<P> The heat of the explosion causes air in the vicinity to become ionized, creating the fireball . The free electrons in the fireball affect radio waves, especially at lower frequencies . This causes a large area of the sky to become opaque to radar, especially those operating in the VHF and UHF frequencies, which is common for long - range early warning radars . The effect is less for higher frequencies in the microwave region, as well as lasting a shorter time--the effect falls off both in strength and the affected frequencies as the fireball cools and the electrons begin to re-form onto free nuclei . </P> <P> A second blackout effect is caused by the emission of beta particles from the fission products . These can travel long distances, following the Earth's magnetic field lines . When they reach the upper atmosphere they cause ionization similar to the fireball, but over a wider area . Calculations demonstrate that one megaton of fission, typical of a two megaton H - bomb, will create enough beta radiation to black out an area 400 kilometres (250 mi) across for five minutes . Careful selection of the burst altitudes and locations can produce an extremely effective radar - blanking effect . </P> <P> The physical effects giving rise to blackout are those that also cause EMP, which itself can cause power blackouts . The two effects are otherwise unrelated, and the similar naming can be confusing . </P> <P> About 5% of the energy released in a nuclear air burst is in the form of ionizing radiation: neutrons, gamma rays, alpha particles and electrons moving at speeds up to the speed of light . Gamma rays are high energy electromagnetic radiation; the others are particles that move slower than light . The neutrons result almost exclusively from the fission and fusion reactions, while the initial gamma radiation includes that arising from these reactions as well as that resulting from the decay of short - lived fission products . </P>

How much damage can the biggest nuclear bomb do