Radiation Fog

Radiation fog forms during all months of the year but is not a detriment to flying except during late fall, winter and early spring. Due to the nearness of water areas, radiation fog forms only in those areas immediately adjacent to KNPA, west through northwest. Inland areas to the west and clockwise through east are ideal formation areas for ground fog. Ground fog formation is most likely to occur with winds of 5-12 knots. Winds of less than 5 knots or greater than 12 knots produce only light ground fog.

Usually a ground fog situation begins 2-3 days after a high or ridge becomes quasi-stationary over the southeastern United States. The advected ground fog becomes a factor at KNPA during two wind situations.

1) The scenario with the earliest onset of fog, the most pronounced fog, and the latest fog dissipation is produced by a surface wind from west-northwest clockwise through east-northeast with a speed of 5-12 knots. This primarily occurs during winter, but may occasionally happen during late fall and early spring. IFR conditions due to reduced visibility begin as early as 2000 local and decrease to below minimums by 2400 local. IFR conditions remain below minimums until about 1 hour after sunrise. VFR conditions are observed between 0900 to 1000 local. Boat operations are hampered in all bay areas and as far out as 3 miles from the coast.

2) A synoptic situation producing a light surface wind can also allow the formation of weaker ground fog inland. In this situation the land/sea breeze is the predominant wind. Relatively light winds occur in the period from sunset until early morning. As the land breeze sets in, usually about 0400 local, visibility begins to decrease, becoming IFR about sunrise. The minimum visibility, 3/4-11/4 mile, occurs about 1 hour after sunrise. Normal insolation dissipates the advected fog usually within 2 hours after sunrise. This occurs mainly during the early fall and late spring.

With conditions favorable for ground fog formation and wind forecast from the east-southeast clockwise through west-southwest, the ground fog remains north of the station. Prevailing visibility remains 4-6 miles with quadrants west through northeast occasionally reducing to 2-4 miles.

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Insitute for Human and Machine Cognition
The University of West Florida