Overall Blast EffectsThe first thing bomb victims experience is the intense flux of photons from the blast, which releases 70-80% of the bomb's energy. The effects go up to third degree burns. Initial deaths are often due to this effect. The next phenomenon is the supersonic blast front. The pressure front has the effect of blowing away anything in its path. After the front comes the overpressure phase. The effect is similar to being a few hundred meters underwater. The pressure gradually diminishes and there is a negative overpressure phase with a reversed blast wind. This reversal is due to the air rushing back in to fill the void left by the explosion. The air gradually returns to normal atmospheric pressure. At this stage, fires caused by electrical destruction often ignite debris turning much of the blast area into a firestorm.
Atmospheric EffectsThe Mushroom CloudThe heat from fusion and fission instantaneously raises the surrounding air to 10 million degrees celsius. This superheated air gives off so much light that it is brighter than the sun and is visible for hundreds of kilometers. The resultant fireball quickly expands, rising at the rate of a few hundred meters per second. After approximately a minute, the fireball has risen several kilometers and has cooled to the extent that it no longer radiates. During expansion, the surrounding cooler air exerts some drag on the rising air which slows the outer edges of the cloud. The unimpeded inner portion rises quicker than the surrounding edges. A vacuum effect occurs when the outer portion occupies the vacuum left by the higher inner portion. The result is a smoke ring. The inner material gradually expands out into a mushroom cloud due to convection. If the explosion occurs close enough to the ground, debris can be sucked up the stem.
Collisions and ionization of the cloud particles result in lightning bolts flickering to the ground. Initially the cloud is orange-red due to a chemical reaction when the air is superheated. As the cloud cools to the surrounding air temperature, water vapor starts to condense turning the clound from red to white. In the final stages, the cloud can reach 100 kilometers wide and 40 kilometers high (for a megaton class explosion).
Electromagnetic Pulse (EMP)A nuclear explosion gives off radiation at all wavelengths of light. Some is in the radio / radar portion of the spectrum. This is known as the EMP effect. The strength of the EMP effect is in part related to the altitude of the initial explosion. The higher the explosion occurs in the atmosphere, the stronger the effect. High altitude explosions can damage electronics by inducing a current surge in closed circuit metallic objects. The damage range can extend to over 1,000 kilometers.Immediate EffectsThe three categories of immediate effects are: blast, thermal radiation (heat), and prompt ionizing of nuclear radiation.
Thermal RadiationThe fraction of a bomb's yield emitted as thermal radiation blast and ionizing radiation if essentially constant for all yields, but the way the different forms of energy interact with air and objects vary dramatically. Air is essentially transparent to thermal radiation. The thermal radiation affects exposed surfaces, producing damage by rapid heating. A bomb that is 100 times larger can produce equal thermal radiation intensities over areas 100 times larger. The area of an (imaginary) sphere centered on the explosion increases with the square of the radius. Thus the destructive radius increases with the square root of the yield (Actually the rate of increase is somewhat less partly due to the fact that larger bombs emit heat more slowly which reduces the damage produced by each calorie of heat). It is important to note that the area subjected to damage by thermal radiation increases almost linearly which yield.
Blast Effect Blast effect is a volume effect. The blast wave deposits energy in the material it passes through, including air. When the blast wave passes through solid material, the energy left behind causes damage. The more matter the energy travels through, the smaller the effect. The amount of matter increases with the volume of the imaginary sphere centered on the explosion. Blast effects thus scale with the inverse cube law which relates radius to volume.
Ionizing RadiationThe intensity of nuclear radiation decreases with the inverse square law much like thermal radiation. However, nuclear radiation is also strongly absorbed by teh air it travels though, which causes the intensity to drop off much more rapidly.
CasualtiesThese scaling laws show the effects of thermal radiation grow rapidly with yield (relative to blast), while those of radiation rapidly decline. In a small nuclear attack (bomb yield approximately 15kT) causualties would be seen from all three causes. Burns would be the most prevelant serious injury (two thirds of those who die the first day would be burn victims) and would occur at the greatest range. Blast and burn injuries would be found in 60-70% of survivors. People close enough to suffer significant radiation illness would be well inside the lethal effects radius for blast and flash burns. As a result, only 30% of injured survivors would show radiation illness. Many of these people would have been sheltered from burns and blast effects. Even so, most victims with radiation illness would also have blast injuries and burns.
Dangerous radiation levels only exist so close to the explosion that surviving the blast is impossible. On the other hand, fatal burns can be inflicted well beyond the range of substantial blast. A 20 megaton bomb can cause potentially fatal third degree burns at a range of 40km, where the blast can do little more than break windows and cause superficial cuts. A conventially rule of thumb for estimating the short-term fatalities from all causes due to nuclear attack is to count everyone inside the 5 psi blast overpressure contour around the hypocenter as a fatality. In reality, substantial numbers of people inside the contour will survive and substantial numbers of people outside the contour will die, but the assumption is that these two groups will be roughly equal in size. This completely ignores any possible fallout effects.
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