Jean M. Bele

Physics Dept., Laboratory for Nuclear Science, MIT

• A typical nuclear weapon detonation produces a huge number of X-rays, which heat the air around the detonation to extremely high temperatures, causing the heated air to expand and form a large fireball within less than one millionth of one second of the weapon's detonation. The size of the nuclear fireball is a function of yield, the height of burst, and the surrounding environment.
• The nuclear fireball is tens of millions of degrees (i.e., as hot as the interior of the sun). Inside the fireball, the temperature and pressure cause a complete desintegration of molecules and atoms.
  For further information see Nuclear weapons effects

• Related to the fireball size is the question of the height of burst at which early (or local) fallout ceases to be a serious problem. (¹).

For example, an explosion of 1000 kilotons(1 megaton yield), it can be found from our calculator that significant local fallout is probable for heights of burst less than about 2,900 feet or 870 meters. 

• The fallout is the radioactive products  when they settle to the ground some time after the nuclear explosion. Radioactive fallout contaminate large areas and is an immediate and extreme biological hazard 

¹Samuel Glasstone and Philip J. Dolan:The Effects of Nuclear Weapons, Prepared and published by the UNITED STATES DEPARTMENT OF DEFENSE and the UNITED STATES DEPARTMENT OF ENERGY

The size of the bomb can be chosen by selecting the weapon’s yield, as measured in kilotons.

Enter the explosion yield in kilotons: Detonate