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"Cloud Gone" – Two Words In The Hiroshima Bombing Logs May Have Some Disastrous Implications For Humanity | IFLScience Researchers examining the flight logs of the Enola Gay as it dropped an atomic bomb on the city of Hiroshima at the end of World War II have homed in on a two-word phrase that may have devastating implications for humanity. On August 6, 1945, at 8:15 am local time, the US detonated "Little Boy" on the Japanese city of Hiroshima, the first time an atomic bomb had been used in warfare. Prior to the act, when the bomb was still in development by the Manhattan Project, J. Robert Oppenheimer and other physicists had worried that the bomb could cause a chain reaction in the atmosphere which would quite rapidly lead to the demise of humanity, along with all the animal and plant life at the whims of human affairs. "Hydrogen nuclei," American physicist Arthur Compton, whom Oppenheimer consulted on the topic, explained in 1959, "are unstable, and they can combine into helium nuclei with a large release of energy, as they do on the sun. To set off such a reaction would require a very high temperature, but might not the enormously high temperature of the atomic bomb be just what was needed to explode hydrogen?" The risks, for Oppenheimer, did not end there. "And if hydrogen, what about hydrogen in sea water? Might not the explosion of the atomic bomb set off an explosion of the ocean itself? Nor was this all that Oppenheimer feared. The nitrogen in the air is also unstable, though in less degree. Might not it, too, be set off by an atomic explosion in the atmosphere?" Nuclear winter Ahead of the bombs being dropped, Oppenheimer was confident that this could not happen in atmospheric conditions, with radiative cooling taking place much too quickly for a reaction to be sustained. And of course, the atmosphere did not catch on fire when the bomb was actually dropped. But that wasn't the only thing that we had to worry about when dropping a giant nuclear weapon. One worry is, of course, the prospect of nuclear winter – a hypothesized period of global cooling after large-scale nuclear war sparks fires and draws carbon and soot into the upper atmosphere, which then blocks and absorbs incoming sunlight. It isn't altogether clear-cut that that would happen following larger-scale nuclear warfare, though scientists do suspect that it is a possibility. The problem is (thank god and/or Stanislav Petrov) that we don't have real-world nuclear explosions to examine, other than the giant plume that was observed over Hiroshima in 1945. Attempts have been made to estimate the size of the pyrocumulus plume – the convective column left behind after the initial mushroom cloud had dissipated, and resulting from the fires down below – by analyzing photographs of it. Some studies have even suggested that the vast majority of black carbon would never make it to the upper atmosphere, where it would have this cooling effect. Rather than an interesting historical query, it could tell us whether nuclear warfare will kill millions of innocent civilians, or pose existential threats to all of us. "Cloud gone" In a new study, researchers looked at the flight logs and descriptions of the crew of the Enola Gay and accompanying aircraft for further clues. Rather than get out of there immediately, the crew remained briefly to observe and photograph the resulting explosion. “I could see the city, and it was being covered with this low, bubbling mass. It looked like bubbling molasses, let's say, spreading out and running up into the foothills, just covering the whole city," tail-gunner George Caron described of the scene. "And fires, I could see fires spring up through this under cast, or whatever you would call it, that was covering the city. Flames in different spots would be springing up.” Of particular interest to the latest team was the two-word phrase "cloud gone" written at 10:52 am, as the aircraft was around 750-765 kilometers (466–475 miles) away, and at an altitude of around 7.6 kilometers (25,000 feet). As a memorandum stated that this “observation was then limited by haze and not curvature of the Earth", this meant that the top of the plume had not dipped below the horizon, allowing the team to estimate a lower bound on its height. Consistent with nuclear winter scenarios This is made a little more complicated by weather conditions, which can alter how much light bends as it travels through the atmosphere. Modeling various scenarios using historical weather data, which affected how far the light from the plume traveled, the team attempted to work out how much the light would have bent on that day in 1945. After running 100,000 simulations, they were able to estimate a lower bound of how tall the plume was. "Visual contact was lost due to haze rather than to the plume fully dipping behind the horizon. Since we know the plume was taller than this lower bound, and since we can say that the lower bound exceeds 11.3 km [7 miles] with 95 percent confidence, we can say that the plume was taller than 11.3 km with better than 95 percent confidence," the team explains in their paper. "Furthermore, the median lower bound is 12.1 km [7.5 miles], so we can say that the plume top was higher than 12.1 km with better than even odds. On the other hand, the 'cloud gone' observation cannot exclude the possibility of the plume being much higher; the observation places no upper bound on the plume height." That height, the team says, is "consistent with nuclear-winter scenarios" where enough material is delivered into the upper atmosphere to cause global cooling. While this modeling involved a number of assumptions, it is the Hiroshima bombing is the only real-world nuclear explosion we have to go off, with the Nagasaki bomb not producing the same firestorm seen in Hiroshima. And it appears that the results were not good; nuclear winter remains a possibility for any species stupid enough to engage in a nuclear war. Fingers crossed we never test again whether this modeling holds up. iflscience.com/cloud-gone-tw…