During which conditions is it most likely for frost to impede airplane operations?

Frost is most likely to form under clear, calm winter nights due to specific atmospheric conditions that are conducive to radiational cooling. During these nights, the ground loses heat rapidly due to the absence of cloud cover, allowing the temperature to drop to the dew point. When the surface temperature falls below freezing, moisture in the air can condense and freeze, leading to frost formation on aircraft surfaces, including wings and control surfaces.

This frost can create a barrier that affects the aerodynamic properties of the aircraft, reducing lift and potentially interfering with safe operations. The calm conditions also mean there is less wind to disturb or dissipate any frost that forms, allowing it to accumulate more heavily.

In contrast, hot, humid days and rainy overcast conditions are typically too warm for frost to occur. While cool, windy nights may have lower temperatures, the wind can prevent frost from forming by mixing the air and maintaining temperatures above freezing, or it may simply prevent the conditions necessary for frost to settle on the aircraft.