G4 (Severe) Geomagnetic Storm of May 23, 1940
A G4 (severe) geomagnetic storm began May 23, 1940, reached Kp 8- and lasted 30 hours, with a daily Ap as high as 78; at that Kp the aurora may be seen from about 47 degrees geomagnetic latitude.
The Kp through the storm
Began May 23, 15:00 UTC, peaked in the block from May 24, 09:00 UTC, ended May 24, 21:00 UTC.
The storm
- Peak Kp
- 8- (7.67)
- NOAA level
- G4 Severe
- Duration
- 30 hours
- Highest daily Ap
- 78
- Highest 3-hour ap
- 179
- Rank since 1932
- 240 of 4,139
How far south the aurora could have been seen
At Kp 8- NOAA’s scales put the aurora’s reach at about 47 degrees geomagnetic latitude: 38°N in the central US, 37°N in the eastern US, 41°N in the western US and 47°N in northern Europe.
NOAA says a G4 storm’s aurora has been seen as low as Alabama and northern California.
US cities within reach, the southernmost first
- Richmond, VA 46.7° geomag.
- Lexington, KY 47.0° geomag.
- Lexington-Fayette, KY 47.0° geomag.
- Louisville, KY 47.2° geomag.
- Lakewood, CO 47.3° geomag.
- Denver, CO 47.3° geomag.
- Aurora, CO 47.3° geomag.
- St. Louis, MO 47.3° geomag.
- Overland Park, KS 47.4° geomag.
- Meads, KY 47.5° geomag.
71 US cities of 150,000 people or more sit at or poleward of that line. Low on the northern horizon under a dark, clear sky; not a record of sightings.
Day by day
The daily Ap is the mean of the day’s eight three-hour ap values.
| UT day | Daily Ap | Highest Kp that day |
|---|---|---|
| May 23, 1940 | 18 | 5+ |
| May 24, 1940 | 78 | 8- |
More
The storm before, May 22The storm after, May 26Every storm of 1940The strongest on recordThis day in weather historyTonight’s aurora forecast
Sources
Kp and ap from GFZ Helmholtz Centre for Geosciences, Geomagnetic Observatory Niemegk, licensed CC BY 4.0: Matzka, J., Stolle, C., Yamazaki, Y., Bronkalla, O. and Morschhauser, A., 2021, The geomagnetic Kp index and derived indices of geomagnetic activity, Space Weather, doi:10.1029/2020SW002641; data doi:10.5880/Kp.0001. Storms, levels and aurora reach worked out by WeatherOverTime from those values and NOAA’s space weather scales. Definitive through August 31, 2026; later values are preliminary. Rebuilt 2026-09-24.