"I often arrive at my home airport late at night after a business trip and our airport is notorious for developing fog after sunset. It has caught me by surprise a few times. The TAFs don't seem to help and there is no automated weather on the field. What causes it and are there any forecasts that can help?" - Jack C.
“The basic rules that work for us during the day don’t always apply during the graveyard shift. A perfect example is radiation fog.
Radiation fog can catch even the most prepared pilot by surprise. Unlike fog associated with a warm front, it is not as predictable. It is the most common and dense form of fog, and even the most astute forecaster can miss the event entirely. It will typically develop several hours after sunset, first in the rural areas especially near bodies of water as the earth begins to cool by radiation on a clear night. This produces a nocturnal temperature inversion just above the surface providing for very stable conditions.
Terminal forecasts are not always the best source to warn a pilot for an impending radiation fog event since radiation fog can be a very localized event. It is possible that an airport in an urban area is spared while satellite airports around the city are experiencing a low IFR event.
Take some extra time examining the weather if you are flying late at night into the early morning hours. Read the area forecast discussions. They will often help you understand more about the forecaster’s thought process and what is expected to occur throughout the local region.”