A nuclear explosion is the very rapid release of a large amount of energy in a small space, [1] and it unfolds in a fixed order. A fireball forms in under a millionth of a second; a flash of light and heat races out at the speed of light; a shock wave follows through the air; and a mushroom cloud rises for about ten minutes. The times and distances below are for a 1-megaton air burst, from the standard reference The Effects of Nuclear Weapons (Glasstone & Dolan).
What happens, in order
In the first instant, the weapon's energy forms a fireball — a spherical mass of incandescent air and bomb residues — in less than a millionth of a second. [1] Within about 0.7 millisecond the fireball of a 1-megaton burst is roughly 440 feet across; by 10 seconds it has grown to about 5,700 feet. [1] The flash of light and heat leaves at the speed of light, [1] so it reaches the eyes before anything else.
A shock front races ahead of the fireball: about 3 miles out at 10 seconds and about 12 miles out at 50 seconds, by then moving at roughly 1,150 feet per second, a little faster than sound at sea level. [1] Initial nuclear radiation — the gamma rays and neutrons released in roughly the first minute — arrives over the same interval. [1] The cloud top reaches about 3 miles at 30 seconds and about 5 miles within a minute, climbing to its maximum height after about 10 minutes. [1]
How hot it gets
At the first instant the weapon residues reach several tens of millions of degrees — hotter than the centre of the Sun. [1] That heat is what forms the fireball and drives the thermal flash. As the fireball expands and cools it radiates most of its energy as light and infrared, the pulse that causes burns and starts fires at a distance.
How big it gets
The fireball of a 1-megaton burst grows to about 5,700 feet across within 10 seconds, [1] but the destructive radii reach much further. How far the blast and heat extend depends on yield, and for that the nuclear blast radius calculator computes the fireball, blast and thermal rings for any weapon from 1 kiloton to 50 megatons on a map of any city.
The sound and the shock wave
The shock wave is what people hear as the explosion, and it arrives well after the flash because it travels at roughly the speed of sound rather than the speed of light. By 50 seconds after a 1-megaton burst the shock front is about 12 miles out and moving at about 1,150 feet per second. [1] Closer in, it arrives in seconds: roughly 1.4 seconds at 1 mile and 4.5 seconds at 2 miles. [1] Because the flash arrives first, someone a few miles away sees the light, then feels the blast seconds later.
Underwater and in space
Not every nuclear explosion is an air burst over a city. In the 1946 Crossroads Baker test, a 21-kiloton device was detonated 90 feet underwater at Bikini Atoll on 25 July (local time), throwing up a column of water and a base surge of radioactive spray. [2, 3] (The US Navy history office gives the yield as 23 kilotons; this page uses the 21-kiloton figure from the DOE test catalogue. [4])
A nuclear explosion in space behaves differently again. On 9 July 1962 (UTC) the Starfish Prime test detonated a 1.4-megaton warhead about 400 km above Johnston Island in the Pacific. [5, 6] With almost no air to carry a shock wave, the main effect was a powerful electromagnetic pulse — the subject of the EMP article, which covers a nuclear explosion in space in detail.
How bright the flash is
The flash of a nuclear explosion is briefly brighter than the Sun and, because light travels at the speed of light, [1] it reaches an observer the instant the fireball forms — long before the sound. At close range the flash can cause temporary flash blindness or permanent retinal burns in anyone looking toward it, which is why it can be seen and felt at far greater distances than the blast damage itself.
What causes a nuclear explosion
A nuclear explosion is powered by one of two nuclear reactions: fission, the splitting of heavy atoms, or fusion, the joining together of light ones. [1] In a fission weapon the energy comes from a self-sustaining chain reaction in uranium or plutonium; [1] a hydrogen bomb adds a fusion stage. For how the two designs work, see how nuclear weapons work. This page describes only the one-line concept, not any weapon design detail.
Where the energy goes
For a fission weapon exploded in the air, the energy divides into about 50% blast, 35% thermal radiation, 5% initial nuclear radiation and 10% residual radiation. [1] That split is why blast and heat dominate the effects of an air burst, while the residual radiation that becomes fallout is a larger factor for a surface burst.