What is hoar frost?

Hoar frost is indeed characterized as a crystalline formation that can cover an aircraft's surface. It typically occurs under certain environmental conditions, usually during clear, cold nights when moisture in the air condenses and freezes upon contact with cold surfaces, leading to the formation of delicate ice crystals.

This phenomenon is significant in aviation because hoar frost can create surface imperfections that affect aerodynamics and can increase drag, as well as potentially disrupt the function of control surfaces. Since hoar frost is light and flaky, it can be challenging to detect and remove, making its presence on an aircraft's surface a critical concern before flight operations.

Other options describe different forms of aircraft icing or weather conditions but do not accurately define hoar frost. Recognizing its unique properties helps pilots and ground crews manage safety and performance effectively during cold-weather operations.