Application Note - BTD-300 for Aviation

Thunderstorms are reported in US Federal Aviation standard according to the location of lightning. For automatic reports, the distances are in nautical miles. Thunderstorms producing lightning within 5 nautical miles (9 km) of the site are considered as overhead, with the METAR code TS. Thunderstorms between 5-10 nautical miles (9-19 km) are defined as being in the vicinity (VCTS). For these two classifications only the proximity is important, with no requirement to report direction. For lightning detected between 10-30 nautical miles (19-56 km), referred to as distant lightning (LTG DSNT) a direction is required, although only as to which octant (N, NE etc.) or octants were affected.