How do I read the exceedance fan?
The exceedance fan plots the next 48 hours of wind against your limit: the primary forecast line, the ensemble median, two spread bands and ember ticks where the odds reach 40%. This guide shows how to read each part.
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Steps
Find the fan in the inspector
Open /app. In the inspector, below the next-exceed row, the section headed "Exceedance fan" holds the chart. On a phone, drag the sheet handle up first. The right of its heading reads "ENS", the ensemble height, gust or speed, and a member count such as 28/31.
Set height and limit first
Use the Height and Limit steppers in the top bar. Height chooses which ensemble level the bands are drawn for. The limit draws a dashed ember horizontal line; with no limit set the line is absent and the readout says "no limit".
Identify the lines and bands
The filament-coloured line is the primary forecast: gust at 10 m when your height is exactly 10 m, otherwise mean speed at your height. The pale line is the ensemble median, P50. The darker band covers P25 to P75 and the wider, fainter band covers P10 to P90.
Read the readout line
Hover over the chart, or move along it, and the line beneath it changes, for example "Sat 14:00 site · 22 mph · P50 19 mph · P(>25 mph) 36% · 64 members". It gives the hour, primary value, median, odds against your limit and member count. With a mouse, clicking the chart also selects that hour for the whole instrument.
Look for ember ticks
Short ember marks just under the baseline show hours where the odds are 40% or more. They line up with the ember cells on the time strip. Hours with fewer than 10 members show a gap in the bands rather than a guess.
Tip: On touch screens, tapping previews an hour but does not select it; use the time strip to select.
01What the fan shows
Each column is one hour, starting from the current hour. The window is the next 48 hours, extended if you scrub the time strip further ahead, so that you always see the hour you selected and 12 hours beyond it. The y axis is in your display unit. Labels along the bottom mark midnight and noon of site time. Everything is plotted from modelled data; nothing on the fan is an observation.
02Primary line and median
The primary line comes from the best-match forecast read at your working height. The median is different: it is the middle value of the ensemble members at the ensemble level. The two often differ, because one is a single run and the other is the centre of many. When the primary line sits well above the median, the main forecast is more aggressive than most of its siblings, which is worth noticing.
03The two bands
The darker band holds the middle half of ensemble members, P25 to P75. The wider band holds the middle 80%, P10 to P90. A narrow band means members agree. A band that widens with lead time is normal. A wide band crossing your dashed limit line means members disagree about that hour, and some reach your limit. A band hugging the limit from below means most members are close to it.
04Which level the bands use
The ensemble offers speed at 10, 80, 100 and 120 m and gust only at 10 m. The bands are drawn for the nearest of those, not interpolated to your exact height, while the primary line is read at your height. When these differ, a small note under the legend says so, for instance that the ensemble is at 80 m for an asset at 70 m, or that no ensemble was available and the 10 m one is shown. The legend reads "primary gust 10 m" or "primary" with your height.
05Gaps, loading and unknown
"Reading the ensemble…" appears while the first request is under way. A gap in the bands, and a dash for the odds, means that hour had fewer than 10 members or no ensemble data. That is unknown, not calm, and a gap is never filled with zero. If the ensemble cannot be loaded at all, a line in ember beneath the legend gives the reason. The message "No ensemble spread at this height/hour" belongs to the wind-profile glass in the inspector, not to the fan.
06How the fan relates to the rest
The hero readout, the next-exceed row and the time strip all read the same ensemble. The fan adds the shape over time. Use it to spot where a breach starts and how fast the spread opens, then check the lamp, the observation and the models chip for context. The lamp is a separate calculation that also weighs the plain forecast against your limit, so the two can disagree for an hour or two.
07Mistakes to avoid
Do not read a band below your limit as a promise of calm; ensembles can be too narrow. Do not read the dashed limit line as a measured threshold: it is the number you typed. Do not compare the fan across different heights without noting the ensemble level. And do not rely on the median alone. The fan is evidence for your decision, and the decision stays with the person responsible on site.
Questions
What is the difference between the median and the primary line?
The primary line is a single best-match forecast at your height. The median is the middle of the pooled ensemble members at the ensemble level. They answer different questions, and when they diverge it is a sign of forecast uncertainty about that hour.
Why is the fan sometimes very wide?
Ensemble members disagree about that hour, usually because it is far ahead or because a front or squall makes the timing uncertain. A wide fan that crosses your limit means some members reach it and some do not. Look at the odds in the readout line for the figure.
Does the fan change with my height?
Yes, in two ways. The primary line is recomputed at your height, and the ensemble level behind the bands can change, since only 10, 80, 100 and 120 m exist. Heights under 45 m use 10 m. The header text above the chart and the note below it tell you which level is in use.
Why are there gaps in the bands?
The ensemble returned fewer than 10 members for those hours, or none. The fan leaves the bands out instead of guessing, and the odds show as a dash. Treat those hours as unknown.
Can I click the fan to move through time?
With a mouse, yes: clicking selects that hour for the hero, the map and any route or tee sheet. On touch screens a tap only previews the readout, so use the time strip to move through the hours.