Space Weather R, G, S Scales Explained

Space Weather R, G, S Scales ExplainedNOAA's three space-weather scales — Radio blackout (R), Geomagnetic storm (G), Solar radiation storm (S) — plus solar wind, CMEs, and SDO imagery.NOAA's Space Weather Prediction Center (SWPC) classifies space-weather events on three scales — Radio blackout (R), Geomagnetic storm (G), and Solar radiation storm (S) — each running 1 to 5. Most space-weather data is global (the Sun affects all of Earth roughly the same), with the exception of the aurora forecast which is computed per-county.R-scale — Radio blackoutDriven by solar X-ray flares. The flare ionizes Earth's upper atmosphere within minutes, disrupting HF (high-frequency) radio propagation on the sunlit hemisphere.

R1: Minor — minor HF degradation on sunlit side.

R2: Moderate — limited HF blackout, some loss of contact.

R3: Strong — wide-area HF blackout, low-frequency navigation degraded.

R4: Severe — HF blackout most of sunlit Earth for hours.

R5: Extreme — complete HF blackout entire sunlit side.

Who cares: amateur radio operators (SKYWARN nets, emergency comms), aviation HF over oceans/polar routes, maritime communications, military.G-scale — Geomagnetic stormEarth's magnetic-field disturbance, usually triggered by a CME impact or high-speed solar wind. Driven by the planetary K-index (Kp), 0–9.

G1 (Kp 5): Minor — aurora at high latitudes, minor grid fluctuations.

G2 (Kp 6): Moderate — aurora to mid-latitudes; high-latitude HF noise.

G3 (Kp 7): Strong — voltage corrections needed; GPS error grows; aurora to ~50°N.

G4 (Kp 8): Severe — widespread voltage problems; HF radio sporadic; aurora to ~45°N.

G5 (Kp 9): Extreme — grid systems can fail; aurora as far south as Florida/Texas.

Who cares: power-grid operators, GPS-dependent industries (precision agriculture, surveying, drones), pipeline operators, satellite ops, aurora chasers.K-index (Kp) — the underlying numberKp is the global summary number for geomagnetic activity, derived every 3 hours from a network of magnetometers. Each whole step is roughly a 2× jump in disturbance — so Kp 6 isn't just "one more" than Kp 5, it's twice as energetic.

Kp 0–2: Quiet — no magnetic disturbance worth mentioning.

Kp 3: Unsettled — minor wobble, no operational impact.

Kp 4: Active — elevated geomagnetic activity below storm threshold; aurora possible at very high latitudes (Alaska, northern Canada).

Kp 5: Storm onset (G1) — aurora visible at high latitudes; minor grid noise.

Kp 6+: Significant storm (G2+) — aurora pushes to mid-latitudes; real grid + GPS effects.

SWPC issues "ALERT/WARNING: Kp X" messages whenever Kp crosses an observed or forecast threshold (most commonly Kp 4, 5, 6, 7, 8). Kp 4 alerts on the Solar tab mean the magnetic field is restless but not yet in storm territory — an early heads-up that conditions could escalate, not an action signal for mid-latitude operators.S-scale — Solar radiation stormEnergetic protons accelerated by solar events. Rare but consequential.

S1: Minor — minor impacts to HF in polar regions.

S2: Moderate — some polar HF degradation; airlines may reroute.

S3: Strong — astronauts shelter; total HF polar blackout.

S4: Severe — satellite single-event upsets common.

S5: Extreme — complete polar HF blackout for days.

Solar wind metrics

Bz: Interplanetary magnetic field component along Earth's magnetic axis. Sustained negative (southward) Bz lets solar wind couple to Earth's magnetosphere — the precursor to geomagnetic storms. Bz ≤ −10 nT for ≥ 30 minutes is a textbook storm signal.

Bt: Total magnetic field strength.

Speed: Solar wind speed in km/s. Background is 350–450 km/s; high-speed streams ≥ 600 km/s drive moderate G-storms; CME shocks can push speeds past 1,000 km/s briefly.

Density: Proton density (protons/cm³). Pressure spikes at CME arrivals.

Coronal Mass Ejections (CMEs)Massive bursts of plasma and magnetic field ejected from the Sun. Take 1–3 days to reach Earth. The CME pipeline panel shows analyzed CMEs from NASA's DONKI catalog with predicted arrival times and Kp impact — the most actionable advance warning we can give for grid/comms/GPS preparation.SDO imagery — different wavelengths show different layers

Visible (HMI): White-light view of the photosphere — sunspots visible as dark patches.

304 Å (chromosphere): Helium emission from the lower atmosphere — filaments, prominences, flare ribbons.

171 Å (corona): Iron emission from the million-degree corona — active region loops, coronal holes.

Magnetogram (HMIB): Black-and-white map of magnetic polarity (white = north, black = south). Sunspot complexes show as bipolar pairs.

Part of the BloomWX learn library — beginner-friendly explainers covering every surface of the BloomWX weather dashboard. Open BloomWX to see live data for any U.S. county.