The live windrose

In addition to the usual observation types (which include windrun), there is a special windrose observation type (new in 6.0, replacing the experimental windrun_<direction> types) for building a live NOAA-style windrose. For each period, loopdata accumulates a matrix of sixteen compass bins (N, NNE, NE, … NNW — clockwise from north) by N wind-speed bands, tracking both time and distance per cell, plus a directionless “calm” total.

windrose works with every period except current and trend — including week, month, year, rainyear and alltime. Span periods seed at startup from the archive, so even alltime.windrose starts fully primed after a restart.

The four aggregates

Four aggregates project the matrix into json:

• day.windrose.sum — sixteen distances (the classic windrun rose), unit-converted to the target report’s distance unit; supports a unit override (day.windrose.sum.km) and .formatted (an array of report-formatted strings). Note: wind below the calm threshold counts as calm time, not distance — there is no reliable direction to attribute it to — so on a near-calm day .sum can be all zeros while windrun shows a small total.
• day.windrose.time — sixteen durations in seconds (a frequency rose).
• day.windrose.banded — the full 16-by-N matrix of seconds per speed band (the NOAA rose; divide by total time for percentages).
• day.windrose.calm — seconds of calm: wind below the calm threshold, or no wind direction. Render it as the rose’s center-circle percentage.

(day is just the example — any period except current and trend works: month.windrose.banded, alltime.windrose.sum, 10m.windrose.time, …)

Arrays are emitted as json numbers (charting javascript wants numbers, not strings); .round(n) applies per element (e.g. day.windrose.sum.round(2)).

Two helper keys come along:

• Whenever any windrose field is configured, loopdata also emits a windrose.bands key holding the band edges in the target report’s windSpeed unit, so a page’s legend never hardcodes them.
• unit.label.windrose yields the distance unit label for .sum.

The speed bands

The speed bands default to the classic WRPLOT/NOAA bands — 0.5, 2.1, 3.6, 5.7, 8.8 and 11.1 m/s, converted to the target report’s windSpeed unit and rounded to one decimal. The first edge doubles as the calm threshold. To override, list ascending edges (in the target report’s windSpeed unit) in the [LoopData] section of weewx.conf:

[LoopData]
    windrose_bands = 1, 4, 8, 13, 19, 25

Choosing your bands

The default bands suit a windy site; on a light-wind site most of the rose can land in the lowest band (or below the calm threshold entirely). Your archive already knows what your site does — one query shows how your time divides across wind speeds (values are in your database’s unit; convert if your report unit differs):

sqlite3 weewx.sdb "SELECT CAST(windSpeed AS INT) AS speed, \
  ROUND(100.0 * SUM(interval) / (SELECT SUM(interval) FROM archive \
  WHERE windSpeed IS NOT NULL), 1) AS pct_of_time \
  FROM archive WHERE windSpeed IS NOT NULL GROUP BY speed;"

Then place the edges by three rules:

• Calm threshold (the first edge): at or just below the smallest non-zero speed your console reports. Integer-reporting consoles (e.g. Davis Vantage, in mph) spend a lot of time at exactly 1 — an edge of 1.1 silently counts all of it as calm, an edge of 1 keeps it in the rose. Calm percentages of 10–40% are normal on real roses.
• Middle edges: place them so each band carries a real share of the non-calm time (a few percent at minimum) — a band that never fills is a wasted legend entry, and one band holding most of the time is a monochrome rose.
• Top edge: rare on purpose — around the speed you exceed only a few hours a year (the 99.5th percentile or so), so the darkest color flags genuinely notable wind rather than never appearing at all.

Round numbers in the report unit make the best legend. Iterating is cheap: changing the bands is just an edit and a weewxd restart — every period reseeds from the archive at startup, so the whole rose re-buckets under the new edges.

Drawing a rose

The sample skin draws a live canvas windrose from day.windrose.banded and day.windrose.calm — see skins/LoopData/realtime_updater.inc for javascript to crib from.

Upgrading from windrun_<direction> fields

The experimental windrun_<dir> observation types (loopdata ≤ 5.x) are gone; fields naming them are ignored. The mapping is by compass order: day.windrun_N.sum is now element 0 of day.windrose.sum, day.windrun_NNE.sum element 1, and so on clockwise through NNW (element 15). Unlike windrun_<dir>, windrose accumulators seed every period from the archive at startup, so buckets are no longer empty after a weewxd restart.