Aurora forecast FAQ: Kp, OVATION, cloud and darkness

Kp is NOAA's planet-wide measure of geomagnetic activity, from 0 to 9. NOAA describes Kp 0 to 2 as dim aurora far north, Kp 3 to 5 as brighter aurora that moves further from the poles, Kp 6 to 7 as bright and active aurora that can reach the northern edge of the United States, and Kp 8 to 9 as very bright aurora seen far toward the equator. The higher the Kp, the further south the auroral oval reaches.

Not exactly. It is NOAA's modelled aurora intensity for the one-degree grid cell at your place, read as a viewing probability for the sky overhead. It looks 30 to 90 minutes ahead. It knows nothing about clouds, moonlight or daylight, so it can be high at noon and low under a clear night sky that still shows a faint glow on the horizon. Use it together with the cloud and darkness figures.

Four things can get in the way: the sun is up, cloud covers the sky, the place is too far south for that Kp, or the activity ended before it got dark where you are. Kp is a three-hour index for the whole planet, so it can be high in the hours when your sky was bright or overcast. The hourly strip checks darkness, cloud and Kp together for this reason.

It depends on geomagnetic latitude. NOAA puts the equatorward edge of the oval near 66 degrees geomagnetic latitude at Kp 0 and moves it about 2 degrees south per Kp step. Aurorix converts each place with the 2026 IGRF pole and shows the first whole Kp at which the edge reaches it. Each destination page has the figure and the arithmetic. Treat it as a guide: the dipole conversion can be off by up to two Kp steps in some regions.

The shortest forecast is NOAA's OVATION model, which looks 30 to 90 minutes ahead because that is how long the solar wind takes to travel from the L1 monitoring point to Earth. NOAA also publishes a 3-day Kp forecast and a 27-day outlook of geomagnetic indices. Skill falls quickly with lead time: a Kp forecast for tomorrow is a guide, not a promise, and a bright event from a solar eruption can only be seen coming roughly an hour in advance at L1.

Cloud cover of 40 percent or less. That is our own rule of thumb applied to MET Norway's total cloud area fraction. It is deliberately loose, because aurora can shine through thin cloud and gaps over the northern horizon matter more than the sky average. The strip shows the actual percentage on every cell's tooltip and in the cloud tile.

Aurorix treats every dark hour the same, because it has no open data source that weights the time of night. Aurora can appear at any dark hour when activity is high. The strip therefore shows only whether an hour is dark, clear and above the Kp threshold.

It runs from when nights get dark again in late summer to when they stop being dark in spring. The darkness table on each destination page lists the hours with the sun below −12 degrees on the 15th of each month. Midsummer has none at any of the northern places, and the polar-night months have the most.

Aurora follows geomagnetic latitude, not geographic latitude. The geomagnetic pole sits at about 80.9 N, 72.8 W, on the Canadian side of the Arctic, so places in Canada and Iceland are closer to it than their geographic latitude suggests and places in Finland are further away. By the dipole formula Yellowknife (62.5 N) is near 68 degrees geomagnetic latitude and Rovaniemi (66.5 N) near 64. The guide on geomagnetic latitude has the full table.

A scheduled job downloads NOAA and MET Norway data every hour and the site is rebuilt from it. Each page shows the time its data were fetched and, once it loads in your browser, how many minutes or hours ago that was. After three hours it says the snapshot is ageing and after 24 hours it warns that it may no longer match the sky. For the live minute-by-minute value, open NOAA SWPC.

Apps mix several products. Aurorix shows NOAA's one-minute estimated Kp as "now", and draws the three-hour blocks, observed and predicted, from NOAA's Kp series. A different app may show the last official three-hour value, a forecast block or a different agency's index. Values can differ by a third of a step even when the sources agree.

NOAA's geomagnetic storm scale maps Kp to levels: G1 is Kp 5, G2 is Kp 6, G3 is Kp 7, G4 is Kp 8 including 9-, and G5 is Kp 9. NOAA says aurora is commonly visible at high latitudes at G1, has been seen as far south as New York and Idaho at G2, and as low as Florida and southern Texas at G5. Over the Nordic countries a storm level is not needed: it only matters for places much further south.

No. The six destinations are all in the northern hemisphere and the map shows the northern OVATION grid from 45 degrees north to the pole. NOAA publishes a southern grid, but Aurorix does not read it.