Fireball of H-bomb explosion after test blast over Bikini Atoll. (Photo by Time Life Pictures/Department Of Defence (DOD)/Time Life Pictures/Getty Images). The location of this image is Marshall Islands. Copyright: Time & Life Pictures

The energy released from a nuclear weapon detonated in the troposphere can be divided into four basic categories:
However, depending on the design of the weapon and the environment in which it is detonated the energy distributed to these categories can be increased or decreased. The blast effect is created by the coupling of immense amounts of energy, spanning the electromagnetic spectrum , with the surroundings. Locations such as submarine, surface, airburst, or exo-atmospheric determine how much energy is produced at blast and how much as radiation. In general, denser media around the bomb, like water, absorb more energy, and create more powerful shockwaves while at the same time limiting the area of its effect.
The dominant effects of a nuclear weapon where people are likely to be affected (blast and thermal radiation) are identical physical damage mechanisms to conventional explosives . However, the energy produced by a nuclear explosive is millions of times more powerful per gram and the temperatures reached are briefly in the tens of millions of degrees.
Energy from a nuclear explosive is initially released in several forms of penetrating radiation. When there is a surrounding material such as air, rock, or water, this radiation interacts with and rapidly heats it to an equilibrium temperature (i.e. so that the matter is at the same temperature as the atomic bomb's matter). This causes vaporization of surrounding material resulting in its rapid expansion. Kinetic energy created by this expansion contributes to the formation of a shockwave. When a nuclear detonation occurs in air near sea level, much of the released energy interacts with the atmosphere and creates a shockwave which expands spherically from the hypocenter. Intense thermal radiation at the hypocenter forms a fireball and if the burst is low enough, its often associated mushroom cloud . In a burst at high altitudes, where the air density is low, more energy is released as ionizing gamma radiation and x-rays than an atmosphere-displacing shockwave.
In 1942 there was some initial speculation among the scientists developing the first nuclear weapons that there might be a possibility of igniting the Earth's atmosphere with a large enough nuclear explosion. This would concern a nuclear reaction of two nitrogen atoms forming a carbon and an oxygen atom, with release of energy. This energy would heat up the remaining nitrogen enough to keep the reaction going until all nitrogen atoms were consumed. This was, however, quickly shown to be impossible, due to inverse Compton effect cooling of the fireball. Nevertheless, the notion has persisted as a rumour for many years.

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