The NOAA G-Scale: A Practical Guide to Geomagnetic Storm Classifications

When NOAA issues a geomagnetic storm watch or warning, it uses a five-level scale — G1 through G5 — to communicate how intense the storm is expected to be. Each level carries specific implications for where aurora will be visible, how active it will appear, and what effects the storm may have on technology and infrastructure. For aurora travelers and photographers, understanding what each classification actually means in practice is more useful than the label alone.

What the G-Scale Is

The NOAA G-scale is a severity classification system for geomagnetic storms, introduced alongside similar scales for solar radiation storms (S-scale) and radio blackouts (R-scale). It runs from G1 (minor) to G5 (extreme), with each level corresponding to a range of Kp index values and a set of expected effects on aurora visibility, power systems, GPS accuracy, and radio communications.

What helped me picture the scale: think of it like the Saffir-Simpson scale for hurricanes. A Category 1 and a Category 5 are both hurricanes, but the difference in impact is enormous. Similarly, a G1 and a G5 geomagnetic storm are both storms — but G5 events are rare, historically significant occurrences, while G1 events happen dozens of times per year during active solar periods.

G1 Through G5: What Each Level Means for Aurora

A G1 storm (Kp 5) represents the entry point for geomagnetic storm classification. Aurora is reliably visible from high-latitude locations — Alaska, northern Canada, Iceland, northern Scandinavia — and may be seen on the horizon from the northern tier of US states under very dark skies. For travelers beneath the auroral oval in Fairbanks, a G1 event is a productive night.

G2 storms (Kp 6) push aurora further equatorward — into the northern United States and central Europe — and produce more intense displays at high-latitude locations. Substorm activity increases. Color variety begins to expand beyond the dominant green.

G3 storms (Kp 7) bring aurora to mid-latitudes reliably. At high-latitude destinations, displays can be fast-moving and multi-colored, with red tops and blue-purple bases appearing consistently. These are the events that generate social media attention across the northern United States and Canada.

G4 storms (Kp 8) are significant events — aurora visible as far south as the southern United States and southern Europe, with intense, rapidly shifting displays at high latitudes. GPS degradation and power grid impacts become measurable.

G5 storms (Kp 9) are the maximum classification — rare, historically notable events. The May 2024 storm that brought aurora to tropical latitudes was a G5. At high-latitude destinations, these events produce some of the most dramatic aurora conditions on record: full-sky corona, vivid color across the entire visible spectrum, and substorm cycles that can persist for hours.

For context on how storm frequency relates to the solar cycle, see our overview of solar cycles and the northern lights and our guide on the best time to see the northern lights in Alaska.

What the G-Scale Means for Photographers

The G-scale is a useful planning reference for setting exposure expectations before a night in the field. At G1–G2, aurora is active enough to photograph comfortably with standard wide-angle aurora settings — ISO 1600–3200, f/2.8, 8–12 seconds. At G3 and above, the pace of aurora movement accelerates and shutter speeds need to come down to 3–6 seconds to preserve structure during substorm onsets.

Color is the other variable the G-scale predicts reasonably well. G1 events are predominantly green. G3 events reliably produce red and purple. G5 events can produce displays where red dominates the upper sky and cameras capture layers of color that the naked eye perceives only partially.

One important calibration for photographers: the G-scale describes conditions globally, but what you see overhead depends on your position relative to the auroral oval. A G2 storm produces exceptional photography from Fairbanks; the same storm may produce only a faint glow on the horizon from Seattle. Our Northern Lights Tour in Fairbanks places guests directly beneath the oval — which means the G-scale ceiling for any given night translates more directly into what appears overhead.

Return to the full Northern Lights Glossary to continue through the Geomagnetic Indices and Measurements section.