When a nuclear explosion occurs, radiation, heat or thermal effects, and blast will occur. When detonated at low altitudes, a nuclear weapon will form a fireball as a result of the sudden release of immense quantities of energy. The initial temperature of the fireball ranges in the millions of degrees, and the initial pressure to millions of atmospheres. Most of the energy from the detonation of a nuclear weapon appears as nuclear radiation, thermal radiation and blast, or shock vawe.In the detonation of a typical fission-type nuclear weapon, the percentage of the total energy appearing as nuclear radiation, blast, or thermal radiation depends on the altitude at which the weapon is detonated (subsurface, surface, or air) . For detonations within a few kilometers above the earth's surface, slightly more that 50 percent of the energy may appear as blast, approximately 35 percent as thermal energy, and approximately 15 percent as nuclear radiation. Weather, i.e., a clear day, or haze, fog, snow, and to some extent topology, may influence the effects of nuclear explosions in relation to the destructive power on the targeted areas.

Nuclear Radiation - A usual result of a nuclear detonation is the mushroom formation bearing, in particular if resulting from low altitude bursts, radioactive materials which are formed from the materials sucked from the ground and the weapon's debris. Radioactive materials which may later reach the earth's surface as fallout are carried by the wind and the height of the burst is important in this respect, since low altitude bursts will create more local fallout within relatively short periods of time while high altitude bursts will carry radioactive materials, which will fall to the earth after extended periods of time, for long distances. Weather and topology as related to altitude of burst are also important in this connection. Neutron and gamma radiation from the weapon's detonation produce casualties and oftentimes material damage as well. Ionized regions which result when the atmosphere absorbs nuclear radiation may also interfere with the propagation of electromagnetic waves associated with communications systems.
Thermal Radiation -
The fireball emits an intense thermal radiation, resulting from the heath and light of the explosion due to the immediate release of an enormous amount of energy in a very small space which results in an initial fireball temperature that ranges into million of degrees. The total amount of thermal radiation is directly proportional to the yield for a given type of weapon. A thermal wave within the atmosphere travels in straight lines at the speed of light and, depending on atmospheric conditions, can be scattered, reflected or absorbed since any condition which significantly alters the transparency of the air, i.e., clouds, smog, fog or rain will effect its propagation even to large extents.
Blast Wave - A blast wave, i.e., a pulse of air in which the pressure increases sharply at the front, accompanied by winds propagated from an explosion with accompanying drag effects travels outwards from the burst site. Most casualties and damages are caused by the destructive power of the blast wave traveling outward at high velocities through the atmosphere and which is caused by the expansion of the exceedingly hot gases at extremely high pressures within the fireball. An overpressure develops due to the abrupt rise in pressure with respect to the normal atmospheric pressure. The velocity of the shock front is initially many times the speed of sound and, as it progresses outwards, it slows down and moves with the speed of sound. The magnitude of the effects from the blast, or, shock wave, depend on the weapon's yield, altitude of detonation and distance from ground zero. Weather, surface conditions, topography, all affect the blast wave which may last from a fraction of a second to many seconds depending on the weapon's yield and distance from the explosion.
Wind Speed and Direction -
Since contaminated debris and particles deposit downwind, wind speed and its direction determine the shape, size, location and intensity of fallout and surface winds, as well, have an important role in the final deposition of fallout. Just as snow falls on pavements or frozen surfaces and surface winds pile it in drifts, so, too, can local winds cause localized fallout in crevices, ditches, or against curbs and ledges. Air density and clouds have no significant effect on fallout patterns.
Rainout - Removal of the radioactive particles due to precipitation, known as rainout, may take place in the proximity of ground zero or the contamination may be carried aloft for tens of kilometers before deposition downwind. Rainout , in particular, may occur in the case of subsurface or surface bursts and it may cause the fallout area to increase or decrease as well as causing radioactive hot spots; for airburst residual contamination hazards may be increased by rainout. Normally the hazard from an air burst is a neutron induced contamination around ground zero, however rainout can cause additional contamination areas in diverse locations. Yields of 10 kilotons or less present the greatest potential for rainout, and yield of 60 kilotons or more offer the least. Besides, rainout may be produced by yields within 10 kilotons and 60 kilotons if the nuclear cloud remains at or below rain cloud height. Rain on an area contaminated by surface bursts changes the pattern of radioactive intensities by washing off higher elevations, buildings, materials and vegetation. This reduces intensities in some areas and possibly increase intensities in drainage systems, on low ground, and in flat, poorly drained areas.
Topology - Some protection may be afforded in an area contaminated by gamma radiation by ditches, gullies, small hills and ridges. Since terrain contours also cause winds systems to develop these winds will affect the final disposition of fallout creating both hot spots and areas of low intensity within the pattern.
However, concerning the shock wave, it is not heavily dependent on line-of-sight considerations because the shock waves will bend or diffract around obstacles. Some protection may be afforded from flying debris from hills, which may decrease dynamic pressures, or robust structures.
Shelters - Due to the extreme power of the shock wave and the penetrating power of ionizing radiations, in particular gamma rays, shelters should be several tens of kilometers from a possible nuclear target. However the main considerations, concerning shelters, are: structural strength, to withstand an eventual shock wave, which implies armored concrete or buried structures and, concerning protection from intense radiation or fallout, due consideration should be given to the density and thickness of the materials employed for sheltering, since dense materials, .i.e., iron or armored concrete, are more effective in stopping radiation while at the same time their protective property is directly proportional to the thickness of the materials employed for the shielding purpose and, also in these case, buried structures afford better protection.
Concerning radioactivity, and in particular incoming or delayed fallout, due consideration should be given to the possibility of being confined in a shelter for prolonged periods of time, i.e., weeks, and hence the availability of survival facilities in terms of alimentary and medical supplies, air filtration, lighting and hygienic considerations.

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