The Kp your latitude needs

Most aurora forecasts give one number for a whole country. The number that matters is the one for your own horizon, and between Riga and Tromsø it changes by six full storm levels. Here are 12 places, each one computed rather than looked up.

Sorted by the Kp needed. “Dark nights” counts nights when that Kp is reached and the sky at that latitude can reach astronomical darkness — clear weather not included, because nobody can promise that.
Place	Geomag. lat.	Kp needed	Kp days / yr	Dark nights / yr
Tromsø	67.5°	—	365	191
Murmansk	65.0°	0.5	361	191
Rovaniemi	63.7°	1.2	333	188
Oulu	62.3°	1.8	283	170
Oslo	59.7°	3.1	148	104
Tampere	59.2°	3.4	118	79
Stockholm	58.0°	4.0	90	64
Helsinki	57.8°	4.1	68	47
Tallinn	57.1°	4.4	50	35
Narva	56.6°	4.7	36	25
Tartu	55.8°	5.1	26	18
Riga	54.8°	5.6	18	13

Why one number cannot serve a whole country

The oval sits near 66° magnetic latitude when the field is quiet and moves about 2° toward the equator for every step of Kp. That is a rule with a place in it: it says nothing at all until you say where you are standing. Between Tromsø and Riga there are more than twelve degrees of magnetic latitude, which is six Kp steps — the difference between “any clear night” and “a severe storm”.

Why these night counts are lower than the ones you have seen

Almost every aurora page quotes NOAA's storm scale: a Kp 5 storm on 900 days per solar cycle, about 82 days a year. We checked it against the Kp record before repeating it, and it does not describe the sky we currently have. Cycles 18 to 22 delivered 73–85 such days a year and NOAA's figure matches them exactly. Cycle 23 delivered 48, cycle 24 delivered 25 — the weakest in the record — and cycle 25 has managed 34 so far. Every number in the table is measured over the last 30 years instead.

Darkness is the other half

Far north, the Kp is not the limiting factor for months at a time: the sky simply never gets dark. Rovaniemi loses a third of its year to that and Riga barely a quarter, so two places with similar storm counts can offer very different numbers of usable nights. The last column is the only one that accounts for it.

Your own horizon, not the nearest city

A list of cities is a compromise. SkyFlux does the same calculation for the exact spot you are standing on, against the live Kp forecast, and adds the two things a threshold cannot tell you: when it gets dark tonight, and whether the cloud will let you see anything. No account, no ads, and it all runs on the phone.

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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°.