Radiation Fog

Radiation fog is a common type of fog that can significantly impact visibility near terrain.

Formation

Radiation fog forms overnight as the air near the ground cools and reaches saturation. It is most likely to develop in sheltered valleys with little wind and near bodies of water[2]. The fog will first form at the surface and then deepen and extend upward as the air above it also cools.

Characteristics

• Usually patchy and tends to stay in one place
• Forms at low levels beneath a ground inversion, often visible as a white blanket or soft mist in valleys in the morning
• Dissipates under the sun's rays the next day
• Most prevalent during fall and winter

Recognizing Radiation Fog

Pilots can anticipate radiation fog by observing the following signs:

• Clear skies overnight allowing heat to radiate from the ground
• Light winds enabling the fog to form and persist
• Cooler air settling in low-lying areas like valleys
• Presence of a low-level inversion trapping the fog

Radiation fog typically "burns off" as the sun rises and heats the ground. However, thicker fog may linger longer into the day[2]. Pilots should exercise caution and wait for improved visibility before flying in areas prone to radiation fog formation.