Kp Index
Also called: planetary K-index, K-index
The Kp index is a 0 to 9 scale of global geomagnetic activity, reported in 3-hour intervals from ground-based magnetometers. A Kp of 5 or higher indicates a geomagnetic storm, and the values map onto NOAA's G-scale from G1 (Kp 5) to G5 (Kp 9). The higher the Kp, the farther from the poles the aurora can be seen.
Kp Values and the NOAA Geomagnetic Storm Scale
| Kp | NOAA scale | Aurora may be seen as low as |
|---|---|---|
| 5 | G1 (Minor) | High latitudes, such as northern Michigan and Maine |
| 6 | G2 (Moderate) | New York and Idaho |
| 7 | G3 (Strong) | Illinois and Oregon |
| 8, including 9- | G4 (Severe) | Alabama and northern California |
| 9 | G5 (Extreme) | Florida and southern Texas |
Over an 11-year solar cycle, NOAA counts about 1,700 G1 storms and about 4 G5 storms.
Source: NOAA Space Weather Prediction Center: NOAA Space Weather Scales
How It Is Measured
- Each station's K-index measures the largest swing in the horizontal magnetic field during a 3-hour interval.
- Kp is the mean of the standardized K-indices from 13 geomagnetic observatories between 44° and 60° geomagnetic latitude, north and south.
- Kp below 5 is not a storm. Kp of 5 is a G1 storm, and each step up in Kp is a step up the G-scale.
- Kp describes activity for the whole planet. Seeing the aurora also takes darkness, clear skies, and a location far enough from the equator for that level of activity.
Related Terms
Zulu Time
Zulu time, written with a Z, is the name used in aviation and meteorology for Coordinated Universal Time (UTC), the time at the Greenwich prime meridian. Weather observations, model runs, and aviation reports all use it so that a time means the same thing everywhere. 12Z is 8 in the morning Eastern Daylight Time and 7 in the morning Eastern Standard Time.
UV Index
The UV index is a forecast of the intensity of skin-damaging ultraviolet radiation expected to reach the ground at solar noon, when the sun is highest. It is 0 at night and rises to 11 or higher, and the higher the number, the faster unprotected skin and eyes are harmed. Sun protection is recommended at 3 and above.
Kp Index: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
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In the WeatherAI App
WeatherAI's Space Weather section shows current NOAA SWPC geomagnetic and aurora information. Optional Geomagnetic Storm notifications are triggered by SWPC's observed K-index alerts, at a minimum level you choose: G1, G2, or G3.
Availability, coverage, update frequency, and alert delivery vary by data source, region, subscription plan, device permissions, and service conditions. WeatherAI supplements official sources and is not an emergency notification service. For life-safety decisions, rely on your national weather service, local emergency management, and official warning channels.