How the scores work

Every score on this site comes from one transparent formula — no machine learning, no hidden weights, nothing you could not recompute yourself. Four conditions have to cooperate for you to see the aurora, so the score multiplies four factors, each scaled 0 to 1:

Dark sky

Full value when the sun is 10° or more below the horizon, zero when it is higher than 4° below. During the midnight-sun months this factor is zero all night — which is why whole summer months score 0% whatever the sun is doing.

Solar activity

NOAA's forecast Kp index compared with each destination's threshold — the Kp at which the auroral oval typically reaches that latitude (Yellowknife needs about 0.5, Reykjavík about 2). The ramp is soft: a bit below threshold can still mean lights low on the northern horizon.

Clear sky

Hourly cloud forecasts, with low cloud blocking fully, mid-level cloud at 70% and thin cirrus at 30%. This factor kills more aurora nights than anything else.

Moonlight

A full moon above the horizon dims the score by up to a quarter. It never zeroes a night — strong displays shine straight through moonlight.

The night score

Each 3-hour block of the night gets its own score; the night's headline number is the best block, because one good window is all you need. The weakest factor becomes the plain-language reason — "clouds are the problem" beats a mystery number.

Monthly odds

The best-time pages multiply two independent, destination-specific probabilities: the share of days each calendar month with geomagnetic activity above the local threshold (from GFZ Potsdam's Kp record, 1996–2025 — about 2.7 solar cycles, so no single cycle phase skews a month), and the share of nights offering at least one dark hour under 40% cloud (hourly reanalysis, 2015–2024). Months without astronomical darkness are zero by definition.

Honest limits

Kp forecasts are only detailed about three days out; nights four to seven lean on NOAA's rough 27-day outlook and are labelled accordingly. Substorms — the bursts that actually paint the sky — are not predictable hours ahead by anyone. Treat a high score as "be outside and looking", not a guarantee, and a low score as "sleep well", not a ban.

Data sources

Kp forecast and 27-day outlook: NOAA Space Weather Prediction Center. Historical Kp: GFZ German Research Centre for Geosciences. Cloud and weather, current and historical: Open-Meteo. Sun and moon positions are computed astronomically.