When the Trinity test occurred early in the morning of 16 July 1945, it created a fireball whose temperature reached 8430 kelvin, or 14,710 F, hotter than the sun’s surface temperature (5778 K). Enrico Fermi estimated that the explosive yield was 10 kilotons (TNT equivalent), but that was incorrect by a factor of 2 and Groves later reported that the yield was between 15 and 20 kilotons. In recent years, the Department of Energy’s official estimate became 21 kilotons, although a recent unofficial assessment postulates 24.8 kilotons. Whatever the exact yield was, the explosion spewed dust and other particles, including unfissioned plutonium, into the air and the fallout spread northeast of Ground Zero.
After the test occurred there were worrisome moments. On 16 July 1945, radiation monitor Arthur Breslow wrote that there was the “danger of an immediate evacuation and the [radiation] count was rising rapidly.” He decided otherwise, and the consensus was that radiation levels were not high enough to necessitate evacuation. The special detachment was disbanded. All the same, a few days after the test, Dr. Stafford Warren wrote to General Groves that “the dust outfall from the various portions of the cloud was potentially a very serious hazard over a band almost 30 miles wide extending almost 90 miles northeast of the site.”
Warren and others discovered that pre-test organizers had overlooked nearby people who were at risk of over-exposure. Notably, the Ratliff family, grandparents and their grandson, lived near a “Hot Canyon” where radiation levels were high. Los Alamos medical staffers would visit this downwinder family and a few others during the months that followed the test and quietly checked on them. They concluded that they were healthy, although cattle and other farm animals in the area, including the Chupadera Mesa, had been exposed to fallout and experienced burns on their skin because they had been out in the open. The cattle would be the subject of clinical studies. Unnoticed, however, was that other downwinders, including young girls at nearby camp, had also been exposed to dangerous levels of fallout.
Six weeks after the test, Victor Weisskopf and other radiation experts corroborated Warren’s findings about the scope of fallout exposure. They found a “swath of fairly high radioactivity on the ground covering an area of about 100 miles long by 30 miles wide” [See Document 23]. In a likely reference to the Ratliffs, they reported that, “One ranch house east of Bingham received an initial radiation intensity calculated to be 7 r/hr [roentgen per hour],” although the intensities reached “tolerance” within a month. Moreover, after the test, “gamma radiation was found in measurable but very low intensities in Santa Fe, Las Vegas, Raton and even in Trinidad, Colorado (260 miles from zero point).” The report concluded that the radiation levels near the ranch house and elsewhere were not high enough to be dangerous.