The Kp index, explained
The Kp index is a scale from 0 to 9 that describes how disturbed Earth's magnetic field is over a three hour window. The higher the number, the further from the poles the aurora reaches. Most of the northern United States needs Kp 5 or higher before the lights come into range, and the far north of Alaska sees them at Kp 0.
Right nowKp 1.00as of 10:56 AM EDTSee every state
What each number means
Kp is a planetary average, so it describes the whole globe rather than your back garden. As it rises, the auroral oval widens and its southern edge pushes toward the equator. The geomagnetic latitude column below is the oval's equatorward edge at that Kp, which is why states at the same map latitude do not all see the aurora at the same time.
| Kp | NOAA storm level | Oval edge (geomagnetic) | Typically in range |
|---|---|---|---|
| 0 to 2 | None | 62 to 67 degrees | Northern Alaska, northern Canada |
| 3 | None | 60 degrees | Upper Michigan, northern Maine, northern Minnesota |
| 4 | None | 58 degrees | Much of Minnesota, Wisconsin, Vermont, Washington |
| 5 | G1 Minor | 56 degrees | Boston, Seattle, Minneapolis, Portland Maine |
| 6 | G2 Moderate | 54 degrees | New York City, Chicago, northern Ohio |
| 7 | G3 Strong | 52 degrees | Denver, Columbus, Kansas City |
| 8 | G4 Severe | 50 degrees | Northern California, Oklahoma, Virginia |
| 9 | G5 Extreme | 48 degrees | Deep into the southern states, rare |
The right hand column is a guide, not a promise. Aurora sits between roughly 100 and 300 kilometres up, so it stays visible low on the horizon for observers a few degrees further south than the oval itself. That margin is the difference between a faint green glow over the treeline and nothing at all.
Where the number comes from
Kp stands for planetarische Kennziffer, German for planetary index. It is worked out from thirteen geomagnetic observatories spread around the world, each measuring how much the local magnetic field has wandered from its quiet day baseline. Those readings are combined into a single figure for every three hour block, from 00:00 to 03:00 UT and so on through the day.
Because a settled three hour value arrives after the fact, NOAA's Space Weather Prediction Center also publishes an estimated Kp updated every minute. That estimate is what WeatherTotals shows, so the number on the page reflects conditions now rather than an average of the last three hours. Every reading on this site carries the time it was taken.
Kp and the G scale
NOAA also runs a five step geomagnetic storm scale, from G1 to G5. It maps directly onto the top of the Kp scale, so the two are the same information under different names. News coverage tends to use the G scale because a severe geomagnetic storm sounds more urgent than Kp 8.
- G1 Minor is Kp 5. Aurora reaches the northern tier of the United States.
- G2 Moderate is Kp 6. It reaches New York and Chicago.
- G3 Strong is Kp 7. It reaches Denver and Columbus.
- G4 Severe is Kp 8. It reaches well into the middle of the country.
- G5 Extreme is Kp 9. Sightings from the deep south become possible, and these are rare.
Why a high Kp still does not guarantee a sighting
Kp tells you whether the aurora is overhead somewhere at your latitude. It says nothing about whether you can actually see it, which is why a promising forecast so often ends in a cold field and no photographs.
- Cloud. This is the usual spoiler, and it is why every state page here shows tonight's cloud cover next to the Kp reading.
- Daylight. A Kp 7 at two in the afternoon does nothing for you.
- The Moon. A full moon washes out faint aurora much as it washes out the Milky Way.
- Light pollution. From the middle of a city you will miss displays that are obvious twenty miles out.
- Timing. Activity is usually strongest around magnetic midnight, roughly between 10pm and 2am local time.