Northern Lights Forecast Explained: How to Read a Forecast

A live aurora forecast contains half a dozen numbers — and most people only look at one of them. Here's what each metric means and which ones actually matter.

Most aurora apps show a number, colour it green or red, and call it a forecast. That number — usually the KP index — is a useful starting point. But a proper aurora forecast combines half a dozen data streams, and understanding them separately is the difference between knowing that "tonight might be active" and knowing whether you should actually drive out to a dark location right now.

KP Index — The Headline Number

The KP index measures global geomagnetic disturbance on a 0–9 scale, updated every three hours. It's the most widely communicated aurora metric because it's simple and universal. The key thing to understand: it's a lagging indicator. The KP value you see was calculated from magnetometer data collected over the past three hours. It doesn't tell you what's happening right now — it tells you what happened.

For short-term planning (should I go out in the next hour?), real-time Bz data is more useful. For day-level planning (is tomorrow a good aurora night?), the KP forecast is appropriate.

Bz — The Real-Time Aurora Switch

Bz is the north-south component of the interplanetary magnetic field carried by the solar wind. When Bz is negative (pointing south, opposite to Earth's field), magnetic reconnection occurs and aurora activity rises. When Bz is positive, activity quietens. The DSCOVR satellite at L1 measures Bz in real time, giving 15–60 minutes of lead time before conditions arrive at Earth.

A Bz of −10 nT or more negative for an extended period typically produces KP 4–5 activity. Values of −20 to −30 nT drive G3–G4 storms (KP 7–8). The record during the May 2024 storm reached approximately −50 nT. Watching Bz trend increasingly negative while it's already producing aurora is a reliable signal to get outside quickly.

Solar Wind Speed — The Energy Multiplier

Solar wind speed is measured in km/s and typically ranges from 300 to 800 km/s, with occasional bursts to 1,000+ km/s during major CME events. Speed amplifies the effect of negative Bz — a solar wind of 700 km/s with Bz of −10 nT produces more geomagnetic disturbance than 400 km/s at the same Bz. Speed alone doesn't cause aurora; it has to combine with southward Bz.

When you see solar wind speed spike to 600 km/s or above on a forecast, watch Bz carefully. If it also turns negative, activity can escalate rapidly. This is the combination that produces the surprise rapid-onset storms that light up aurora unexpectedly on otherwise quiet nights.

Solar Wind Density — Pressure on the Magnetosphere

Proton density (measured in particles per cubic centimetre) tells you how "thick" the solar wind is. Higher density increases the pressure on the magnetosphere, compressing it and making it more reactive to negative Bz. A density spike from 5 to 25+ p/cc often accompanies a sudden commencement event — a rapid rise in geomagnetic activity visible on magnetometers worldwide.

Density spikes often precede or accompany the arrival of a CME's leading edge. If you see density jump while Bz turns negative and speed rises simultaneously, conditions for a significant aurora event have arrived.

Cloud Cover — The Aurora Killer

It doesn't matter what KP is if you can't see through the clouds. Cloud cover is the single most important local factor for aurora viewing and the one most easily ignored by people focused on the space weather data. A KP 7 storm with 100% cloud cover is a complete loss; a KP 2 night with clear skies from a dark location can produce a memorable experience.

Use a satellite-based cloud cover tool rather than a standard weather app, which models clouds with lower spatial resolution and accuracy. EUMETSAT, Sat24, and the Windy cloud layer show real satellite imagery updated every 15 minutes. Look for the actual cloud position, not a modelled forecast — models are wrong often enough that satellite confirmation matters.

Aurora Probability — Putting It Together

PolarForecast's aurora probability percentage combines all of the above — KP, Bz, solar wind speed and density — with your specific geomagnetic latitude and the cloud cover forecast for your location. It's designed to answer the specific question "what are the odds I personally will see aurora tonight from where I am?" rather than the global "how active is space weather?" that KP alone answers.

A high global KP but your location under thick cloud produces a low probability. A moderate KP but clear skies at 67°N produces a high probability. The probability number integrates these factors in a way that a raw KP reading doesn't.

Putting It Into Practice

• Check the 3-day KP forecast to decide whether your travel timing is broadly right.
• On the day, watch Bz — is it trending negative? Has it been below −10 nT for 30+ minutes?
• Check satellite cloud imagery for your specific location — not a model, actual current satellite data.
• Once outside, give your eyes 10–15 minutes to dark-adapt before concluding there's nothing to see.
• Set a KP alert for a threshold relevant to your latitude so you don't miss overnight activity while sleeping.