|Themes > Science > Chemistry > Nuclear Chemistry > Nuclear Weapons > The First Nuclear Chain Reaction > Effects of Nuclear Explosions > Air Bursts and Surface Bursts|
It might seem logical that the most destructive way of using a nuclear weapon would be to explode it right in the middle of its target - i.e. ground level. But for most uses this is not true. Generally nuclear weapons are designed to explode above the ground - as air bursts (the point directly below the burst point is called the hypocenter). Surface (and sub-surface) bursts can be used for special purposes.
When an explosion occurs it sends out a shock wave like an expanding soap bubble. If the explosion occurs above the ground the bubble expands and when it reaches the ground it is reflected - i.e. the shock front bounces off the ground to form a second shock wave travelling behind the first. This second shock wave travels faster than the first, or direct, shock wave since it is travelling through air already moving at high speed due to the passage of the direct wave. The reflected shock wave tends to overtake the direct shock wave and when it does they combine to form a single reinforced wave.
This is called the Mach Effect, and produces a skirt around the base of the shock wave bubble where the two shock waves have combined. This skirt sweeps outward as an expanding circle along the ground with an amplified effect compared to the single shock wave produced by a ground burst.
The higher the burst altitude, the weaker the shock wave is when it first reaches the ground. On the other hand, the shock wave will also affect a larger area. Air bursts therefore reduce the peak intensity of the shock wave, but increase the area over which the blast is felt. For a given explosion yield, and a given blast pressure, there is a unique burst altitude at which the area subjected to that pressure is maximized. This is called the optimum burst height for that yield and pressure.
All targets have some level of vulnerability to blast effects. When some threshold of blast pressure is reached the target is completely destroyed. Subjecting the target to pressures higher than that accomplishes nothing. By selecting an appropriate burst height, an air burst can destroy a much larger area for most targets than can surface bursts.
The Mach Effect enhances shock waves with pressures below 50 psi. At or above this pressure the effect provides very little enhancement, so air bursts have little advantage if very high blast pressures are desired.
An additional effect of air bursts is that thermal radiation is also distributed in a more damaging fashion. Since the fireball is formed above the earth, the radiation arrives at a steeper angle and is less likely to be blocked by intervening obstacles and low altitude haze.
Surface bursts are useful if local fallout is desired, or if the blast is intended to destroy a buried or very hard structure like a missile silo or a dam. Shock waves are transmitted through the soil more effectively if the bomb is exploded in immediate contact with it, so ground bursts would be used for destroying buried command centers and the like. Some targets, like earth-fill dams, require actual cratering to be destroyed and would be ground burst targets.
Exploding a bomb below ground level can be even more effective for producing craters and destroying buried structures. It can also eliminate thermal radiation and reduce the range of blast effects substantially. The problem, of course is getting the bomb underground. Earth-penetrating bombs have been developed that can punch over one hundred feet into the earth.