G5 (Extreme) Geomagnetic Storm of May 11, 1938
A G5 (extreme) geomagnetic storm began May 11, 1938, reached Kp 9- and lasted 18 hours, with a daily Ap as high as 103; at that Kp the aurora may be seen from about 42 degrees geomagnetic latitude.
The Kp through the storm
Began May 11, 15:00 UTC, peaked in the block from May 11, 18:00 UTC, ended May 12, 09:00 UTC.
The storm
- Peak Kp
- 9- (8.67)
- NOAA level
- G5 Extreme
- Duration
- 18 hours
- Highest daily Ap
- 103
- Highest 3-hour ap
- 300
- Rank since 1932
- 60 of 4,139
How far south the aurora could have been seen
At Kp 9- NOAA’s scales put the aurora’s reach at about 42 degrees geomagnetic latitude: 33°N in the central US, 32°N in the eastern US, 36°N in the western US and 42°N in northern Europe.
NOAA says a G5 storm’s aurora has been seen as low as Florida and southern Texas.
US cities within reach, the southernmost first
- Salinas, CA 42.4° geomag.
- Birmingham, AL 42.4° geomag.
- Albuquerque, NM 42.5° geomag.
- Henderson, NV 42.6° geomag.
- Spring Valley, NV 42.6° geomag.
- Paradise, NV 42.6° geomag.
- Fresno, CA 42.7° geomag.
- Las Vegas, NV 42.7° geomag.
- North Las Vegas, NV 42.7° geomag.
- Sunrise Manor, NV 42.7° geomag.
122 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 11, 1938 | 103 | 9- |
| May 12, 1938 | 76 | 7+ |
More
The storm before, May 4The storm after, May 12Every storm of 1938The 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.