The weather at an airport affects the ability of an aircraft to land in so far as it controls the pilot's vision of the runway as he approaches it. This aspect of weather is at present described by (a) Runway Visual Range (RVR) as measured along the runway by Air Traffic Control, (b) Visibility as measured from and by the Meteorological Office, and (c) the amount and height above the airfield of the lowest cloud. Weather minima are specified in terms of RVR or visibility and of cloudbase by each Company for each type of aircraft using each available type of approach aid on each runway at each airport, such that an aircraft should not try to land in worse conditions. A landing forecast known as a TAF is issued regularly for each airport and specifies among other things visibility and cloudbase, giving either single values for each, or subsidiary ‘possible’ values indicating temporal variations or values with a small probability of occurrence. Examples are: ‘Visibility 10 miles TEMPO (temporarily) showers Visibility 2 miles’. ‘Fog Visibility 500 yards GRADU (gradually becoming between) 0400–0600Z mist Visibility 1 mile’. The TAF for the destination is seen by the aircraft captain before departure and during flight: if it is unfavourable he may decide to increase the fuel reserve reducing the payload or to delay departure or to divert. A wrong TAF may have costly and occasionally dangerous results.