1. Solar activity — the Kp index
The Sun constantly sends charged particles toward Earth. When they hit our magnetic field they light up the sky near the poles — that's the aurora. Scientists measure how disturbed the magnetic field is on a 0-to-9 scale called Kp, updated every 3 hours.
- Low Kp (0–2): the aurora sits in a ring right around the magnetic poles. You'll only see it from far-north places like Tromsø, Fairbanks or Yellowknife.
- Medium Kp (3–5): the ring widens toward the equator — now reachable from southern Norway, Iceland, much of Canada.
- High Kp (6–9): a real storm. The lights can reach the northern US, the UK, even mid-latitudes.
NOAA publishes a 3-day Kp forecast. We compare it to each destination's own threshold — because a Kp 4 night that's spectacular in Scotland is just an average night in Tromsø.
2. Darkness — you need a dark sky
The aurora is always there, but you can only see it against a dark sky. In high-latitude summer the sun barely sets — the "midnight sun" — so even a huge storm is invisible. That's why the season runs roughly late September to early April up north (and the opposite half of the year for the southern lights). When a destination is in its bright season, we say so plainly: out of season.
3. Clear skies — the dealbreaker
The aurora glows about 100 km up, far above the clouds. An overcast sky hides it completely, no matter how strong the storm. Cloud cover is the most common reason a promising night fizzles — so our forecast weights it heavily, using hourly cloud predictions for each exact location.
How we turn that into a score
For every destination we take the forecast Kp, compare it to that place's visibility threshold, weight it by the cloud forecast over the dark-hours window, and rule out any night with no real darkness. The result is a 0–100 score and a plain label: Low, Fair, Good, or Excellent. It's a probability, not a promise — see our full methodology.