In this eyewitness account of the Trinity test, carried out at Alamogordo, New Mexico, on July 16, 1945, physicist Luis W. Alvarez documented the explosion from his perch between the pilot and co-pilot in a B-29 flying near the blast.
Alvarez, who worked on the bomb's detonators, was also a observer on a B-29 that flew in formation with the Enola Gay when it bombed Hiroshima in August, 1945. Physicist Lawrence Johnston, then Alvarez’s assistant, accompanied him on both missions. Johnston remembered that Alvarez got the observer job after telling J. Robert Oppenheimer
that he would like to get involved in the Pacific operations…Oppenheimer needed someone to measure the explosive energy of the bombs which were to be dropped on Japan, and Luie [Luis] took the job in April 1945.
The Trinity test was meant to be a practice run for the observation at Hiroshima. Johnston recalled that Alvarez had wanted to fly in a B-29 directly behind the plane that dropped the bomb, so that he could release parachute-borne microphones that would record changes in pressure and send them telemetrically to receivers in the plane. At the last minute, Oppenheimer decided that this plan was too dangerous, and demanded that the group stay 25 miles distant.
As might be expected, Alvarez’ account and his sketches, which were classified until the 1960s, are strictly scientific in nature. While Johnston wrote, in his later memory of the experience, of his relief that their detonators worked, Alvarez made no comment here on anything beyond the exact look of the cloud in all of its phases.