Those who have watched the videos of atom bomb explosions must have noticed the different type of smoke created by the explosion, rising in the shape of a mushroom and spreading out. The first thing to be noted is that mushroom clouds are not exclusive to nuclear explosions. Mushroom clouds are created in any explosion that releases sufficient energy for certain conditions to develop. It can be formed from the explosion of vacuum bombs or even volcanic eruptions.
Mushroom clouds can be formed only from the explosions occurring in a gravitational field. Without the gravitational force, the resulting gas cloud will retain its initial form of sphere. As for the nuclear detonation, it is conducted at some distance above the ground to maximize its impact. Immediately after the explosion, a pyrocumulus cloud is formed. This massive fireball made of hot gases spread outward swiftly. Since the fireball is much hotter and less dense than the surrounding air, it will begin to rise rapidly. It is the same principle behind the working of a hot-air balloon. And it is this rising ball of burning gases that eventually forms the mushroom cap.
As the fireball rises, a scientific phenomenon called Rayleigh–Taylor instability is formed. It happens when two fluids of different densities are merged and subjected to acceleration. It propels the lighter fluid upwards. As a result, the air is drawn upwards and into the cloud, similar to the case of a chimney. It causes the volume of gases to form the shape of an inverted cup. The upward acceleration of the fireball produces strong air currents known as afterwinds, and these afterwinds will draw the debris from the ground to form what is perceived to be the stem of the mushroom. Once the rising fireball reaches an altitude where its density is equal to that of the surrounding air, it begins to disperse, and the debris drawn upward from the ground will scatter and drift back down. The stabilization point will depend upon the atmospherical features of the place and the altitude at which the detonation takes place.
Mushroom clouds would mainly consist of the fission materials and debris. It would not be formed in the explosions that take place much above the earth’s surface and underground. Mushroom clouds are often accompanied by short-lived vapour clouds, known as ‘Wilson clouds’, which form a ring around the mushroom cap. It is a result of the low pressure created by the negative phase of the shockwaves from explosion which lowers the dew point as well, triggering the formation of temporary clouds.
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