G3 (Strong) Geomagnetic Storm of July 21, 1939
A G3 (strong) geomagnetic storm began July 21, 1939, reached Kp 7+ and lasted 6 hours, with a daily Ap as high as 36; at that Kp the aurora may be seen from about 48 degrees geomagnetic latitude.
The Kp through the storm
Began Jul 21, 09:00 UTC, peaked in the block from Jul 21, 12:00 UTC, ended Jul 21, 15:00 UTC.
The storm
- Peak Kp
- 7+ (7.33)
- NOAA level
- G3 Strong
- Duration
- 6 hours
- Highest daily Ap
- 36
- Highest 3-hour ap
- 154
- Rank since 1932
- 395 of 4,139
How far south the aurora could have been seen
At Kp 7+ NOAA’s scales put the aurora’s reach at about 48 degrees geomagnetic latitude: 40°N in the central US, 39°N in the eastern US, 43°N in the western US and 49°N in northern Europe.
NOAA says a G3 storm’s aurora has been seen as low as Illinois and Oregon.
US cities within reach, the southernmost first
- Baltimore, MD 48.5° geomag.
- Indianapolis, IN 48.7° geomag.
- Columbus, OH 49.0° geomag.
- Lincoln, NE 49.1° geomag.
- Philadelphia, PA 49.1° geomag.
- Eugene, OR 49.4° geomag.
- Pittsburgh, PA 49.6° geomag.
- Omaha, NE 49.6° geomag.
- Staten Island, NY 49.8° geomag.
- Brooklyn, NY 49.8° geomag.
53 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 |
|---|---|---|
| July 21, 1939 | 36 | 7+ |
More
The storm before, Jul 19The storm after, Jul 26Every storm of 1939The 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.