How the Aurorama model works
Every figure on Aurorama comes from one rule applied to every hour of every night from 1995 to 2025: was the sky dark, clear and active enough to see the aurora? The model is version v0. It uses two open archives, ERA5 cloud cover from the Copernicus Climate Change Service and the Kp index from GFZ Potsdam, and nothing from tour operators.
The night
A night runs from 16:00 to 08:00 on the local clock of each place: Norway's and Sweden's time in Tromsø, Alta, Svolvær, Abisko and Kiruna, Finland's in Rovaniemi and Iceland's in the four Icelandic places. The model checks every whole hour in that span and gives the night one verdict.
Dark, clear and active hours
An hour is dark when the sun is more than 12 degrees below the horizon at the place. That is nautical twilight: the brightest aurora can show earlier, but a faint one needs this much darkness.
An hour is clear when the total cloud cover in the weather-grid cell is at or below 40%. Thin or broken cloud above that share often still lets the aurora through, so the rule is strict on purpose.
An hour is active when the Kp index for its three-hour period is 1.0 or higher.
The four verdicts
Each night gets one of four verdicts:
- Seen. At least one hour was dark, clear and active. The site calls this a good night.
- Cloud. There were dark hours, and every one of them was cloudy.
- Quiet. At least one dark hour was clear, but activity stayed below the threshold.
- Too light. No hour of the night was dark. In the Arctic summer this is every night.
Why the activity threshold is provisional
The activity bar of Kp 1.0 was set from one night, 12 March 2022 in Tromsø, when the author watched a strong display at a low global Kp. It is a weak basis and the site says so. The same bar is used for every place, although the oval sits differently over Iceland than over northern Scandinavia.
How much it matters: with a bar of Kp 2.0, the share of good March nights in Tromsø falls to 17%; with Kp 3.0, to 10%. Local magnetometer records will replace the global index in a later version.
Which weather-grid cell each place uses
ERA5 gives cloud cover on a grid of 0.25 degrees, about 31 km from north to south at these latitudes. Each place uses the nearest cell. The table shows how far the centre of that cell lies from the town.
| Place | Latitude | Longitude | Cell latitude | Cell longitude | Distance (km) | Clock |
|---|---|---|---|---|---|---|
| Tromsø | 69.65 | 18.96 | 69.75 | 19.0 | 11 | Europe/Oslo |
| Alta | 69.9689 | 23.2717 | 70.0 | 23.25 | 4 | Europe/Oslo |
| Svolvær | 68.234 | 14.568 | 68.25 | 14.5 | 3 | Europe/Oslo |
| Abisko | 68.35 | 18.8167 | 68.25 | 18.75 | 11 | Europe/Stockholm |
| Kiruna | 67.8558 | 20.2253 | 67.75 | 20.25 | 12 | Europe/Stockholm |
| Rovaniemi | 66.5039 | 25.7294 | 66.5 | 25.75 | 1 | Europe/Helsinki |
| Reykjavík | 64.1466 | -21.9426 | 64.25 | -22.0 | 12 | Atlantic/Reykjavik |
| Akureyri | 65.6835 | -18.09 | 65.75 | -18.0 | 8 | Atlantic/Reykjavik |
| Mývatn | 65.6436 | -16.9086 | 65.75 | -17.0 | 13 | Atlantic/Reykjavik |
| Höfn | 64.2539 | -15.2121 | 64.25 | -15.25 | 2 | Atlantic/Reykjavik |
What the model does not see
City lights, local fog, the moon and clear patches smaller than one grid cell, about 31 km across, are not in the verdicts. Mountains and lakes inside a cell are averaged with everything else, which matters most at Abisko and in the fjords. The model was tested against one real trip, with two nights out of three in agreement: see the Tromsø trip of March 2022. The data files behind every page are listed on the data page.