What the Cuts Keep and Throw Away
The undercast record is cut out of the hourly remarks the Mount Washington Observatory staff append to the KMWN METAR — a TPS LWR group giving the coverage and top height of any cloud deck below the 6,288 ft summit, plus the visibility and the sky above the station. Five conditions on those numbers decide whether an hour counts.
Written down the cuts sound reasonable. This page checks them against the thing they are a proxy for: the webcam frame taken at the same moment. Every day below reported something below the summit, so there was a decision to make. They are grouped by what the cuts did with it.
The five cuts, in the order they are applied:
Reading this
Four of the five cuts are free on this sample: visibility, the lid overhead, the deck depth and the lowest layer overhead throw out fourteen days between them and not one is an undercast. The coverage cut is the only real trade, and it is a big one — it removes twenty-nine days on its own and four of those are genuine undercasts, all of them scattered decks the observer called SCT.
The gallery also makes the ceiling visible. Look at the days that pass and the days rejected for coverage side by side: several look alike, because the remark records how much of the sky the deck covers and nothing about how much of the horizon it fills. BKN is written the same way for an unbroken floor of cloud and for a scatter of cumulus with the valleys open in between. No threshold on these numbers separates those two, which is why the cuts top out where they do.