From Glossary of Meteorology

G scale

A National Oceanic and Atmospheric Administration classification scheme for geomagnetic storm severity and a means of communicating to the general public the possible effects on built systems. The G-scale values correlate to the estimated and/or 3-hour-determined Kp index and are as follows: G1 (minor), Kp = 5; G2 (moderate), Kp = 6; G3 (strong), Kp = 7; G4 (severe), Kp = 8 and 9−; and G5 (extreme), Kp = 9.

Term edited 14 August 2018.