The FAA has published Information for Operators 18011 to explain updated codes that will soon be used for indicating snow pellets and small hail in terminal area weather reports (Metars).
Currently, snow pellets and/or small hail have a single code of “GS” being used to report two different precipitation types, which also requires a weather observer to augment the Metar or include an aviation special weather report (Speci). “This has often resulted in flight crews being unable to interpret holdover time limitations,” the agency said.
The first change will be the use of “GR” to refer to all hail. All GR reports must include hailstone size diameter in the remarks section of the Metar/Speci in increments of a quarter of an inch. When small hail less than a quarter inch in size is occurring, the hailstone is reported in the remarks as, “GR LESS THAN ¼ inch.” Small hail will also result in the issuance of a Speci. The second change will reflect “GS” for snow pellets only when observed and will include appropriate holdover times.
Providing a single code for all hail regardless of size will solve operators’ de-icing issues and preserve the climate record, the FAA believes. Aircraft are not permitted to be operated in large hail but can operate in small hail when the crew applies deicing/anti-icing procedures.