G4 (Severe) Geomagnetic Storm of August 15, 1947

August 15 to 23, 1947 · 201 hours

A G4 (severe) geomagnetic storm began August 15, 1947, reached Kp 8+ and lasted 201 hours, with a daily Ap as high as 71; at that Kp the aurora may be seen from about 43 degrees geomagnetic latitude.

The Kp through the storm

Began Aug 15, 09:00 UTC, peaked in the block from Aug 22, 09:00 UTC, ended Aug 23, 18:00 UTC.

From 00 UT the day before to the end of the day after. Blocks in the storm are colored by level; the rest are gray.

The storm

Peak Kp: 8+ (8.33)
NOAA level: G4 Severe
Duration: 201 hours
Highest daily Ap: 71
Highest 3-hour ap: 236
Rank since 1932: 71 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 43 degrees geomagnetic latitude: 35°N in the central US, 34°N in the eastern US, 37°N in the western US and 44°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

San Francisco, CA 43.3° geomag.
Little Rock, AR 43.3° geomag.
Oakland, CA 43.4° geomag.
Modesto, CA 43.4° geomag.
Huntsville, AL 43.6° geomag.
Stockton, CA 43.7° geomag.
Oklahoma City, OK 43.7° geomag.
New South Memphis, TN 43.8° geomag.
Memphis, TN 43.9° geomag.
Santa Rosa, CA 44.0° geomag.

106 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.

Tonight’s aurora forecast

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
August 15, 1947	55	7+
August 16, 1947	54	7+
August 17, 1947	58	6
August 18, 1947	60	6+
August 19, 1947	52	6+
August 20, 1947	44	6
August 21, 1947	41	5+
August 22, 1947	71	8+
August 23, 1947	55	7

The storm before, Aug 13 The storm after, Aug 25 Every storm of 1947 The strongest on record This day in weather history Tonight’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.