Why does an atomic bomb make a mushroom cloud? Because of the speed of the blast and the heat from the fireball.
We have all seen images of nuclear bombs being detonated and they always have a very distinctive cloud that is in the shape of a mushroom. These mushroom clouds can be tens of kilometers high and very wide. The largest ever atomic bomb detonated was the Tsar bomb. It was a 58 megaton bomb detonated in 1961. To put that in perspective, the bomb dropped on Hiroshima was 15 kilotons. The Tsar bomb was 3,866 times larger than the Hiroshima bomb. Kiloton and megaton are measures of the strengths of bombs and they refer tons of TNT. 1 kiloton is 1,000 tons of TNT and 1 megaton is 1,000,000 tons of TNT. The Tsar bomb was the equivalent of 58,000,000 tons of TNT. Its mushroom cloud stretched 67 km into the sky and spread out over 95 km. 67 km is very high. The fireball was 8 km high and could be seen 1,000 km away in Alaska. The shockwave travelled three times round the Earth.
So, why do these bombs make the distinctive mushroom cloud? Well, any large enough bomb can make a mushroom cloud. It doesn’t have to be an atomic bomb, but let’s focus on nuclear weapons. The Tsar bomb was a thermonuclear bomb. Thermonuclear weapons get their immense power from nuclear fusion, the same process that happens naturally in the sun, but that cannot be initiated without great heat and pressure. To get that heat and pressure, the weapon starts with a regular fission bomb. Here’s what happened with the Tsar bomb.
The bomb weighed 27 tons and it was released from a heavily modified plane at 10,500 m. The bomb was fitted with a parachute to slow its descent and give the planes a chance to fly to safety. They were only given a 50% chance of survival. The bomb detonated at 11:32, at 4,200 m. The bomb had barometric sensors to detect the height. A chemical explosive was triggered that started a fission reaction in a uranium core. The temperature from the fission reaction increases and the pressure inside the bomb increases. There is another uranium core that is compressed by the incredible pressure of the fission reaction. This starts a fusion reaction going in the uranium core. The Tsar bomb was a three stage thermonuclear bomb, which meant that the fusion reaction of the second uranium core was used to compress a third, even larger uranium core, which gave the final fusion reaction. The fireball went out in all directions, but was forced back up by the bomb’s own shockwave, reaching 8 km into the atmosphere. The mushroom cloud began to form straight away.
Mushroom clouds only form because we have an atmosphere. If you detonated a nuclear bomb on the moon, there wouldn’t be a mushroom cloud. The first part of the explosion is the fireball, which rises straight up into the sky because warm air is less dense. The sudden energy from the bomb vaporizes the gases in the air. Nuclear bombs are usually detonated at height so their blast isn’t deflected by buildings or mountains. This means the fireball can go higher. The fireball is a hot bubble of burning gases that rises very quickly. As it goes up, it sucks air up with it and creates a vacuum behind it. Smoke and debris from the bomb rush into this vacuum, getting sucked up behind the fireball in a column, which is the stalk of the mushroom. The top part of the fireball is still moving upwards and the burning gases within it rotate in a circular motion away from the center, forming a donut shape known as a torus. The center is an area of low pressure, which draws the smoke and debris in the stalk up into it.
The burning gas burns itself out fairly quickly and the smoke and debris keep rising until it reaches a height where the air is too cold and too dense for it to go any higher. This is known as its equilibrium level. Once the smoke can’t go any higher, it starts to spread out, forming the cap of the mushroom. The cloud will stay there until the winds in the atmosphere can disperse it. One of the greatest problems with nuclear bombs is that the burning gas and smoke carries radioactive material with it up into the mushroom cloud. If this gets into the upper atmosphere, it can be carried far around the world on the jet stream or the trade winds. And this is what I learned today.
Image By https://web.archive.org/web/20080518004931/http://www.atomicforum.org/russia/tsarbomba.html (photograph belongs to Russian department of Atomic Energy Minatom), Fair use, https://en.wikipedia.org/w/index.php?curid=16874908
Sources
https://www.britannica.com/story/why-does-a-mushroom-cloud-look-like-a-mushroom
https://en.wikipedia.org/wiki/Mushroom_cloud
https://www.livescience.com/why-nuclear-bomb-mushroom-cloud.html
https://en.wikipedia.org/wiki/Tsar_Bomba
https://www.ucsusa.org/resources/how-nuclear-weapons-work