Subscriber question:
"When an ASOS reports 'distant lightning,' how far from the airport is the lightning strike?" — Mike S.
Scott:
“An ASOS will format a routine observation (METAR) or special observation (SPECI) for lightning in one of three ways:
If the cloud-to-ground lightning strike is within five miles of the ASOS, the ASOS will make a special observation and carry ‘TS’ (for thunderstorms) in the body of the special observation in the present weather field.
If the cloud-to-ground lightning strike is between 5 miles and 10 miles of the ASOS, the ASOS will make a special observation, and carry ‘VCTS’ (for thunderstorms in the vicinity) in the present weather field.
Separately and independently of those conditions, if the cloud-to-ground lightning strike is between 10 miles and 30 miles of the ASOS, the ASOS will carry a distant lightning remark (LTG DSNT) with the lightning direction in octants being appended at the end of the remarks on all special and routine observations.
Some ASOS sites have a single-site lightning sensor. If there isn’t a lightning sensor at the site, it is still possible for the ASOS to report lightning. For FAA-sponsored ASOS sites without a lightning sensor, lightning data is made available through the Automated Lightning Detection and Ranging System (ALDARS), which is a ground-based lightning detection system.
Of course, at locations with a human observer, the ASOS observation can be overridden including adding remarks such as, ‘frequent lightning in clouds and cloud-to-ground overhead, thunderstorms overhead, moving northeast’ (FRQ LTGICCG OHD TS OHD MOV NE).”
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