VOLCLO ?
Yes, take LGW/EGKK METARs, for example.
As I write, the current one (1550z) is giving CAVOK. I'm not surprised, because here on the North Downs I've yet to SEE any deposit on the ground, and have no means of detecting it in the local air. Presumably, the Met observer at Gatwick is similarly handicapped? Visibility, >10k, is presumably too good to justify a Haze report.
Normally, the only Met phenomenon that justifies the closure of an airfield is lying snow. in that case, the METAR has the remark: "SNOCLO". The latter is not an observation of the actual weather. In this case, Gatwick is closed (correct me if I'm wrong) because NATS has closed certain types of airspace, including that - from the surface upwards - at Gatwick. Again, not necessarily because of the current weather observed at the station.
Would it not make practical sense to annotate METARS with "VOLCLO", or equivalent, as a warning to young aviators?
Chris