Understanding the aurora forecast – Kp index, Bz and clouds

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In short: Three things count for an aurora night, in this order: clouds, darkness and geomagnetic activity. The Kp index (0–9) is a rough measure of global activity; in the far north – Tromsø, Lofoten, Lapland, Iceland – low values are often enough. In the short term, real-time solar wind data (especially a negative Bz) and local magnetometers matter more. As of October 2026.

What the Kp index measures

The Kp index describes how disturbed Earth’s magnetic field is within a three-hour window. It is calculated from measurements at several magnetometer stations around the world and runs from 0 (quiet) to 9 (extreme storm). The scale is not linear: a step from Kp 4 to Kp 5 means considerably more disturbance than a step from 1 to 2.

The US National Oceanic and Atmospheric Administration (NOAA) translates high values into a storm scale: G1 corresponds to Kp 5, G2 to Kp 6, G3 to Kp 7, G4 to Kp 8 and G5 to Kp 9. Storms like these are the nights when auroras can be seen far to the south – as far as Central Europe.

Which Kp is enough where?

Auroras appear mainly in a ring-shaped band around the magnetic north pole, the auroral oval. The higher the activity, the further south the oval expands.

Important: the Kp index is a global three-hour average. It does not tell you whether the sky right above you will light up in the next 20 minutes. That is why real-time data are worth a look.

Solar wind and Bz: the real-time values

Satellites between Earth and Sun measure the solar wind before it reaches Earth – depending on its speed, the warning time is roughly half an hour to an hour. Three values are especially telling for auroras:

• Bz (north–south component of the interplanetary magnetic field): if Bz stays negative (southward) for a while, energy can enter Earth’s magnetic field more easily. This is the most important signal for flaring activity.
• Solar wind speed: higher values usually mean more energy.
• Density: more particles amplify the effect when Bz points south.

No single value is a guarantee. But a Bz that stays negative for a longer period combined with increased speed is the moment to put your jacket on.

Rules of thumb for real-time values

The following values are rough rules of thumb, not physical limits. They help you interpret NOAA satellite data:

In the far north the sky can light up even with „rather quiet“ values – especially when a so-called substorm begins: energy stored in Earth’s magnetic tail is suddenly released, and a quiet arc turns bright and lively within minutes. Local magnetometers often announce these moments better than the Kp index.

The best sources

• Clouds first: for Norway yr.no (Norwegian Meteorological Institute with NRK), for Finland the FMI weather service, for Iceland the cloud and aurora map on vedur.is.
• 30-minute forecast: the Ovation model from NOAA SWPC shows the expected position and strength of the oval.
• 3-day and 27-day outlook: NOAA publishes Kp forecasts for the coming days and a 27-day outlook. Because the Sun rotates roughly once every 27 days, active regions (e.g. coronal holes) can recur – a rough hint, not a basis for planning.
• Local magnetometers: the Auroras Now! service of the Finnish Meteorological Institute shows whether the magnetic field over Lapland is currently disturbed.
• Alert apps bundle these data. They are handy but no substitute for looking at the cloud map.

How to plan an aurora night

1. In the afternoon: check the cloud forecast for the coming hours. Where will it be clear – on the coast or inland?
2. Early evening: look at the Kp forecast and solar wind. If NOAA expects increased activity, head out early.
3. Choose a spot: dark, clear view north, safe parking – never stop on the road.
4. Outside: wait warmly dressed, activate your alert app, look north regularly. Activity often comes in bursts of 15–30 minutes.
5. If it is cloudy: look for gaps – a drive inland or to the other side of a fjord often helps.

When in the year your chances are best is covered in Best time for northern lights in Norway – month by month; how to capture the result on camera in Photographing the northern lights with phone and camera – settings that work.

Common mistakes when reading forecasts

• Treating a multi-day Kp forecast as a promise: forecasts several days out are based on solar observations and are uncertain. A predicted storm may not come – and a quiet forecast can bring a beautiful night.
• Reading Kp the wrong way round: in the far north a very high Kp is not automatically better. During big storms the oval shifts south, and the brightest forms may then be south of you. For Tromsø, Lofoten and Lapland a moderate value is usually enough.
• Relying only on apps: alerts are often delayed or calculated for another location. Look north regularly yourself.
• Ignoring clouds: the most important map is the cloud forecast. A glance upwards beats any number.
• Giving up too early: activity comes in bursts. After a quiet hour the sky can suddenly erupt.
• Forgetting the moon and town lights: at full moon and in lit places faint auroras disappear.

Checklist for the evening

1. Cloud map for the next 6 hours checked
2. NOAA 30-minute forecast and solar wind checked
3. Dark spot facing north with safe parking chosen
4. Warm clothes, thermos, head torch with red light packed – see Packing list for −30 °C and the polar night – also with kids
5. Camera ready: tripod, focus at infinity, spare battery

Frequently asked questions