Aurora Forecast for Your Area: The Kp Index Explained (What Kp Do You Need?)

Every aurora forecast leads with a number between 0 and 9: the Kp index. It is useful, but on its own it does not tell you whether you will see the northern lights tonight. That depends on Kp, how far north you are, the time of night, cloud cover and the moon. This guide explains each, so you can read any aurora forecast for your area with confidence.

What the Kp index measures

Kp is a global measure of how disturbed the Earth's magnetic field is, published by NOAA's Space Weather Prediction Center and updated in three-hour blocks. It runs from 0 (quiet) to 9 (extreme). When the field is disturbed, the auroral oval (the ring of aurora around each magnetic pole) gets brighter and expands toward the equator. That is why a higher Kp means the aurora can be seen further from the Arctic.

Kp 5 and above is officially a geomagnetic storm, graded G1 (Kp 5) to G5 (Kp 9). The storms you hear about on the news, when people far south suddenly see red and pink skies, are usually G4 or G5.

What Kp do you need to see the aurora?

NOAA's own rule of thumb for the northern hemisphere, as the lowest places the aurora may be seen (often low on the northern horizon, and brighter on camera than to the eye):

• Kp 3 to 4: northern Norway, Sweden and Finland, Iceland, Alaska, northern Canada. In these places the aurora is common on most clear, dark nights.
• Kp 5 (G1): Scotland, southern Norway and Sweden, the Baltic states, and US border states such as northern Michigan and Maine.
• Kp 6 (G2): northern England, Denmark, northern Germany and Poland; New York and Idaho in the US.
• Kp 7 (G3): much of central Europe; Illinois and Oregon.
• Kp 8 to 9 (G4 to G5): southern Europe, and in the US as far as Alabama, northern California and, in extreme storms, Florida and southern Texas.

What matters is magnetic latitude, not geographic latitude. North America is tilted toward the magnetic pole, so a given Kp reaches further south there than in Europe or Asia.

Kp is not the whole story: the OVATION map

Kp is one number for the whole planet. NOAA's OVATION model is more useful locally: it maps the probability of aurora over a grid around the poles, updated every few minutes from real-time solar wind measurements. If the oval on the map reaches your latitude, or sits just north of you (the aurora can be seen up to several hundred kilometres away, low on the horizon), your chances are real.

The solar wind also matters minute to minute. When the interplanetary magnetic field (Bz) turns strongly south, the aurora can flare up within half an hour even if the three-hour Kp looked modest. That is why forecasts change during the night.

The 3-day aurora forecast

NOAA also publishes a 3-day Kp forecast. It is based on what has left the Sun: coronal mass ejections take one to three days to arrive, and fast solar wind from coronal holes returns every 27 days as the Sun rotates. Use it to plan which night to drive somewhere dark, then check the live forecast on the evening itself. Arrival times of a CME are uncertain by many hours.

Clouds and the moon decide more nights than Kp

A Kp 7 storm under full cloud is invisible. Always check cloud cover for the next few hours alongside the aurora forecast, and look for gaps: a partly cloudy night can still work. A full or nearly full moon washes out faint aurora and stars but does not hide a strong display; a new moon is best.

One number for your exact location

Checking Kp, the OVATION map, cloud cover and the moon phase separately every evening gets old. Night Cam, the app I built for this, combines them into one aurora chance for where you are: it reads the OVATION grid around your position and toward the pole, discounts it by cloud cover for the next 12 hours, and shows the moon phase and the 3-day Kp outlook. Optional alerts send at most one notification a night when it is worth going out. The forecast is free, and the same app has camera presets for photographing what you see. For the camera side, see iPhone camera settings for the northern lights.

Quick reference

• Kp 0 to 2: quiet. Aurora only in the far north, if at all.
• Kp 3 to 4: active. Good nights in the Arctic.
• Kp 5 to 6 (G1 to G2): storm. Scotland, southern Scandinavia, northern US states.
• Kp 7+ (G3 and above): strong storm. Central Europe and mid-latitude US; check the sky even if you rarely see aurora.
• Always check clouds and the moon too. Best hours are usually 10 p.m. to 2 a.m. local time.