Question: Does an automated surface observation system (ASOS) have a built-in lightning detection system? Without an observer, how does it know to add a thunderstorm (TS), thunderstorms in the vicinity (VCTS), or tell you about lightning in the distance?
Answer: Some ASOS sites do have a single-site lightning sensor in the array. 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 to the ASOS through the automated lightning detection and ranging system (ALDARS), which is a ground-based lightning detection system. ALDARS is not coresident with the sensor and sends the data to the ASOS. Here’s how it all works at a simplified level.
Without a human observer logged in to the ASOS terminal, it will format a METAR or SPECI (special observation) for lightning in one of three ways: TS, VCTS, or lightning in the distance.
1. If the cloud-to-ground lightning strike is detected within 5 miles of the ASOS (usually within the airport’s terminal area), the ASOS will make a special observation (SPECI) and carry “TS” in the body of the special observation in the present weather field. If precipitation, such as rain or freezing rain, is also detected by the sensor array, the observation will include RA (or perhaps freezing rain or drizzle) along with the precipitation intensity (e.g., +TSRA for heavy rain and thunderstorms). “TS” will continue to be carried in the present weather field in subsequent observations until no cloud-to-ground lightning strikes are observed for a 15-minute period within that 5-mile radius. At that time, the ASOS will make a second SPECI observation and officially end the thunderstorm (removes the TS from the METAR).
2. If the cloud-to-ground lightning strike is between 5 miles and 10 miles of the ASOS (the vicinity of the terminal area), the ASOS will make a SPECI observation and carry “VCTS” in the body of the observation in the present weather field. “VCTS” will continue to be carried in the present weather field in subsequent METAR observations until no cloud-to-ground lightning strikes are observed for a 15-minute period in the vicinity. At that time, the ASOS will make a SPECI and officially end the report of thunderstorms in the airport’s vicinity.
3. Separate from above, or independently, if the cloud-to-ground lightning strike is between 10 miles and 30 miles of the ASOS, the ASOS will carry a “LTG DSNT XX” remark, indicating distant lightning, with “XX” being the direction of the lightning in octants. This will be appended as appropriate on all SPECI and/or METAR observations.
For stations with a human observer logged into the ASOS terminal, the observation can be overridden or augmented to include adding remarks, such as FRQ LTGICCG OHD TS OHD MOV NE, which translates to frequent (FRQ) lightning (LGT) in clouds (IC) and cloud-to-ground (CG) overhead (OHD) with thunderstorms (TS) overhead (OHD) and moving (MOV) to the northeast (NE). Lastly, in the remarks of the METAR or SPECI, you will see a group that provides the exact time the thunderstorm begins and ends, such as TSB0159E30, which means the thunderstorm began (TSB) at 0159Z and ended (E) at 30 minutes after the next hour or 0230Z.
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