How the chance is worked out

For each spot and each night we take three things that anyone can check, and combine them into a number from 0 to 100. We keep it simple on purpose.

1. Darkness. We only count hours when the sun is more than 12 degrees below the horizon. In summer there are none, and the page says "Too light".
2. Cloud. Hourly cloud cover for the spot from MET Norway (Norwegian Meteorological Institute). Clear sky is what matters most: no gap in the cloud, no lights.
3. Geomagnetic activity. The Kp forecast from NOAA SWPC. Higher Kp means stronger aurora. NOAA only forecasts about three days ahead, so later nights use a middling Kp and are marked "cloud only".
4. Moon. A bright moon above the horizon lowers the score a little.

The score is the best two hour window of the night. Roughly: 65 or more is good, 40 to 64 is possible, 15 to 39 is unlikely, under 15 is poor.

What it cannot do

It cannot see the actual sky, and weather forecasts are wrong sometimes, especially more than two days out. Aurora is also patchy: a spot with a high score can still be disappointing. The Icelandic Met Office publishes its own aurora forecast; check it too, and check road conditions before you drive at night.

Data and credit

Weather data from MET Norway under their open licence. Geomagnetic data from NOAA Space Weather Prediction Center. Sun and moon positions are calculated here.