Radiation fog most commonly forms under which conditions?

Radiation fog forms by the ground losing heat to space and cooling the air immediately above it to its dewpoint. Four conditions are needed together. Clear skies allow the radiational cooling to happen, since cloud cover would radiate heat back down. Light wind — roughly two to five knots — is essential in both directions: dead calm produces only dew or a very shallow layer, while stronger wind mixes the cooled air upward and prevents fog forming. Moist air near the surface supplies the water, which is why river valleys, low ground and recently irrigated or rained-on land are the classic locations. And a long cool night gives the process time. The result is a shallow, dense layer that forms overnight, is at its worst around sunrise, and burns off as the sun warms the ground — which is precisely the window a remote pilot planning an early flight cares about, since the fog often appears in the hour when the crew is setting up.

Radiation fog forms when clear skies allow the ground to cool, light wind provides gentle mixing without dispersal, and moist surface air is cooled to its dewpoint over a long night.

Radiation fog forms overnight under clear skies with light wind over moist ground, as the surface cools the adjacent air to its dewpoint. It is shallow, densest near sunrise, and dissipates with surface heating.

Clear, Calm, Cool, Moist. Take away any one of the four and the fog does not form.

Know the four fog types by mechanism: radiation from ground cooling, advection from warm air over a cold surface, upslope from ascent, precipitation-induced from evaporation into cool air.

• •Expecting radiation fog under strong wind
• •Confusing it with advection fog
• •Overlooking that it is worst at the sunrise setup window