SkyFlux
Northern lights in Stockholm
Stockholm lies at geomagnetic latitude 58.0°. The aurora's oval has to reach that far south before anything appears above the northern horizon here — and that takes a Kp of about 4.0.
All three are ceilings, not promises. The night count says the Kp was reached at some point in those 24 hours — the peak still has to fall in your dark hours, and the sky still has to be clear. Halve it for cloud and you are nearer the truth.
Why 58.0° and not 59.33°
The aurora answers to the Earth's magnetic field, and the magnetic pole sits in northern Canada rather than at the geographic one. That tilt is worth several degrees: Stockholm is at 59.33°N geographically and 58.0° magnetically, and it is the second number the oval reaches down to. The figure here is the standard dipole approximation. Measured against stations whose corrected geomagnetic latitude is published, it reads about 0.6° too far poleward — roughly a third of a Kp step, and it errs on the optimistic side. We would rather write that down than round it away.
How often that actually happens
Over the last 30 years — solar cycles 23, 24 and 25 so far — the planetary Kp index reached 4.0 on about 90 days a year, which is 25% of all days. At this latitude 64 of them fall in a part of the year when the sky can go astronomically dark at all.
That figure is measured rather than quoted. NOAA's published storm scale puts a Kp 5 storm at 900 days per solar cycle, about 82 days a year, and that is right for the cycles it was drawn from. It is not right for ours: the record gives 48 days a year for cycle 23, 25 for cycle 24 — the weakest on record — and 34 for cycle 25 so far. Quoting the table would have inflated every number on this page two to three times over.
The other half of the answer is darkness
A storm you cannot see is not an event. Here there are 118 nights a year, from April 24 to August 19, when the sun never gets 18° below the horizon and the sky never reaches astronomical night. Aurora in those weeks has to be far brighter than these numbers assume to show through the twilight — which is why the count above is the two measurements multiplied together, not the Kp figure on its own.
What the app does with this
SkyFlux runs the same calculation for wherever you are actually standing, not for the nearest city on a list, and puts the live Kp forecast against it. It also tells you when the sky goes dark tonight, what the cloud is doing hour by hour, and where the nearest genuinely dark ground is. No account, no ads, and the arithmetic runs on the phone.
Other places
Kp index: GFZ Helmholtz Centre for Geosciences, Geomagnetic Observatory Niemegk, CC BY 4.0 (Matzka et al. 2021). Storm scale and the 66°/2° rule: NOAA Space Weather Prediction Center. Geomagnetic latitude: dipole approximation. Darkness: solar altitude at lower culmination, −18°.