How many geomagnetic storm days were there in 1940?

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

What was the strongest geomagnetic storm of 1940?

The highest 3-hour Kp was 9, first reached on March 24, 1940.

How far south could aurora reach in 1940?

At Kp 9 SWPC's rule puts the average oval edge near 48.0 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 · 1940

Geomagnetic storms in 1940

Answer

In 1940 the GFZ Potsdam record has 71 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 1940 32nd of 94 by storm days.

days at Kp 5 or more

GFZ Kp, 366 days

Storm days by month

March was the stormiest month with 11 storm days. Every month had at least one storm day. March, April, September and October together account for 26 of the 71, the equinox months SWPC links with larger storms.

Storm levels against the median year

By level, the year counts 71 at G1, 30 at G2, 18 at G3, 8 at G4, 1 at G5. The highest 3-hour value was Kp 9, G5 Extreme on NOAA's scale. The average day peaked at Kp 3.68, against 3.36 in the median year.

Days per NOAA G level, 1940 and the median of 94 complete years
Level	Kp	1940	Median year
G1 Minor	5	71	51.5
G2 Moderate	6	30	20
G3 Strong	7	18	6.5
G4 Severe	8	8	2
G5 Extreme	9	1	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 48.0 degrees geomagnetic. 1196 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 1940
Date (UTC)	Highest Kp
March 24, 1940	9
March 25, 1940	9-
March 30, 1940	9-
March 29, 1940	8+
March 31, 1940	8+

G4 storm events

5 separate periods reached G4 (Kp 8) or more: March 24, 1940 (peak Kp 9); March 29, 1940 (peak Kp 9-); April 3, 1940 (peak Kp 8); April 25, 1940 (peak Kp 8); June 25, 1940 (peak Kp 8+). For context, 1939 had 76 storm days and 1941 had 83 storm days.

Questions

How many geomagnetic storm days were there in 1940?: 71 UTC days reached Kp 5 (G1) or more in the GFZ record. 8 reached Kp 8 (G4).
What was the strongest geomagnetic storm of 1940?: The highest 3-hour Kp was 9, first reached on March 24, 1940.
How far south could aurora reach in 1940?: At Kp 9 SWPC's rule puts the average oval edge near 48.0 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.