FAA Instrument Flight Instructor (FII)WeatherMedium
A pilot is planning an early-morning departure. The previous evening had clear skies, light winds, and high relative humidity. Which type of fog most likely formed overnight at the departure airport?
- AAdvection fog
- BRadiation fog
- CUpslope fog
- DSteam fog
Show answer & explanationAnswer & explanation
Correct answer: B. Radiation fog
Radiation fog forms on clear, calm nights with high humidity as the ground rapidly radiates heat away, cooling the air near the surface to its dewpoint. It typically dissipates within a few hours after sunrise as solar heating warms the ground.
Why the other options are wrong
- A. Advection fog requires warm, moist air moving horizontally over a colder surface, not the clear/calm conditions described.
- C. Upslope fog forms when moist air is forced up rising terrain, not indicated in this scenario.
- D. Steam fog forms when cold air moves over much warmer water, not consistent with the described conditions.
Radiation Fog
Radiation fog forms over land on clear, calm nights with high relative humidity as the surface cools by radiational heat loss, chilling the adjacent air to its dewpoint.
- Requires clear skies and light/calm winds
- Forms overnight, typically over low-lying areas
- Usually dissipates by mid-morning as the sun heats the ground
- Can be dense enough to produce IFR conditions
Memory trick: Radiation fog = the ground shivers itself into a blanket overnight