Aurora forecast, Kp index and clouds
Every evening in Tromsø, thousands of people stare at aurora apps. Most of them look at the wrong number. This far north the cloud map matters more than the Kp index, and the guides read both.
Two forecasts, not one
To see the aurora you need two things at once: activity in the upper atmosphere, and a clear sky between you and it. Aurora forecasts predict the first. Weather forecasts predict the second. In Tromsø, the second is almost always the harder one.
What the Kp index means
The Kp index is a scale from 0 to 9 that measures how disturbed the Earth's magnetic field is. The higher the number, the stronger the aurora and the farther south it can be seen. Southern Norway, Scotland or the northern United States may need Kp 5 or more. Tromsø is different, because it lies right under the auroral oval, the ring where the lights are most frequent, so even a Kp of 1 to 3 can bring a good display overhead. A high Kp is welcome, but a low one is no reason to stay in.
Very high values can even push the strongest aurora south of Tromsø, so that you see it on the southern horizon rather than straight above.
Why the cloud map matters more
A strong aurora behind a thick layer of cloud is invisible. Tromsø sits on the coast, where clouds and showers come in from the sea, while the inland valleys toward Sweden and Finland are often drier and clearer. That is the whole idea behind a chase. Our pick, Snow Travels, uses real-time satellite data and more than 40 live cameras across the region, and Arctic Glow follows webcams and weather data from across the north all evening. They look for gaps, not for Kp, and on a cloudy night that can mean a long drive into Finland.
Let the guide read the clouds
Snow Travels backs every chase with satellite data, live cameras and an operations team in town, and sets no fixed limit on the distance. From ~$168.
How to read the forecast yourself
- Check the cloud forecast first. The Norwegian weather service yr.no shows cloud cover by hour and by altitude. Low, middle and high cloud all matter.
- Then the aurora. Most apps show a Kp forecast for the next hours and days, and some show live data from satellites measuring the solar wind.
- Look at the solar wind if you want more. A southward magnetic field, often shown as negative Bz, tends to switch the aurora on. It changes within minutes, which is why forecasts are rough.
- Do not trust a three-day aurora forecast too much. They are rough guesses. The weather forecast a day ahead is more useful for planning.
When to cancel, and when not to
All six tours on our page offer free cancellation up to 24 hours before. If the cloud forecast for the whole region, coast and inland, is solid for your night, consider moving your booking to another evening. If only Tromsø is cloudy, keep it, because that is exactly the night a chase earns its price. The big bus with free repeats is the most forgiving if you have several nights. What each tour does if you see nothing.
Boat and camp: less room to move
The reindeer camp always goes to the same place, and the electric boat can only pick among the fjords near town. For both, the local cloud forecast for the evening tells you most of what you need to know. Chase or camp compares them.
The forecast is a guide, not a promise
Even good forecasts miss. Aurora can flare on a night rated quiet, and fade on a night everyone expected a storm. The guides keep it simple. Get out of the city lights, and be patient. The best months and hours are in our season guide.