How to Read Space Weather Forecasts

Space weather forecasts are full of letters and numbers that look interchangeable and absolutely are not. Here’s what each one means, and which ones actually matter to you.

TL;DR

• Kp — how big the storm is, 0 to 9
• Bz — whether the storm can couple to Earth at all (negative is what you want)
• Speed — how hard the solar wind is pushing
• G / S / R — three separate scales for three unrelated hazards

The three NOAA scales are not the same thing

This is the most common confusion, and it’s worth clearing up first. NOAA publishes three scales, each 1 to 5, and they measure completely different phenomena:

• G (Geomagnetic) — storms in Earth’s magnetic field. This is the aurora one. G1 through G5, tied directly to Kp: G1 = Kp 5, G5 = Kp 9.
• S (Solar Radiation) — energetic protons arriving from the Sun. Matters for astronauts, polar flights and satellites. Does not cause aurora.
• R (Radio Blackout) — X-rays from flares ionising the dayside upper atmosphere, disrupting HF radio. Also not an aurora indicator.

An R3 blackout headline sounds dramatic but says nothing about your chances tonight. Look at G.

Kp: the storm size

Kp runs 0 to 9 and describes how disturbed Earth’s magnetic field is globally, in three-hour blocks. Roughly:

• 0–2 — quiet
• 3–4 — unsettled to active
• 5+ — storm (this is where G1 starts)

Two caveats worth knowing. First, Kp is an average of the whole planet — it says nothing about your specific sky. Whether you can see anything depends on your geomagnetic latitude, which is not the same as your latitude on a map. Look up your location’s threshold once and you’ll never have to guess again.

Second, Kp is backward-looking. It’s computed from magnetometer readings after the fact. Which is why the next number matters more if you want warning.

Bz: the on/off switch

Bz is the north–south component of the magnetic field carried in the solar wind, measured in nanotesla (nT).

When Bz is negative — southward — it can link up with Earth’s field and energy flows in. When it’s positive, that coupling is much weaker and the same solar wind produces far less. A sustained −10 nT or below is a strong signal.

The reason Bz is so useful is timing. It’s measured by satellites at the L1 point, about 1.5 million km upstream, giving you roughly 15 to 60 minutes of real warning before the effect reaches Earth. Kp tells you what already happened; Bz tells you what’s coming. It’s live on the solar wind dashboard.

Speed, density and Bt

Supporting details rather than headline numbers:

• Speed — typical is 300–400 km/s. 500+ is elevated and helps.
• Bt — total magnetic field strength. High Bt means there’s a lot of field available; it only pays off if Bz goes negative.
• Density — can sharpen the impact, especially spiking alongside favourable Bz.

Reading a 3-day forecast

NOAA’s 3-day outlook gives predicted Kp in three-hour blocks. Treat it as a planning tool, not a promise — CME arrival times routinely shift by six hours or more in either direction.

Use the forecast to decide which night to keep free. Use Bz on the night itself to decide whether to actually go out.

A simple go / no-go check

1. Is predicted Kp at or above your location’s threshold?
2. Is Bz negative and staying there?
3. Is the sky dark where you are — genuinely dark, not just after sunset?
4. Is it clear?

All four yes, go. Any one no, stay in — and don’t let a big Kp number talk you out of the other three.