Radiational cooling is long-wave radiation emitted from the surface to the atmosphere. After sunset with clear conditions, the lack of solar radiation produces a net effect of radiation leaving the surface.
Radiation fog can occur in a variety of synoptic conditions.
Radiation fog forms at night, under calm and clear conditions associated with high pressure. These conditions provide a stable environment with little or no mixing along the valley floor. During late summer and early fall, there is enough time for cooling at the surface to occur, providing a better opportunity for the fog formation process. As nighttime progresses, mixing ends, the atmosphere becomes stable, and the cold denser air from above flows down the hillsides and settles underneath the warmer air in the valley. The cold air mixes slightly with the surface that has been cooled.