wave woke people from its center. A typical weapon consists of a container packed with a fuel substance, the center of which has a small conventional-explosive "scatter charge". Fuels are chosen on the basis of the exothermicity of their oxidation, ranging from powdered metals, such as aluminum or magnesium, to organic materials, possibly with a self-contained partial oxidant. The most recent development involves the use of nanofuels. A thermobaric bomb's effective yield depends on a combination of a number of factors such as how well the fuel is dispersed, how rapidly it mixes with the surrounding atmosphere and the initiation of the igniter and its position relative to the container of fuel. In some designs, strong munitions cases allow the blast pressure to be contained long enough for the fuel to be heated well above its autoignition temperature so that once the container bursts, the superheated fuel autoignites progressively as it comes into contact with atmospheric oxygen. Conventional upper and lower limits of flammability apply to such weapons. Close in, blast from the dispersal charge, compressing and heating the surrounding atmosphere, has some influence on the lower limit. The upper limit has been demonstrated to influence the ignition of fogs above pools of oil strongly. That weakness may be eliminated by designs in which the fuel is preheated well above its ignition temperature so that its cooling during its dispersion still results in a minimal ignition delay on mixing. The continual combustion of the outer layer of fuel molecules, as they come into contact with the air, generates added heat which maintains the temperature of the interior of the fireball, and thus sustains the detonation. In confinement, a series of reflective shock waves is generated, which maintain the fireball and can extend its duration to between 10 and 50 ms as exothermic recombination reactions occur. Further damage can result as the gases cool and pressure drops sharply, leading to a partial vacuum. This rarefaction effect has given rise to the misnomer "vacuum bomb". Piston-type afterburning is also believed to occur in such structures, as flame-fronts accelerate through it. Fuel–air explosive A fuel–air explosive (FAE) device consists of a container of fuel and two separate explosive charges. After the munition is dropped or fired, the first explosive charge bursts open the container at a predetermined height and disperses the fuel (and possibly ionizes it, depending on whether a fused quartz dispersal