Cloud cover and aurora: how to judge a clear sky
What total cloud cover means in MET Norway forecasts, how percent maps to oktas, and how Aurorix decides that a sky is clear enough.
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Aktualisiert am 17 October 2026
Cloud is the usual spoiler
Aurora happens 100 km and more above the weather, so any cloud between you and the sky blocks it. A strong show under overcast is invisible and a weak show under a clear sky is a good evening. For this reason Aurorix puts cloud cover next to the Kp and the OVATION value.
What the forecast number is
Aurorix reads cloud_area_fraction from MET Norway's Locationforecast 2.0. It is the percentage of the sky covered by cloud at the forecast time for the place coordinates, all cloud layers combined.
Meteorologists traditionally report the same quantity in oktas, eighths of the sky. One okta is 12.5 percent. The table below shows how Aurorix groups the percentages.
The series in the snapshot is hourly for the first two and a half days and then steps to every six hours. Aurorix holds a value until the next one arrives and shows no data across a gap longer than seven hours, rather than guessing what happens in between.
The Aurorix rule
Cloud cover of 40 percent or less counts as clear enough, a little over three oktas. Between 40 and 75 percent the sky is called partly cloudy and above 75 percent mostly cloudy. The cut-off is our own editorial choice: it is loose enough that a forecast of scattered cloud over a mountain valley does not rule out an evening that could have worked.
In the hourly strip, an hour that is dark and above the Kp threshold but over 40 percent cloud is marked "dark but cloudy". Its cell uses a hatched pattern as well as colour, so it can be told apart without relying on colour.
What the average hides
- Where the gaps are. A sky that is 60 percent covered with the clear part to the north can be better than one that is 30 percent covered by a bank of cloud on the northern horizon.
- Thin cloud. High, thin cloud dims stars but bright aurora can still show through.
- Distance. Forecasts are for the place centre. A valley, a coast or a lake 30 to 100 km away can have a very different sky, and that is why aurora trips often involve a drive.
- Timing. Cloud forecasts are updated more often than the Aurorix snapshot. Check a live satellite or weather radar view before you drive anywhere.
Licence and credit
MET Norway publishes its open data under the CC BY 4.0 licence and asks that users credit the source and identify themselves in requests. Aurorix credits MET Norway in the footer of every page and identifies itself in the User-Agent of the download job.
How Aurorix reads cloud cover
| Cloud cover | Label | In the hourly strip |
|---|---|---|
| 0 to 40 % | Mostly clear | Can count as a good hour |
| 41 to 75 % | Partly cloudy | Dark but cloudy |
| 76 to 100 % | Mostly cloudy | Dark but cloudy |
The 40 % limit is our own rule of thumb, not a NOAA or MET Norway value. Cloud cover is a percentage of the sky; a gap in the cloud can still show aurora.