The Kp index, explained
Kp runs from 0 (quiet) to 9 (extreme storm). Each step moves the aurora about 2° of latitude, roughly 230 km, further from the pole.
What Kp measures
Kp is the planetary K-index: every 3 hours, magnetometers at about a dozen observatories measure how much the horizontal magnetic field swings, each on a 0 to 9 scale tuned to its latitude, and the readings are averaged. NOAA publishes it in thirds (4.33 is "4+", 4.67 is "5−"). A big swing means electric currents are pouring into the upper atmosphere: the same currents that light up the aurora. So Kp is a stand-in for how big and how bright the auroral oval is.
Kp is global and 3-hourly. It cannot tell you that the aurora is bright over Edinburgh at 11:40 pm; for that, look at the live OVATION map on northern lights tonight, which is updated from solar wind measured about an hour upstream of Earth.
The scale
| Kp | Level | Where the aurora shows |
|---|---|---|
| 0–2 | Quiet | Aurora only near the Arctic Circle: Alaska, northern Canada, northern Scandinavia, Iceland. |
| 3 | Unsettled | Overhead in Fairbanks and the Yukon; low in the north from Winnipeg, Calgary and Grand Forks. |
| 4 | Active | Low on the horizon from Minneapolis, Vancouver and Aberdeen. |
| 5 (G1) | Minor storm | Low in the north from Seattle, Toronto, Glasgow and Edinburgh. About 900 days per 11-year cycle. |
| 6 (G2) | Moderate storm | Low in the north from Chicago, Boston, Manchester and Dublin; overhead in Minneapolis and Aberdeen. About 360 days per cycle. |
| 7 (G3) | Strong storm | Low in the north from New York, Denver, Birmingham and Cork; overhead in Scotland and Seattle. About 130 days per cycle. |
| 8 (G4) | Severe storm | Low in the north from London and Washington DC; overhead over Detroit and the Great Lakes. About 60 days per cycle. |
| 9 (G5) | Extreme storm | Seen far south: in May 2024 the aurora was photographed from Florida and southern Europe. About 4 days per cycle. |
Storm frequencies: NOAA space weather scales (average days per 11-year solar cycle). Places: typical, from the rule below.
Kp needed at your latitude
| Kp | Overhead from | On the horizon from | First visible (low in the north) from |
|---|---|---|---|
| 0 | 66° | 62° | north of our cities |
| 1 | 64° | 60° | Anchorage · Edmonton |
| 2 | 62° | 58° | Winnipeg |
| 3 | 60° | 56° | Grand Forks · Calgary |
| 4 | 58° | 54° | Minneapolis · Vancouver · Aberdeen |
| 5 · G1 | 56° | 52° | Seattle · Toronto · Glasgow |
| 6 · G2 | 54° | 50° | Chicago · Halifax · Manchester · Dublin |
| 7 · G3 | 52° | 48° | New York · Birmingham · Cork |
| 8 · G4 | 50° | 46° | Washington · London |
| 9 · G5 | 48° | 44° | north of our cities |
Magnetic latitude. Rule of thumb: the oval's southern edge near 66.5° − 2.04 × Kp; seen on the horizon from about 4° further south. Real storms are lumpy: the edge can sit a few degrees off on any night.
Why magnetic latitude, not latitude
The aurora circles the north magnetic pole, which sits over the Canadian Arctic. So a city in North America is magnetically further north than a European city at the same latitude: Minneapolis (45°N) sits at magnetic latitude 54°, London (51.5°N) at 48°. We use AACGM-v2 magnetic coordinates for every city on the site.
How reliable is a Kp forecast?
NOAA's forecasters predict Kp up to 3 days ahead, mostly from solar storms (coronal mass ejections) seen leaving the Sun. The arrival time is often off by 6 to 12 hours, and the strength depends on the storm's magnetic field direction, which is only known when it passes the monitoring spacecraft about 1.5 million km upstream, 30 to 90 minutes before it hits. So treat a forecast Kp 6 as "worth watching tonight", not a promise.
What else matters
- Darkness. At least nautical twilight (Sun 12° below the horizon).
- Clouds. The biggest spoiler; check your local forecast.
- The Moon. A bright Moon hides a faint aurora but not a strong one.
- Light pollution. Get out of town and face north.
- A camera. Phone night modes pick up aurora the eye sees only as a grey glow.
Questions
What is the Kp index?
A 0 to 9 measure of how disturbed Earth's magnetic field is, averaged over 3 hours from magnetometers around the world. The higher the Kp, the further from the poles the aurora reaches.
What Kp is needed to see the northern lights?
It depends on your magnetic latitude. Use the table on this page, or your city's page, which works it out for you.
What is the Kp right now?
Kp 2.67 (estimated) for the 3 hours from 00:00 UTC, per NOAA.
What do G1 to G5 mean?
NOAA's geomagnetic storm scale. G1 is Kp 5, G2 Kp 6, G3 Kp 7, G4 Kp 8, G5 Kp 9. Storms G1 and up are when aurora watchers in the US and UK get their chance.
Data: NOAA Space Weather Prediction Center (planetary K-index forecast, 3-day forecast, 27-day outlook, OVATION aurora model). Magnetic latitudes: AACGM-v2. Darkness and Moon computed with Skyfield and JPL DE440. We do not forecast clouds: you need a clear view to the north. How the verdict is made.