How many geomagnetic storm days were there in 1939?

76 UTC days reached Kp 5 (G1) or more in the GFZ record. 4 reached Kp 8 (G4).

What was the strongest geomagnetic storm of 1939?

The highest 3-hour Kp was 9-, first reached on April 24, 1939.

How far south could aurora reach in 1939?

At Kp 9- SWPC's rule puts the average oval edge near 48.7 degrees geomagnetic. Viewing reaches farther, since aurora can be seen from as far as about 1,000 km away.

Does Glow count 5- as a storm?

No. A level counts from the whole Kp value, so 5- (4.67) is below G1 and 8- is below G4. Other sites may round differently.

GFZ Potsdam Kp record · 1939

Geomagnetic storms in 1939

Answer

In 1939 the GFZ Potsdam record has 76 UTC days whose highest 3-hour Kp reached 5 or more, the G1 storm level, against a median of 51.5 for the 94 complete years since 1932. That ranks 1939 28th of 94 by storm days.

days at Kp 5 or more

GFZ Kp, 365 days

Storm days by month

May was the stormiest month with 14 storm days. 1 of 12 months had none. March, April, September and October together account for 35 of the 76, the equinox months SWPC links with larger storms.

Storm levels against the median year

By level, the year counts 76 at G1, 37 at G2, 17 at G3, 4 at G4, 0 at G5. The highest 3-hour value was Kp 9-, G4 Severe on NOAA's scale. The average day peaked at Kp 3.75, against 3.36 in the median year.

Days per NOAA G level, 1939 and the median of 94 complete years
Level	Kp	1939	Median year
G1 Minor	5	76	51.5
G2 Moderate	6	37	20
G3 Strong	7	17	6.5
G4 Severe	8	4	2
G5 Extreme	9	0	0

A day counts at a level when its highest 3-hour Kp reached the whole value; 5- does not count as G1.

How far the oval reached

At the year's peak, Kp 9-, SWPC's rule of thumb puts the average oval edge at 48.7 degrees geomagnetic. 1192 of Glow's 1218 towns sit at or poleward of that line, including 328 in United Kingdom, 246 in United States, 181 in Canada.

Highest days of 1939
Date (UTC)	Highest Kp
April 24, 1939	9-
April 17, 1939	8
October 13, 1939	8
October 15, 1939	8
April 23, 1939	8-

G4 storm events

4 separate periods reached G4 (Kp 8) or more: April 17, 1939 (peak Kp 8); April 24, 1939 (peak Kp 9-); October 13, 1939 (peak Kp 8); October 15, 1939 (peak Kp 8). For context, 1938 had 76 storm days and 1940 had 71 storm days.

Questions

How many geomagnetic storm days were there in 1939?: 76 UTC days reached Kp 5 (G1) or more in the GFZ record. 4 reached Kp 8 (G4).
What was the strongest geomagnetic storm of 1939?: The highest 3-hour Kp was 9-, first reached on April 24, 1939.
How far south could aurora reach in 1939?: At Kp 9- SWPC's rule puts the average oval edge near 48.7 degrees geomagnetic. Viewing reaches farther, since aurora can be seen from as far as about 1,000 km away.
Does Glow count 5- as a storm?: No. A level counts from the whole Kp value, so 5- (4.67) is below G1 and 8- is below G4. Other sites may round differently.

Sources and verification

GFZ Potsdam, Kp index datahttps://kp.gfz.de/en/data
Matzka et al. 2021, Geomagnetic Kp index, GFZ Data Serviceshttps://doi.org/10.5880/Kp.0001
NOAA SWPC, Tips on viewing the aurorahttps://www.spaceweather.gov/content/tips-viewing-aurora
NOAA Space Weather Scaleshttps://www.spaceweather.gov/noaa-scales-explanation

Quoted figures last checked against their sources on 2026-09-30; next review 2026-12-31. Computed figures come from SunCalc and the IGRF-14 dipole as described in the methodology. Every quote is listed in the sources ledger.