Northern lights in Sweden

The Kp index measures how disturbed the Earth's magnetic field is, from 0 (quiet) to 9 (extreme storm), in three-hour steps. The stronger the disturbance, the further the auroral oval — the ring of light around each magnetic pole — spreads towards the equator.

What counts is the magnetic, not the geographic, latitude. The magnetic pole lies over northern Canada, so North America sees the lights much further south than Europe or Asia at the same latitude. For every place we compute its magnetic latitude and from it the Kp at which the edge of the oval appears low on the horizon and the Kp at which it is overhead.

You also need true darkness (the sun at least 12° below the horizon — around midsummer there is none in the far north), a clear sky and ideally no bright moon. The monthly odds on each page combine thirty years of measured Kp during the dark hours with the place's usual cloud cover.