Dark hours and clear skies, by base and month

Two numbers per base per month, and both of them ours. How many hours the sun spends more than twelve degrees below the horizon, and how often an observer at that base's own weather station actually saw a clear sky in those hours.

Northern Lights Index editorial team

Source and method.

A chase needs two things from the sky before anything else can go right: it has to be dark, and it has to be clear. Both vary by base and by month in ways that no booking page will tell you, and both can be worked out without anyone's permission.

Dark hours are arithmetic. The sun's position is computed from the date and the station's coordinates, and the figure is the mean number of hours per day with the sun below minus twelve degrees, the point at which the sky is dark enough for a faint aurora to read against it. The clear-sky share is observed: of the hourly observations that fall inside those dark hours, the share where the observer at that station recorded two eighths of cloud or less. Ten years, 2015 to 2024.

What this is not is a forecast, and it is not a probability of seeing the aurora. Darkness and a clear sky are necessary for a sighting. Solar activity, which this page says nothing about, decides whether there is anything to see once you have them.