dew point (the temperature at which air becomes saturated and water vapor condenses). When that happens, fog forms.
Two conditions must be present for radiation fog to develop: high relative humidity and light or calm winds. Wind stronger than about 5–7 knots will mix the cooled surface air with warmer air above, preventing fog from forming. Valleys and low-lying basins are especially prone to radiation fog because cold, dense air drains downhill and collects there.
Radiation fog is typically shallow. It often ranges from just a few feet to a few hundred feet thick. After sunrise, solar heating warms the ground again, which then warms the air from below. The fog burns off from the ground up, usually within one to three hours after sunrise.
Example in Aviation#
A student pilot checks the weather the night before a planned morning flight. The forecast shows clear skies, calm winds, and high humidity. She arrives at the airport at 7:00 a.m. and finds the field socked in with fog, visibility below one statute mile and ceiling near zero.
By 9:30 a.m., the sun has warmed the surface and the fog has lifted. The ceiling rises quickly, visibility improves, and she departs under Visual Flight Rules (VFR). The entire sequence is a textbook radiation fog event.
Why It Matters#
Radiation fog can appear suddenly and reduce visibility to near zero in minutes. A pilot who departs or lands without accounting for it risks operating in conditions far below VFR minimums, which require at least 3 statute miles visibility and a 1,000-foot ceiling in controlled airspace.
Understanding the conditions that produce radiation fog lets pilots read a forecast and anticipate problems before they develop. Recognizing the telltale setup (clear sky, calm wind, high humidity, cool night) is a practical skill that separates a cautious pilot from a complacent one.
Key Takeaways#
- Radiation fog forms when the ground cools calm, humid air to its dew point overnight.
- Clear skies and light winds are the two key ingredients.
- Valleys and low areas collect cold air and fog more readily than elevated terrain.
- Fog typically burns off from the ground up within hours after sunrise.
- Pilots should anticipate radiation fog whenever overnight conditions show high humidity, calm winds, and clear skies.