United States

Anchorage is not a bad sky in absolute terms: 36.4% would put it ahead of every European base except Kiruna. What it lacks is position. At 61°13′N the city sits south of the auroral oval on a quiet night, so seeing anything usually needs the oval to expand, which happens when geomagnetic activity climbs. Fairbanks needs no such help.

So the practical consequence is one sentence: if the northern lights are why you are flying to Alaska, keep going north. Fairbanks is 45 minutes by air from Anchorage or twelve hours on the Aurora Winter Train, and it changes your odds more than any choice of tour, operator or price bracket could.

Two things decide how much aurora time a month contains: how often the sky is usable, and how many hours of real darkness there are to use it in. Multiplied together, Alaska produces a season that is the opposite of the European one, and the two American bases do not even agree with each other.

Fairbanks peaks in January and Anchorage in November, and the month Fairbanks is best is the month Anchorage is second worst. Five hundred and seventy kilometres apart, and no shared calendar.

The Fairbanks column is the striking one: 4.01 expected usable dark hours in January is the highest figure of any month at any of the fourteen destinations we measure, and its worst month, September at 1.36, still beats Tromsø's best. That is what dry continental air under a favourable magnetic latitude produces, and it is why the advice to avoid midwinter, which is sound in Scandinavia, is simply wrong here.

September is the weakest month in Alaska as it is everywhere, though by the narrowest margin in this catalogue at Anchorage, which as the southernmost base has the most September darkness of anywhere: 3.8 hours.

One limit, stated because it argues against this ranking: 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, favouring September and March. At Fairbanks that does not come close to overturning January; at Anchorage, where the oval also has to expand south to reach you, March is probably better than fourth place suggests.

Six kinds of night are sold here, and reading them as answers to problems rather than as products makes the choice much easier.

Lodges and glass cabins solve endurance. All of them are on high ground, which around Fairbanks means warmer than the town, because cold air pools in the valley and a summit 700 metres up can be ten to twenty degrees milder. They are also the cheapest thing in Alaska, from 87 EUR, and the format whose best month is unambiguously January, because a heated room is the direct answer to the only argument against coming then.

Road hunts solve cloud, and five of them also sell the Dalton Highway, a 1974 pipeline haul road running 414 miles to the Arctic Ocean. Ice fishing solves the waiting. The hot springs at Chena solve nothing at all and are worth doing anyway. Photography solves the picture, and unusually all five of those departures include the photographs free. Husky sledding solves the daylight hours, and the dogs work best in exactly the cold that makes the sky clear.

An aurora display does not brighten gradually. It sits as a quiet arc for an hour or two, then breaks up in the space of a minute, floods westward, fills the sky and fades. That sequence, the auroral substorm, was first described from Fairbanks in 1964, and it is the single most useful thing to know before an Alaskan night.

It explains why guides here will not promise an end time, why the good operators keep going past midnight, and why the format matters more than the price. It is also why the most common way to miss the aurora in Alaska is not cloud, which on half the nights of the year is not the problem. It is deciding at eleven o'clock that nothing is going to happen.