From AMS Glossary
(Also called fitness number.) In the United Kingdom, a measure of the fitness of the weather at an airport for the safe landing of aircraft.
The figure F is computed on the basis of corrected values of visibility and cloud height. Observed visibility is adjusted according to intensity of precipitation; and cloud height is corrected for height of nearby obstructions and cloud amount. Further corrections are applied for the cross- runway component of the wind. The most comparable device widely used in the United States is the sliding scale of ceiling and visibility.