Mt. Rainer, WashingtonJune 1947

On June 24, 1947, Kenneth Arnold, a private pilot and fire equipment salesman from Boise, Idaho, spotted nine shiny, disc-shaped objects flying near Mount Rainier in Washington. Arnold was cruising at about 9,500 feet when he noticed a bright flash on his windshield. Looking to the left and north of the mountain, he saw a chain of nine peculiar objects approaching rapidly at what he estimated as 9,500 feet elevation. They were flying from north to south, moving toward the mountain peaks. Arnold thought at first that they might be jet aircraft, but he couldn't find any tails. He watched as they seemed to dip or shift course every few seconds, catching the sun and reflecting brightly. The objects moved past the southern edge of Mount Rainier and down a mountain ridge. Arnold estimated the distance between his plane and the objects to be about 25 miles.

Using a cowling tool as his measuring reference, Arnold guessed that the objects were about two-thirds the size of a DC-4 aircraft. He observed them passing a high snow-covered ridge between Mount Rainier and Mount Adams. Measuring later, that ridge is roughly five miles long, so Arnold estimated the chain of objects was about five miles long. He timed the objects as they traveled between Mount Rainier and Mount Adams, a distance he believed to be 47 miles. His plane's clock showed one minute and forty-two seconds had elapsed. This calculation produced a speed of approximately 1,656.71 miles per hour. Arnold described the objects to reporters that evening as looking like saucers skipping on water. Newspapers shortened this to "flying saucers," and the term caught fire.

The Air Force concluded that what Arnold saw was likely a mirage. Arnold had mentioned that the air was very smooth and crystal clear, conditions that are associated with stable atmospheric inversions, which increase the refraction of light. The file contains multiple analyses of Arnold's observations, including calculations by a scientist showing mathematical inconsistencies in Arnold's estimates of distance and size. If the objects were as far away as Arnold claimed and visible in the detail he described, they would have had to be enormous, over 2,000 feet long. Conversely, if they were reasonably sized at around 400 feet long, they would have been much closer than 20-25 miles away. Some researchers later theorized that Arnold may have observed wave clouds, lenticular formations that can move rapidly in suitable atmospheric conditions and sometimes appear to skip across mountain ridges much like Arnold described.

Arnold profited from his account by selling his story to Fate magazine for publication. The full case file is reproduced below as held by the National Archives, spanning 61 pages of declassified materials.