page_title: Nuclear blackout
section_title: Within the atmosphere
hierarchical_section_title: Nuclear blackout / Bomb effects / Within the atmosphere
caption_attribution_description: Operation Hardtack/Lea. Rapatronic picture.
context_page_description: Nuclear blackout, also known as fireball blackout or radar blackout, is an effect caused by explosions of nuclear weapons that disturbs radio communications and causes radar systems to be blacked out or heavily refracted so they can no longer be used for accurate tracking and guidance. Within the atmosphere, the effect is caused by the large volume of ionized air created by the energy of the explosion, while above the atmosphere it is due to the action of high-energy beta particles released from the decaying bomb debris. At high altitudes, the effect can spread over large areas, hundreds of kilometers. The effect slowly fades as the fireball dissipates.
  The effect was known from the earliest days of nuclear testing when radar systems were used to track the nuclear mushroom clouds at very long distances. Its extended effects when exploded outside the atmosphere were first noticed in 1958 as part of the Hardtack and Argus nuclear tests, which caused widespread radio interference extending over thousands of kilometers.
context_section_description: When a nuclear bomb is exploded near ground level, the dense atmosphere interacts with many of the subatomic particles being released. This normally takes place within a short distance, on the order of meters. This energy heats the air, promptly ionizing it to incandescence and causing a roughly spherical fireball to form within microseconds.
  Proceeding at a slower speed is the actual explosion, which creates a powerful shock wave moving outward. The energy released by the shock wave is enough to compression heat the air into incandescence, creating a second fireball. This second fireball continues to expand, passing the radiative one. As it expands, the amount of energy in the shock wave drops according to the inverse-square law, while additional energy is lost through direct radiation in the visible and ultraviolet spectrum. Eventually the shock wave loses so much energy that it no longer heats the air enough to cause it to glow. At that point, known as breakaway, the shock front becomes transparent, and the fireball stops growing.
  The diameter of the fireball for a bomb exploded clear of the ground can be estimated using the formula:
   kilometers
  Where  is the yield in megatons, and  is the ratio of the sea level air density to the air density at altitude. So, a 1 megatonne of TNT (4.2 PJ) bomb exploded at a burst altitude around 5,000 feet (1,500 m) will expand to about 1 kilometre (3,300 ft). The ratio  can be calculated over a wide range by assuming an exponential relationship:
  where  is the altitude of the burst in feet. So the same burst at 50,000 feet (15,000 m) will be at a pressure of about 0.1 atmospheres, resulting in a fireball on the order of 2,150 metres (7,050 ft) in diameter, about twice the size of one near the ground. For a high altitude burst, say 250,000 feet (76 km), the fireball will expand to about 46 kilometres (29 mi) in diameter.
caption: This image of the Hardtack II Lea test shot was taken milliseconds after detonation. The radiative fireball has already formed and the expanding shock wave is continuing the expansion. The spikes at the bottom are due to the rope trick effect.
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