Geomagnetic Activity — Kp, A-Index, G-Scale Explained for Hams

Geomagnetic Activity — Kp, A-Index, G-Scale Explained for HamsThe K-index, A-index, and NOAA G-scale storm rating — what they measure, what each level means at the radio (HF flutter, polar paths, aurora propagation on VHF).How disturbed Earth's magnetic field is. Quiet = clean HF propagation. Disturbed = noisy bands, fading, polar paths degraded.K-indexLogarithmic 0-9 scale, recomputed every 3 hours. SWPC publishes a planetary average (Kp).

K = 0-2: Quiet

K = 3: Unsettled

K = 4: Active

K = 5: G1 storm

K = 6: G2 storm

K = 7+: G3+, widespread HF disruption, aurora visible

A-indexLinear daily average of the eight 3-hour ap values. Single digits = quiet day; A >= 30 = active; A >= 100 = severe storm.G-scaleNOAA storm rating 0-5. G1 = Kp 5, G2 = Kp 6, G3 = Kp 7, G4 = Kp 8, G5 = Kp 9.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.