Certified Flight Instructor - Flight Instructor Airplane Practice Exam

Which factor mainly contributes to the formation of radiation fog?

1. Strong winds during the day.

2. High humidity at night with light winds.

3. The presence of thunderstorms nearby.

The correct answer is: High humidity at night with light winds.

Radiation fog primarily occurs under conditions of high humidity combined with light winds at night. This type of fog forms when the ground cools rapidly after sunset, causing moisture in the air near the surface to condense. As the temperature drops, the air's capacity to hold moisture decreases, leading to saturation. In the absence of strong winds, the cooling process is more pronounced, allowing the air closest to the ground to reach its dew point, where water vapor condenses into tiny water droplets, creating fog. The light winds help distribute the cooler air and moisture evenly, maintaining the humid conditions required for fog development. When conditions include strong winds, the mixing of air layers disrupts the cooling process, preventing the air near the surface from reaching the necessary temperature for fog formation. Similarly, the presence of thunderstorms typically indicates turbulent weather, which also does not favor the calm conditions needed for radiation fog to develop. Thus, the combination of high humidity at night with light winds is a key factor in the formation of radiation fog.