Space weather explained

What is the Kp index?

The Kp index is a single number that tells you how disturbed Earth's magnetic field is right now. Understanding it tells you whether the northern lights are likely to reach your location tonight.

What the Kp index measures

The Kp index - short for "planetarische Kennziffer," German for planetary index - is a global measure of geomagnetic activity on a scale from 0 (quiet) to 9 (extreme storm). It is updated every three hours based on magnetometer readings at observatories around the world.

Thirteen stations at mid-latitudes measure how much Earth's magnetic field deviates from its normal quiet-day value. These deviations are averaged and converted to the 0-9 scale. A reading of 0 or 1 means very little activity. Kp 5 and above indicates a geomagnetic storm is in progress.

The number is useful because it has a direct relationship with aurora visibility. The higher the Kp, the further south the auroral oval expands - meaning the lights become visible at lower and lower latitudes.

The Kp scale and what each level means

Kp 0–2 Quiet (G0)

No geomagnetic storm. Aurora visible only above 70°N - deep in the Arctic. Not visible from the UK.

Kp 3–4 Unsettled (G0)

Mild disturbance. Aurora may be visible above 65°N on a clear night. Northern Scotland possible on Kp 3-4 if skies are exceptional.

Kp 5 Minor storm (G1)

Threshold for a geomagnetic storm. Visible above roughly 60°N. Northern Scotland has a genuine chance on a clear night.

Kp 6 Moderate storm (G2)

Visible above around 55°N. Central Scotland, Iceland, and southern Canada. A good evening to look north from Edinburgh.

Kp 7 Strong storm (G3)

Visible above roughly 50°N. Northern England, the Netherlands, and northern US states. Worth heading out from Manchester.

Kp 8 Severe storm (G4)

Visible above around 45°N. England, France, Germany, the central US. A significant storm event.

Kp 9 Extreme storm (G5)

Visible above around 40°N. Southern Europe, southern US, New Zealand. Rare - fewer than 4 times per solar cycle on average.

How Kp is calculated

Geomagnetic observatories record the strength and direction of Earth's magnetic field continuously. Each three-hour period, every station calculates how far its readings deviated from the expected quiet-day baseline. That deviation is converted to a local K index - a logarithmic 0-9 scale specific to each station's location.

The 13 contributing stations are distributed between 44°N and 60°N magnetic latitude - the mid-latitude band most sensitive to geomagnetic disturbances. Their K indices are averaged and standardised to produce the global Kp value.

The logarithmic nature of the scale is important. A jump from Kp 5 to Kp 7 is not twice as strong - it represents roughly 10 times the field disturbance. This is why the aurora expands dramatically southward during high-Kp events.

Limitations of the Kp index for aurora watching

Kp is a three-hour average. The aurora can be active for a much shorter window within that period - bright displays often last 30 to 90 minutes. A Kp reading of 6 for a given window does not mean aurora was visible for the full three hours.

Kp also does not account for cloud cover or light pollution, which are often the deciding factors in whether you actually see anything. A Kp 7 storm in overcast skies gives you nothing. A Kp 4 on a clear, moonless night from a dark hillside in Scotland can be impressive.

Use Kp alongside a cloud cover forecast for your specific location. If the number is high enough for your latitude and the sky is clear, go out. That is the decision the Kp index is designed to help you make.

What Causes the Northern Lights?

How solar wind, CMEs, and the magnetosphere combine to produce aurora.

Tips for Viewing the Northern Lights

Dark skies, clear weather, and timing - what actually makes the difference.

Best Time to See Northern Lights UK

Which months, times of night, and conditions give the best odds in the UK.

Northern Lights UK - Live Forecast

Current aurora visibility across the UK based on live Kp data.

Aurora Forecast Tonight

Live Kp index, geomagnetic storm status, and upcoming CME arrivals.

Common questions

More detail on the Kp index and how it relates to aurora visibility.

What does Kp stand for?

Kp stands for "planetarische Kennziffer" - a German term meaning planetary index. It was introduced by Julius Bartels in 1949 and remains the standard global measure of geomagnetic disturbance.

How is the Kp index measured?

Kp is calculated from magnetometer readings at 13 observatories spread across mid-latitudes worldwide. Each station measures how much the magnetic field deviates from its quiet-day baseline over three-hour intervals. These readings are averaged and converted to a single 0-9 scale, updated eight times per day.

How often is the Kp index updated?

Eight times per day - once every three hours. Each value covers the previous three-hour window, so there is always a short lag between real-world conditions and the published number. During an active storm the Kp can shift significantly from one three-hour period to the next.

What Kp do I need to see the northern lights from the UK?

It depends on your location. From the Scottish Highlands, Kp 3-4 can be enough on a clear night away from light pollution. Edinburgh and southern Scotland typically need Kp 4-5. Newcastle and northern England need around Kp 5-6. London and the south of England need Kp 7 or above - that requires a significant geomagnetic storm.

Is a Kp of 5 a geomagnetic storm?

Yes. Kp 5 marks the threshold for a G1 (minor) geomagnetic storm on the NOAA storm scale. G1 storms can cause minor disruptions to satellite operations and produce aurora visible above around 60°N - that means northern Scotland and Scandinavia in clear conditions.

What is the difference between Kp and the G-scale?

They measure the same thing on different scales. The G-scale runs G1 to G5 and is used for public forecasting. Kp runs 0 to 9. G1 = Kp 5, G2 = Kp 6, G3 = Kp 7, G4 = Kp 8, G5 = Kp 9. Below Kp 5 is G0 - no storm, quiet conditions.

Where does this site get its Kp data from?

All space weather data comes from NASA's DONKI - the Space Weather Database Of Notifications, Knowledge, Information - maintained by the Community Coordinated Modeling Center. Data refreshes every 30 minutes.