Aurora Watch

The problem

A public forecast that is easy to misread

NOAA's Space Weather Prediction Center publishes the aurora forecast for the whole planet as one open file. Reading it right is where the time goes: the map shows a moment about an hour ahead, the number is a derived viewing probability rather than a plain one, and the grid is a set of points on whole degrees, not cells.

Watch it work

Episode 7 — Aurora Watch (6:06)

One public dataset end to end: where the aurora forecast lives, the three details that cost the most time, and the 2D map and 3D night globe it turns into.

What it covers

One source, three details, one map

• The source: the forecast lives in one file from NOAA's Space Weather Prediction Center, the OVATION aurora product. One request returns the whole planet — 65,160 points on a one-degree grid, 360 longitudes by 181 latitudes, about 900 kilobytes — each with an aurora viewing probability from 0 to 100. The file is cached for one minute and a new model run arrives about every five minutes. Beside it sit the planetary Kp index and the solar wind summaries, with the wind speed and the interplanetary magnetic field, Bt and Bz.
• Detail one, the map is ahead of the clock: the file carries an observation time and a forecast time. The gap between them is the lead time — the solar wind's travel time from the upstream monitor to Earth — and it changes with the wind speed. On the measurement day the forecast ran 89 minutes ahead. The header chip shows the forecast time, and the card shows both stamps.
• Detail two, the number is not a plain probability: OVATION forecasts the location and intensity of the aurora, and NOAA derives a viewing probability from that intensity with a straight-line assumption. NOAA publishes no probability at which the aurora becomes visible, so the app does not invent one. The planetary Kp index is reported in thirds: 4.67 is Kp 5-, and NOAA's own forecast labels that a G1 storm, so the storm line on the card sits at 4.67, not at 5.
• Detail three, the grid is points, not cells: the values sit on whole degrees, so each square is drawn centered on its point — drawing from the point to the next degree would shift the whole oval half a degree northeast. Longitudes arrive as 0 to 359 and are shifted to start at -180. On a quiet day about 70 percent of the points are zero; those are dropped at parse time. Points above 85 degrees of latitude are skipped on the 2D map, because Web Mercator cannot draw them and the oval never reaches the pole.
• How it becomes a map: each point becomes a one-degree square on a ramp from deep green through cyan to magenta, with transparency scaled to the value; a threshold slider hides the faintest squares from 3 percent up to 75; North and South swing the view between the two ovals; click any square to read its probability and position; the Space Weather Now card shows Kp with the NOAA storm scale and a three-day sparkline, the solar wind speed and Bt and Bz, with a southward Bz in red; auto-refresh repeats the cycle every five minutes; and a 3D night globe with the real sun position lifts each square into a glowing curtain 100 kilometers up.

What it does not do

• Not a sighting — the map shows a forecast probability; you still need dark, clear skies, and a bright aurora can be seen from up to about 1,000 kilometers away.
• Not a NOAA product — the forecast is NOAA SWPC's public-domain data; the map and the app are All Things Spatial's.
• Not a visibility threshold — NOAA publishes no probability at which the aurora becomes visible, so the app does not set one.

Where the data comes from

Every source in the episode

• NOAA SWPC OVATION aurora forecast (JSON) — services.swpc.noaa.gov/json/ovation_aurora_latest.json
• NOAA SWPC aurora 30-minute forecast product page — swpc.noaa.gov/products/aurora-30-minute-forecast
• NOAA planetary K-index — swpc.noaa.gov/products/planetary-k-index
• NWS disclaimer (public domain) — weather.gov/disclaimer
• Esri dark gray basemap (2D and the 3D globe) — esri.com
• Source sheet with every endpoint — allthingsspatial.io/pages/sources.html

The aurora forecast, the planetary Kp index and the solar wind products are NOAA Space Weather Prediction Center data — National Weather Service information in the public domain, provided as is with no warranty. The credit sits in the Aurora Watch footer, with Esri for the basemap.