G-Scale (Geomagnetic Storm Scale)
Definition
NOAA's public 1-to-5 rating of geomagnetic storms, from G1 (minor) to G5 (extreme).
In plain language
The G-scale translates Kp into plain language: G1 means auroras for the northern US, G3 reaches the mid-latitudes, and G5 is a historic, continent-spanning event.
The science
The G-scale maps Kp values 5 through 9 to G1โG5 and associates each level with typical technological impacts (power grids, satellite operations, HF radio) and equatorward auroral visibility estimates.
Why it matters for aurora viewing
When headlines say "G3 storm watch," you can convert instantly: Kp 7, aurora plausibly visible across the northern half of the US.
See this in action on tonight's data.