"Our airport is notorious for fog after sunset. It's caught me by surprise a few times. The TAFs don't always predict it and there's no automated weather on the field. What causes night fog and what forecasts show it?" — 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 very 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. Check the MOS forecast for fog or converging temperature-dewpoint spreads.
Also, read the area forecast discussions (which are different than the discontinued Area Forecasts). They will often help you understand more about the forecaster’s thought process and what is expected to occur throughout the local region.”
Has unforecast fog ever forced you to divert and land somewhere you didn't plan on going?