The Height Of Hiroshima's Mushroom Cloud

The atomic bombing of Hiroshima on 6 August 1945 was the first time a city had ever been targeted by a nuclear weapon. The bomb, nicknamed 'Little Boy', was dropped from the B-29 Superfortress Enola Gay, exploding at 8:15 am local time at an altitude of around 31,000 feet (9,400 m). The resulting mushroom cloud rose rapidly, reaching over 60,000 feet (18,000 m) in about ten minutes. The cloud contained highly radioactive particles, which remained suspended in the atmosphere even as the water vapour condensed and evaporated.

The Hiroshima mushroom cloud photo

The photograph shows the mushroom cloud as it was seen from Kaitaichi, now part of present-day Kaita, located six miles east of Hiroshima's centre. Reprints of the image appeared in a 1988 Japanese-language publication, but the whereabouts of the original remained unknown for many years. There are only a handful of other photos in existence that capture the cloud from a similar perspective. One notable photograph taken from the Enola Gay aircraft provides a clear view of the separated tiers of the cloud as it flew away from the blast zone.

The person who took the Hiroshima mushroom cloud photo was likely among the first to realise the unprecedented scale and nature of the explosion. It was not a conventional air raid but something far more destructive and fearsome. The photo captures the moment when the citizens of Hiroshima and the world began to comprehend the existence of a bomb unlike any other in human history. The image serves as a stark reminder of the immense power and devastating consequences of nuclear weapons.

How the cloud formed

The iconic mushroom cloud that formed over Hiroshima was the result of a complex interplay of physics and chemistry following the detonation of the atomic bomb. While the specific details of the cloud's formation may vary depending on factors such as altitude, yield of the explosion, and local atmospheric conditions, there are several key processes that contribute to the formation of a mushroom cloud.

Firstly, immediately after the detonation of a nuclear weapon, a rapidly expanding fireball is created. This fireball, consisting of hot, incandescent gases, rises due to buoyancy, similar to how a hot air balloon ascends. As the fireball ascends, it begins to change shape due to atmospheric friction and Rayleigh-Taylor instability. The cool air underneath pushes the bottom gases of the fireball upwards, forming an inverted cup shape, which further evolves into a violently rotating spheroidal vortex.

As the vortex forms, strong air currents known as "afterwinds" are produced. These afterwinds draw in air, smoke, debris, dirt, and condensed water vapour upwards into the cloud, creating a temporary vortex ring with a central column—the "stem" of the mushroom cloud. The colour of the cloud can vary, initially appearing reddish-brown due to the presence of nitrogen dioxide and nitric acid, and later turning white due to water/ice clouds condensing in the fast-flowing air.

The ascent of the mushroom cloud continues until the mass of hot gases reaches its equilibrium level, where it is no longer less dense than the surrounding air. At this point, the cloud stops rising and begins to flatten and spread out, forming the characteristic "cap" of the mushroom shape, often with the assistance of surface growth from decaying turbulence. The height achieved by the mushroom cloud over Hiroshima was remarkable, reaching over 60,000 feet in approximately ten minutes.

The formation and evolution of a mushroom cloud involve multiple phases and are influenced by various factors. While the specific details of the Hiroshima cloud's formation may not be fully known, the fundamental processes described above provide a framework for understanding the creation of one of the most destructive and iconic images of the 20th century.

The cloud's colour

The Hiroshima mushroom cloud, which resulted from the explosion of the world's first atomic bomb, has been described in various ways. The cloud is widely thought to have been white or grey, potentially due to the black-and-white photographs of the cloud. However, some have described the cloud as having "strange, lovely colours like a rainbow", with orange, violet, red, and yellow hues also being mentioned.

The varying descriptions of the cloud's colour may be due to the changing nature of the cloud itself. The initial colour of some radioactive clouds is reddish-brown due to the presence of nitrogen dioxide and nitric acid, formed from initially ionized nitrogen, oxygen, and atmospheric moisture. As the fireball expands, the temperature at its centre drops, causing the colour to shift to red and yellow, which become visible through the cloud. Eventually, when the heat dissipates, the cloud turns white, although it may appear darker when mixed with dirt and sand from the ground.

The Hiroshima cloud rose to over 60,000 feet in about ten minutes, and the city was engulfed in thick black smoke. The cloud and its shadow were used to estimate the time the photograph was taken, indicating that it was taken just before noon, about three hours after the bomb detonated.

The image of the Hiroshima mushroom cloud has become a powerful symbol of the destruction caused by nuclear weapons. It represents a moment in history when the world witnessed the devastating capabilities of a new and fearsome type of bomb, unlike anything ever seen before.

The cloud's height

The cloud that formed over Hiroshima after the atomic bomb exploded on August 6, 1945, is often referred to as a mushroom cloud. However, nuclear experts claim that the image that has become symbolic of the destruction of Hiroshima is not a mushroom cloud but rather billowing smoke from the raging firestorm that followed.

The cloud, which was photographed from the Enola Gay plane, had risen to a height of 45,000 feet within a few minutes of the explosion and was still rising. The plane was flying at 30,000 feet, and the cloud had already risen above them. The cloud is estimated to have risen to over 60,000 feet in about ten minutes.

The formation of a mushroom cloud occurs in several phases. The explosion raises a large amount of moisture-laden air from lower altitudes. As the air rises, its temperature drops, and water vapour condenses as water droplets and later freezes as ice crystals. The phase change releases latent heat, heating the cloud and driving it to higher altitudes. The heads of these clouds consist of highly radioactive particles, primarily fission products and other weapon debris aerosols. The largest and most radioactive particles are deposited by fallout in the first few hours after the blast.

The initial colour of some radioactive clouds can be reddish-brown due to the presence of nitrogen dioxide and nitric acid. This reddish hue is later obscured by the white colour of water/ice clouds and the dark colour of smoke and debris. The distribution of radiation in the mushroom cloud varies with the yield of the explosion, the type of weapon, and the fusion-fission ratio.

The aftermath

On the morning of August 6, 1945, an American B-29 bomber, the Enola Gay, dropped an atomic bomb on the Japanese city of Hiroshima. The blast immediately killed thousands and injured countless others. The city was completely engulfed in a thick black smoke, with fires spreading and winds picking up, creating tornado-like conditions. The mushroom cloud rose to over 60,000 feet in about ten minutes.

Reverend Kiyoshi Tanimoto, who had remained in the city despite sending his wife and baby to a northern suburb, recalled seeing "a tremendous flash of light cut across the sky...from the city towards the hills." He had a few seconds to throw himself between two large rocks in a friend's garden, but the house collapsed anyway, along with the surrounding concrete wall.

Father John A. Siemes, a Catholic priest living about two kilometers away, described the scene as one of chaos: "Everything is on fire, everyone is confused. A lot of people are dead. A lot of people are dead but don't know it yet—they've been exposed to fatal amounts of radiation."

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