Why does it say UNKNOWN instead of zero?

The instrument shows UNKNOWN when data is missing or cannot be worked out. That is not the same as calm wind, which would be a real value near zero. Showing zero for a missing value would suggest a condition nobody has measured or modelled. Hours that lack the core wind fields are dropped and counted, never filled with zeros, and a missing number is carried as unknown along with the reason. Treat UNKNOWN as a prompt to look for another source, not as a sign of light wind. The instrument cannot tell you if it is fine to lift, spray or fly. It reports the wind and the odds of crossing the limit you set.

On the instrument at /app, a wind speed the models cannot supply shows as a dash, with a note reading UNKNOWN and the reason beside it. The status lamp can read UNKNOWN, and the observation line can read OBSERVATION UNKNOWN when no METAR or buoy report is usable within 150 km. The agreement spine uses the word in its detail lines too. On the Fleet board, which is part of the Ops plan, the lamp column can read UNKNOWN, the next-odds column can read UNKNOWN, and on a phone the row says Odds UNKNOWN. A site's reason is printed under its lamp. Cab mode shows UNKNOWN when the saved forecast no longer covers the current hour. In the instrument, having no limit is a different state: it shows NO LIMIT, not UNKNOWN.

Three kinds of gap are common. First, a station: if no METAR or buoy within 150 km has reported recently, the observation is unknown, and the agreement spine says so. Second, a model level: the wind at your height is worked out from the 10, 80, 120 and 180 m levels, and the height reads UNKNOWN only when no level has both speed and direction, or the height itself is not valid. A single missing level is bridged by interpolation or extrapolation, and the height fidelity tells you which was used. Third, ensemble members: the odds are counted from members of three ensembles, so if an hour has too few members the odds are withheld. Each of these gaps is shown as a gap, never as a number.

The odds come from three ensemble systems: GEFS, ECMWF ENS and ICON-EPS. Each contributes many members. For a given hour the odds are only given when at least 10 members are usable. With fewer, the probability for that hour is withheld, and the display shows a dash where the percentage would be, with a short reason. Separately, when some members are missing but enough remain, the readout shows a label such as ENSEMBLE INCOMPLETE 28/31, and the models strip shows how many of the expected members arrived. The odds are then based on fewer members than normal. Neither message is a judgement on the wind. Both describe how much data arrived.

On the Fleet board, a site's lamp is UNKNOWN for one of these reasons, each printed under the lamp: no rule is set, the forecast is unavailable, the ensemble is unavailable so the odds were not evaluated, there are no forecast hours in the shift window, or hours are missing. The order matters. If the primary forecast reaches the limit, the lamp is STOP even when the odds are missing. Where some hours have odds and others do not, the board uses the hours it has, so a site can still show GO from partial data. Check the reason line before you rely on a GO. On a phone, the same states appear as Odds UNKNOWN, No rule set or No crossing in shift.

When you see UNKNOWN, read the reason beside it first, because it usually names the gap. On the instrument, press Retry if the whole forecast failed to load, or choose the place again. Check whether your working height is far outside the 10 to 180 metre range, where the value is extrapolated and labelled. On the Fleet board, open the pencil beside the site if the reason is that no rule is set. Look at the fetched time in the provenance line at the foot of the inspector, to see how recent the data is. If the gap lasts, use another source, such as your site anemometer or the station report, until data returns. How fresh is the data? explains the refresh times.