Project Blue Book File: Kenneth Arnold's 1947 UFO Sighting a

Project Blue Book File: Kenneth Arnold's 1947 UFO Sighting and the Air Force's Temperature Inversion Explanation

A declassified Project Blue Book record details pilot Kenneth Arnold's 1947 sighting of nine mirror-like objects near Mount Rainier, timed at 1,656.71 mph, and the Air Force's official conclusion that the phenomenon was caused by a temperature inversion.

Published

October 5, 2026

Declassified

Date not listed

Source agency

The Black Vault (FOIA archive)

ufofoiaprojectbluebookkennetharnold1947

Kenneth Arnold's 1947 Mount Rainier UFO Sighting File

The Project Book file preserved in The Black Vault's FOIA archive presents a document that is as much an artifact of bureaucratic record-keeping as it is a chronicle of one of the most consequential aerial observations in American history. The record, identified in the archive as the Kenneth Arnold file, centers on a sighting report form — Form TDS SEP 630-329 — whose structured fields demand a conclusion, a count of objects, a length of observation, a type of observation, a course, and notations on whether photographs or physical evidence existed.

71 miles per hour. The record further describes the objects as mirror-like, reflecting the sun, and estimates them at roughly two-thirds the size of a DC-4. Against that detailed witness testimony, the file's stated Air Force conclusion is starkly reductive: the objects were attributed to a temperature inversion.

That juxtaposition — a meticulous pilot narrative set beside a single-line…

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Based on the official release at https://www.theblackvault.com/documentarchive/project-blue-book-the-kenneth-arnold-ufo-sighting-mt-rainier-washington-june-24-1947/

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