Reading cloud forecasts for aurora hunting around Tromsø

Before heading out for the northern lights, look at the hourly clouds for a few places you can reach. You’re looking for an opening that lasts through some of the dark hours you have free. Start with that simple question, then use the cloud layers to understand the forecast a little better.

What does the percentage mean?

MET’s total cloud-cover figure includes clouds at all heights. The separate layers describe low cloud below 2,000 metres, middle cloud between 2,000 and 5,000 metres, and high cloud above 5,000 metres, measured above the ground. These are forecasts of cloud coverage. MET’s cloud definitions

If you see 30 percent cloud cover, that isn’t a 70 percent chance of seeing aurora. It also won’t tell you exactly where the gap will be from the place you’re standing. And there’s no need to add the layers together: clouds at different heights can sit above one another.

Look beyond the little weather icon

A dry forecast is a good start for an evening outdoors, but it can still be cloudy. MET lists precipitation and cloud cover separately, so check both. A night with no rain or snow forecast may still have a covered sky. MET’s forecast variables

In the app’s scoring method, low cloud has a stronger blocking weight than high cloud. Those weights are our modelling choices; they don’t measure how transparent a particular cloud will be when you arrive. The scoring guide explains this, including the limit we apply to heavily overcast forecasts.

Compare places you’d actually like to visit

You could start with the waterfront at Telegrafbukta, the lake setting of Prestvannet, and the fjord at Ersfjordbotn. The two city spots may have similar forecasts. Check the fjord separately to see whether it offers a useful alternative.

You’re choosing between them, so there’s no need to turn the list into a three-stop outing. Work through it like this:

1. Look at the same hours for each place. Include the time you’ll need to get there.
2. Check either side of the clearest hour. A longer opening gives you more room than one promising value squeezed between cloudy ones.
3. Keep the places with a suitable stopping point and a return journey you’re comfortable with.
4. Have another look just before you leave.

We’re building the free iPhone app to bring this forecast comparison together. Our evening checklist helps with the parts the weather can’t decide, such as getting there and home again.

Keep tomorrow’s plan flexible

Forecasts change, and the uncertainty isn’t the same every day. Yr explains this in its uncertainty guide. MET also describes how models need corrections for local features such as coastlines and inversions. A forecast is a useful view of the weather ahead, with some details still to be resolved. Yr’s uncertainty guide, MET’s model documentation

Our suggestion is to keep the exact spot and hour open until you’re within a day of the outing. That’s a planning habit, not an expiry time for the forecast. Earlier in the week, look for possibilities. On the day, narrow them down with fresher information.

If you get there and the sky looks different, trust what’s above you. Check the forecast’s update time and any nearby observations before deciding to move. You may need to wait, change the plan or call it an evening. There’s no benefit in chasing a clear patch that exists only on an old forecast.

Frequently asked questions

Can I see aurora through partly cloudy skies?

Yes. Keep watching the openings; the whole sky doesn’t have to clear. UiT explains that aurora originates above the clouds, which is why a gap can give you a view even while other parts of the sky remain covered. UiT’s cloud and aurora advice

Should I always drive inland when Tromsø is cloudy?

Check the forecast first. Inland is only a useful alternative if conditions look better there during hours you can reach and enjoy. A place’s reputation for dry weather isn’t enough to choose it for this particular night.