Visibility: 9999, 0000 and SM

Prevailing visibility is reported as four digits in metres. 0800 is 800 m, 4000 is 4 km. The figure is the greatest distance at which objects can be identified over at least half the horizon, so it describes the aerodrome generally rather than any one direction.

The two extremes are conventions rather than measurements. 9999 does not mean 9 999 m — it means 10 km or more, the point at which the code stops distinguishing values. 0000 does not mean zero — it means less than 50 m. Both are ceilings on what the group can express, not readings.

North America uses statute miles instead, marked with an SM suffix: 6SM is six statute miles, and fractions appear as 1/2SM or 1 1/2SM. Where visibility differs markedly by direction, a minimum visibility group follows with the lower value and its direction. NDV means no directional variation is available, which is normal from an automatic station that cannot sample around the horizon.