Northern lights tonight: sky and activity for twelve places

What each card shows

Each card has four parts for one place. First is tonight's dark window, the hours when the sky is dark enough. Second is the forecast cloud cover for those hours, from MET Norway. Third is the forecast planetary Kp for the same hours, from NOAA's Space Weather Prediction Center. Last is a verdict in plain language that sums up the two.

Cloud and darkness decide whether there is any sky to look at. Kp is the rough guide to how active the aurora may be.

Why Kp is only a rough guide

Kp is a planet-wide index of magnetic disturbance, on a scale of 0 to 9, issued in three-hour blocks. It is one number for the whole Earth, so it blurs what is happening above your head. NOAA says Kp 0 to 2 means the aurora is far north and quite dim, and that Kp 3 to 5 moves it further from the poles and makes it brighter.

Most of the twelve places sit at or near the usual auroral oval, so a low Kp is normally enough there. Whitehorse, Rovaniemi and Luleå are a little further south in magnetic terms and typically need about Kp 3. Our reading is that cloud matters more than a few points of Kp. For what each value means, read the Kp guide.

Refresh, and what to check before you go

The cards refresh about every 30 minutes. A forecast made hours ahead is a guess about the sky, so use the local official service linked on each card in the last hours before going out. One example is NOAA's OVATION forecast, which gives a 30 to 90 minute outlook of the location and intensity of the aurora.

Reading a card out of season

In late spring and summer there is little or no real darkness at these latitudes, so a card may show a short window or none. Midnight sun runs from 19 May to 27 July in Tromsø and from 7 June to 6 July in Rovaniemi. For planning beyond tonight, use when to go for the month, where to go for the place, and the nights calculator for how long to stay.