What Kp do I need to see the aurora?

It depends entirely on your magnetic latitude: Tromsø needs Kp 2, Edinburgh needs Kp 5, London needs Kp 7. Our city pages state the exact threshold for each of the 53 cities we cover.

Yes — Kp 5 is the first geomagnetic storm level (G1 on the NOAA scale). Each Kp step above 5 maps to a stronger storm: Kp 6 = G2, 7 = G3, 8 = G4, 9 = G5.

How far ahead can Kp be forecast?

Reliably about 3 days, because the forecast follows visible structures on the Sun as it rotates. Beyond that, the 27-day outlook is a research-grade indicator, not a promise.

What Is the Kp Index? The Aurora Number, Explained

The Kp index is a single number, from 0 to 9, that describes how violently Earth's magnetic field is being shaken by the solar wind at a given moment. It is the single most-quoted number in aurora chasing — and the most misunderstood. This page gives you the working knowledge: what it measures, what each level looks like, and the two traps that make people distrust it.

Where the number comes from

Every three hours, observatories around the world compare their local magnetic readings against a quiet-day baseline. The deviations are standardised and averaged into one planetary value — hence "Kp", the planetary K index. The result is published by NOAA's Space Weather Prediction Center (SWPC) alongside a 3-day forecast of where Kp is expected to go. That forecast is what powers the live answer on our city pages.

What each level means

Kp	What it means on the ground
Kp 0–1	Quiet. Aurora confined to the high Arctic. Most of the world sees nothing.
Kp 2	Active arcs over northern Scandinavia, Iceland, northern Alaska. The aurora-zone baseline.
Kp 3	Good aurora-zone night; southern edge of the oval pushes toward cities like Trondheim or Luleå.
Kp 4	"Unsettled". Aurora visible from Edinburgh or Edmonton on a clear night.
Kp 5	First storm level (G1). Mid-latitude locations — Seattle, Copenhagen — get horizon views.
Kp 6	G2 storm. Aurora from the northern US states, Ireland, northern Germany.
Kp 7	G3 storm. London, New York, Toronto can see it low on the horizon.
Kp 8	G4 severe storm. Paris, Berlin, Chicago in play; colours and structure spectacular.
Kp 9	G5 extreme (May 2024 territory). Aurora overhead in Spain, California, Texas.

Trap 1: Kp is an average, not a live picture

Because Kp is a 3-hour average, a brilliant 40-minute substorm can hide inside a "quiet" Kp 2 interval — and a storm can fade to nothing halfway through a Kp 6 window. Treat Kp as the tide, not the waves. If Kp is at or above your city's threshold, check the sky repeatedly instead of once.

Trap 2: Kp is planetary — your location decides what it's worth

Kp 4 is a great night in Tromsø and literally nothing in Madrid. The number only becomes meaningful once you compare it to your city's magnetic latitude. We maintain Kp thresholds for 53 cities and apply them automatically: the live answer on every city page is simply "tonight's forecast Kp vs your threshold, then the sky on top".

Where to check it

NOAA SWPC publishes the observed and forecast Kp every 3 hours. Our 27-day outlook and every city page surface the relevant slice for your location. For real-time burst detection, pair Kp with the Bz reading — see our explainer on solar wind Bz.

Using Kp in practice: a checklist

Before you head out, run this 60-second sequence. 1. Open your city page and read the verdict — it already applies your city's threshold to tonight's forecast Kp. 2. If the verdict is GO or MAYBE, note the "best window" time: it marks the first hour where cloud and darkness cooperate. 3. Set an alarm for that window, not for dusk — aurora before true darkness is invisible anyway. 4. Outside, give your eyes 15–20 minutes to adapt; the first thing most people see is a grey arc, not green. 5. Check the sky every 20–30 minutes for an hour even if nothing happens — substorms turn ordinary nights into extraordinary ones with no warning.

Common Kp mistakes

Chasing a number across the map: "Kp 6 tonight!" means nothing until you compare it to your city's threshold — Kp 6 in Tromsø is a Tuesday, Kp 6 in London is history. Trusting a single Kp hour: the 3-hour block may hide a burst or a fade. Ignoring the moon check: Kp 5 plus a 90% moon is a much weaker night than the number suggests. Waiting for Kp 9: the aurora zone delivers at Kp 2–3 most clear nights; the far-south spectaculars are lottery tickets, not plans.