Which spot tonight: how we score spots, and how to choose
The app’s 0–100 scores are there to make choosing a place easier. They compare cloud forecasts, auroral activity and a few practical factors, so you can start with a manageable shortlist. The numbers are planning judgements, not odds: a score of 80 does not mean an 80 percent chance of seeing aurora.
What goes into the number?
We start with MET Norway’s cloud forecasts for each place and NOAA’s planetary Kp, which describes magnetic activity across a much wider area. Every spot goes through the same calculation. This is the method behind our upcoming free iPhone app. MET’s location forecasts, NOAA’s Kp product
Clouds carry more weight: 70 percent of the initial calculation, compared with 30 percent for a Kp factor adjusted for this region. We chose those weights. Where all three cloud layers are available, the calculation treats low, middle and high cloud differently. Otherwise it uses total cloud cover.
The calculation also accounts for light pollution, distance, darkness and moonlight. The distance adjustment is a rough allowance for longer trips; it doesn’t know your actual driving route, ferry wait or walk from the car. Bright-sky hours lose points. The moon’s estimated brightness and height can also reduce a score, by up to 15 points.
Read the window as well as the score
The app looks at groups of three hourly forecasts and chooses the group with the highest average score. The number shown beside the spot comes from the best hour within that group, rounded to a whole number. It may describe a later hour, so check the window before putting your coat on.
For example, three forecast samples at 21:00, 22:00 and 23:00 appear as a 21:00–23:00 window. That’s a stretch of promising forecast conditions to plan around. Aurora doesn’t follow those start and finish times. If fewer than three hours are available, the app uses that shorter stretch.
Why a cloudy night gets a low score
If total cloud cover at the selected best hour is above 80 percent, we cap the score at 20. The idea is to keep a strong Kp forecast from making a heavily overcast evening look inviting.
This rule is deliberately cautious. It uses total cloud cover even when much of that cover is high cloud, which the earlier part of the calculation treats more generously. The weights and limits are choices we’ve made for planning; they haven’t been calibrated into sighting probabilities.
Make the final choice your own
Compare Prestvannet and Telegrafbukta with Ersfjordbotn. Is the forecast different enough to make the longer outing worthwhile? Can you arrive while the opening is still useful? Which place fits the evening you want?
Check the app’s update time and any data-quality message, then use tonight’s checklist to sort out the journey. If one unusually clear hour seems to explain a high ranking, the cloud guide helps you decide how much to read into it.
Check how fresh the information is
The update time tells you when the results were put together, but there’s one more thing to read: the data-quality message. If a weather or activity source is unavailable, the app can use fallback inputs or retain earlier information. A recently produced result can therefore include older or substitute data.
That keeps the app usable during an interruption, but it’s a reason to check another current source before planning a journey. Yesterday’s ranking may no longer fit the sky outside.
We’re building Aurora Tromsø Northern Lights as a completely free iPhone app. It helps you compare the evening’s options; you bring the local access checks and the decision about where you’d like to spend your time.
Frequently asked questions
Is a score of 90 a guarantee?
No. It means the forecast inputs have produced a high planning score under our chosen weights. Local clouds, obstructions and changes in auroral activity can still affect what you see. Think of it as a promising option to investigate.
Should I always choose the highest-ranked spot?
You don’t have to. A slightly lower-scoring place may be easier to reach, leave you more time outside or make the return simpler. Use the ranking to narrow the choice, then pick the outing that works for you.