Reykjavík

December is the best month to hunt the aurora from Reykjavík, and that sentence is worth pausing on, because in Tromsø December is the worst month of the year. This is the clearest case in our data of advice that does not travel. Iceland's cloud record barely moves across the season, so the darkness peak in midwinter arrives without a penalty attached, while in Norway the same month brings eight hours of dark and a sky that is shut five nights in six.

September is the weakest month here, in spite of having the best cloud figure of the year at 40.7%. There are 2.7 hours of astronomical darkness in a September night at this latitude, against 6.9 in December. A three-hour tour leaving at nine in September spends much of its evening in residual twilight.

The flatness of the first column is still the thing that distinguishes Reykjavík: 31.1% at worst and 40.7% at best, a range of nine points across the entire season, where Tromsø swings seventeen and Rovaniemi thirty. There is no month here where the sky lets you down systematically. What varies is how much dark you get to use it in.

One limit on that ranking, stated because it argues against it. It measures how much dark clear sky a month contains, not the chance the aurora does something inside it. Geomagnetic storms are close to twice as frequent around the equinoxes, an effect described by Russell and McPherron in 1973, which favours September and March. Even doubling September leaves it behind December here.

The median shared seat across all of these is 141 EUR, second cheapest of the nine destinations where we have enough departures to calculate a median at all.

That last row deserves attention, with one qualification. More nights is the single most effective thing anyone can do, but only if the nights are genuinely independent: a weather system parked over the southwest makes three nights in Reykjavík into one night repeated. The multi-day routes that move you 200 km down the south coast are buying separation as well as scenery.

Reykjavík burns almost no fuel to keep warm. Since 1930, when hot water was first piped from the washing springs at Þvottalaugar to a school in the east of town, the city has run on geothermal district heating, and today roughly nine in ten Icelandic homes are heated the same way. The visible consequence on a winter night is an absence: no chimney haze, no coal smoke, nothing between you and the sky except whatever the Atlantic has sent over.

What the city does put in the way is its own light. From Grótta lighthouse on Seltjarnarnes, or from the slope of Öskjuhlíð, a strong display is perfectly visible without leaving the built-up area. A weak one is not, and weak ones outnumber strong ones heavily. That gap is what you are buying a seat to escape: twenty to sixty minutes of driving usually buys two magnitudes of darkness.

On 28 September 2016 the city switched off the street lights across several neighbourhoods for about an hour, after the forecast called for an unusually strong display, and asked residents to turn off their own lights too. Drivers were warned in advance. It is the sort of thing a city of 140,000 can decide to do and a city of eight million cannot, and it says more about how Icelanders treat the aurora than any brochure line: not as an attraction, as weather worth stopping for.

One number worth knowing before you read any forecast: from Reykjavík you do not need a geomagnetic storm. A Kp of 2 or 3 puts the auroral oval overhead. The dramatic Kp 6 and 7 nights that make the news are what lets people in Scotland and Germany see it, not what is required here. If someone tells you activity is too low for Iceland, check the cloud map instead.