Certified Flight Instructor - Flight Instructor Airplane Practice Exam

Radiation fog is most likely to occur under what conditions?

1. Warm, moist air being forced upslope by light winds resulting in the air being cooled and condensed.

2. High humidity during the early evening, cool cloudless night with light winds, and favorable topography.

3. Low temperature/dewpoint spread, calm wind conditions, the presence of hydroscopic nuclei, low overcast, and favorable topography.

The correct answer is: High humidity during the early evening, cool cloudless night with light winds, and favorable topography.

Radiation fog typically forms when high humidity coincides with specific nighttime cooling conditions, particularly on clear nights. During the early evening, the land loses heat through radiation, cooling the surface and subsequently cooling the air just above it. If this air cools to its dew point, condensation occurs, leading to the formation of fog. The calm wind conditions mentioned in this context are significant because they allow the air to remain stable and undisturbed, preventing mixing that could inhibit the desired cooling and condensation process. The presence of favorable topography, such as valleys, can further enhance the likelihood of fog formation as cold air tends to settle in lower areas where moisture accumulates. The combination of these factors—high humidity, nighttime cooling, calm winds, and suitable terrain—creates ideal conditions for radiation fog to develop, making this choice the most accurate representation of the conditions under which radiation fog is likely to occur. Other scenarios, such as those involving warm, moist air being forced upslope or low temperature/dewpoint spreads under a cloud cover, do not accurately depict the specific cooling mechanism responsible for radiation fog formation.