Explainer
The Kp index, in plain English
Kp is a single number from 0 to 9 that says how disturbed Earth’s magnetic field is. The more disturbed it is, the further the aurora spreads from the poles. Try it below — the sky behind the page follows your choice.
Try each level
Pick a Kp value to preview the sky and see what it usually means.
What Kp measures
The planetary K-index summarises geomagnetic disturbance over a three-hour period, using readings from magnetometers at observatories around the world. It was introduced by the German geophysicist Julius Bartels in the 1930s and the planetary version has been used since 1949. The scale is roughly logarithmic: each step up means a considerably bigger disturbance, not a slightly bigger one.
The disturbance comes from the solar wind, the stream of charged particles flowing from the Sun. When the wind is fast, dense, or carries a magnetic field pointing south (the technical term is a negative Bz), energy pours into Earth’s magnetic environment and the aurora brightens and expands.
What Kp does not tell you
It doesn’t measure brightness directly. It is a global average over three hours; a sharp, short burst (a substorm) can light up the sky while Kp barely moves.
It doesn’t account for where you are. The same Kp means very different things in Tromsø and in Edinburgh. The aurora forms an oval around the magnetic pole; Kp changes the oval’s size. Tromsø is already close to the middle of it, so Kp 1–2 can be plenty, while a site far south needs Kp 5 or more.
It can’t see through clouds. The most common reason for a missed aurora in northern Norway isn’t low Kp, it’s cloud. That’s why our verdict puts clear sky first.
The NOAA storm scale
NOAA labels geomagnetic storms G1 to G5, matching Kp 5 to 9.
| Kp | Label |
|---|---|
| 5 | Minor storm · G1 |
| 6 | Moderate storm · G2 |
| 7 | Strong storm · G3 |
| 8 | Severe storm · G4 |
| 9 | Extreme storm · G5 |
Live right now
The most recent Kp value from NOAA’s Space Weather Prediction Center is –. We refresh it hourly.
How we use Kp
Our nightly verdict is a weighted score: 65% clear sky and 35% geomagnetic activity, with a small penalty on bright-moon nights.
- We take the highest Kp forecast in the evening window and cap it at 5 for scoring, so one big number doesn’t outweigh a cloud-covered sky.
- NOAA’s forecast reaches about three days out. Beyond that we assume a typical Kp of 2 and mark it as an estimate.
- Verdicts: GO at 65 and above, MAYBE from 40 to 64, SKIP below 40.
At Arctic latitudes, a clear sky at Kp 2 beats a cloudy one at Kp 6.
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