San Francisco, CaliforniaFebruary 1953

On the evening of February 13, 1953, four members of a Ground Observer Corps post on top of Mount Sutro in San Francisco watched an unidentified flying object for about twenty minutes. The object was round and black with what appeared to be two silver streaks trailing behind it. It moved at a moderately high speed, maneuvering up and down in a zigzag pattern. When the object passed directly overhead the observers, they heard a deep throbbing sound. The sighting began around 1758 (5:58 p.m.) when the object appeared about four miles out over the ocean, heading inland. It disappeared from view over the city of San Francisco around 1820 (6:20 p.m.). The observers estimated the object was flying at approximately 10,000 feet altitude. The weather that evening was clear with unlimited visibility.

The same evening, thousands of other people across the San Francisco Bay Area reported seeing similar objects. A newspaper article from the San Francisco Examiner quoted astronomer Dr. Otto Struve, an astronomer with the University of California, as explaining the sightings as caused by a temperature inversion, a weather phenomenon in which a layer of warm air becomes trapped between two layers of cold air. According to Struve, such an inversion acts as a reflector of light and can create unusual optical effects in the sky. The Weather Bureau confirmed that a mass of warm air had indeed formed between two layers of cold air over the Bay Area at approximately 3,200 feet altitude during the time of the sightings. Many of the witnesses placed the object between 3,000 and 4,000 feet, lower than the Ground Observer Corps estimate.

Despite the plausible weather explanation offered by the astronomer and weather officials, the U.S. Air Force classified the sighting as "unidentified." The file does not explain why the temperature inversion theory was not accepted as the explanation. The full case file is reproduced below as held by the National Archives across 22 pages.