Eyewitness Report by Victor Weisskopf

INTEROFFICE MEMORANDUM, July 24, 1945

To: Lt. Taylor
From: V. Weisskopf

“You have asked me to submit an eyewitness account of the explosion. I was at base camp and watched the phenomenon from a little ridge about 100 yds—east of the water tower. Groups of observers had arranged small wooden sticks at a distance of 10 yds. from our observation place to estimate the explosion’s size. They were set so that their distance corresponded to 1000 ft. at zero point. I looked at the blast through the dark glass but provided an indirect view of the landscape to see the deflected light.

When the explosion went off, I was first dazzled by this indirect light which was much stronger than I anticipated, and I could not concentrate upon the view through the dark glass and missed, therefore, the first stages of the explosion. When I could look through the opaque glass, I saw flames and smoke of an estimated diameter of 1000 yds, slowly decreasing in brightness, seemingly due to more smoke development. At the same time, it rose slightly above the surface. After about three seconds, its intensity was so low I could remove the dark glass and look at it directly. Then I saw a reddish glowing smoke ball rising with a thick stem of dark brown color. This smoke ball was surrounded by a blue glow that indicated intense radioactivity and was undoubtedly due to the gamma rays emitted by the cloud into the surrounding air. At that moment, the cloud had about 1000 billion curies of radioactivity whose radiation must have produced the blue glow.

For the first two or three seconds, I felt very strongly the heat radiation all over the exposed parts of my body. The amount of my retina exposed to the indirect light from the surrounding mountains was blinded entirely, and I could feel traces of the afterimage 30 minutes after the shock.

The reddish cloud darkened after about 10 or 20 seconds and rose rapidly, leaving behind a thick stem of dark brown smoke. After this, I remember seeing a white hemisphere rising above the clouds in continuation of the breakthrough of the explosion cloud through the ordinary cloud level.

The shock wave path through the clouds was visible as an expanding circle all over the sky where clouds covered it.

After about 45 seconds, the sound wave arrived, and it struck me as being much weaker than anticipated.”

V. Weisskopf

Source: U.S. National Archives, Record Group 227, OSRD-S1 Committee, Box 82, folder 6, “Trinity.” Transcription: Thank you, Gene Dannen, for transcribing this document.