Northern lights tonight

Type your town or use your location in the checker on this page. It reads NOAA's Kp forecast for your hours of darkness tonight and your geomagnetic latitude, and tells you whether the aurora is likely overhead, possible low on your poleward horizon, or unlikely. Clouds are the other half: you need a clear, dark sky. (NOAA SWPC, NOAA Space Weather Prediction Center)

Kp measures how disturbed the Earth's magnetic field is over each three hours, on a scale from 0 to 9. NOAA publishes it observed for the past week, estimated for today and predicted three days ahead. The higher it goes, the further the aurora moves from the poles. (NOAA Space Weather Prediction Center, NOAA SWPC)

NOAA's rule of thumb puts the auroral oval's equatorward edge at about 66° geomagnetic latitude at Kp 0, moving about 2° toward the equator for each step of Kp, to 48° at Kp 9. The aurora can also be seen hundreds of kilometres equatorward of where it is. We count it as possible low on your horizon from 500 km beyond the edge. NOAA calls the relation approximate and an average. (NOAA Space Weather Prediction Center)

The oval is centred on the Earth's magnetic pole, which sits off the geographic pole toward Canada, so the same latitude is closer to the aurora in North America than in Europe. We use corrected geomagnetic latitude (AACGM-v2), which follows the real magnetic field: Edinburgh is at about 53° geomagnetic, Chicago at about 51°, though Chicago is 14° further south on the map. (Shepherd, S. G. (2014), NOAA Space Weather Prediction Center)

It is NOAA's OVATION nowcast, a model of the aurora's intensity driven by the solar wind measured 1.6 million km away at L1, giving a 30 to 90 minute forecast. NOAA turns intensity into a viewing probability by assuming a linear relation. The red line is where, on NOAA's Kp rule, the aurora could be seen low on the horizon right now. (NOAA SWPC)

NOAA: usually within an hour or two of midnight, between 10 PM and 2 AM local time, with the active hours widening into the evening and morning when activity is high. It must be properly dark: high latitudes get little or no darkness in summer. (NOAA Space Weather Prediction Center)

A camera collects light for seconds at a time. The Met Office notes that the aurora is often portrayed much brighter in pictures than it appears to the naked eye. (Met Office)

Once a day we check NOAA's 3-day Kp forecast against your place's geomagnetic latitude and its hours of darkness, and email you when the aurora is possible there that night. They are a daily forecast check: they cannot catch a storm that arrives faster than the forecast, so on an active night keep an eye on this page too. (NOAA SWPC)