Instrument Flight Rules (IFR) Practice Exam 2026 – The All-in-One Guide to Master Your IFR Certification!

Advection fog commonly forms when moist air moves over colder surfaces, such as cooler ground or water. As the warm, moist air flows over these colder surfaces, it cools down, causing the moisture in the air to condense into tiny water droplets, resulting in fog. This process is driven by the movement of air, known as advection, which is the horizontal transport of air masses.

The key factor in the formation of advection fog is the temperature difference between the moist air and the surface it is moving over. When the moist air cools sufficiently, the relative humidity reaches 100%, and fog appears. The presence of moisture in the air is essential, as is the cooler surface beneath it.

Understanding this phenomenon helps pilots anticipate weather conditions that may affect visibility during flight operations, as visibility can be severely reduced in fog.