|Fig. 1: Firestorm-made thermal column. Fire (1) superheats air (2) which rises, forming hurricane-force winds that blow (3) towards the fires. (Source: Wikimedia Commons)|
Most people have seen the dramatized scene of worldwide nuclear war: Thousands of ICBMs launched at once, detonating in mushroom clouds all over the world. And, most people are familiar with the after-effects of such a strike - Nuclear Armageddon, cities destroyed, radioactive fallout contaminating food and drinking water. Since the scale of such a war is so massive, dramatizations tend to overlook one of the more interesting small-scale features of detonating a nuclear weapon: the possibility of a firestorm.
According to Phillips, a firestorm occurs "when coalescent fires cause sufficient updraft to form their own wind, blowing inwards from all sides and thereby increasing the intensity of the fire".  Essentially, a collection of fires in the same area gets very hot and converges. This super-fire then heats the air so much that it begins to cause its own self-perpetuating weather patterns with hurricane force winds (see Fig. 1). Most notably, a small firestorm occurred at Hiroshima (Fig. 2) after the US detonated a nuclear bomb over the city during World War Two. [2,3]
The physics behind this phenomena is simple. After the initial fiery blast outwards, air rushes back inwards towards the epicenter of the explosion at upwards of 200 mph.  This adds fuel to the flames, which start to burn hotter. The air, superheated by the numerous fires rises quickly. This causes air to be sucked back towards the epicenter of the fires, to be heated again and perpetuate the process until the fire runs out of fuel. At its peak, a firestorm can cause hurricane force winds and level entire cities. 
|Fig. 2: A Pyro-cumulus cloud (as a result of a firestorm) forming as Hiroshima was burning. Taken after the mushroom cloud had cleared, by members of the US Air Force.  (Courtesy of the U.S. Army. Source: Wikimedia Commons)|
Although firestorms are devastating occurrences, would they really be ranked among the most destructive after-effects of detonating a nuclear weapon? To answer this question, we must simply consider the likelihood of a firestorm developing and evaluate from there. In the early years of nuclear weapons, it was thought that the combination of weather factors necessary for a firestorm to form were rare.  However, the bomb dropped on Hiroshima possessed an equivalent of around 15 kilotons of TNT.  The B53 thermonuclear bombs the US manufactured at the peak of the cold war were equivalent to around 9 megatons of TNT - roughly 600 times the explosive power of the bomb dropped on Hiroshima.  Since all that's really necessary for a firestorm to form after a nuclear detonation is fuel to burn and a sufficiently large mass of fire in the same area, we can conclude that the increase in power of nuclear weapons since the World War Two era makes the occurrence of firestorms very likely after detonating a nuclear weapon. Even compared to the likelihood (and severity) of other effects such as nuclear fallout, blast damage, and long-lasting radiation, firestorms are especially terrible consequence of nuclear warfare.
I have examined both the cause of firestorms, as well as the likelihood of their forming. Additionally, it should be noted that firestorms can (and most often occur) outside the detonation of nuclear weapons. For example, the firebombing of Hamburg caused a massive firestorm throughout much of the city.  Ultimately, firebombing can be considered an extremely destructive side-effect of nuclear detonations.
© Charlie Furrer. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 A. F. Phillips, "The Effects of a Nuclear Bomb Explosion on a City," Peace Res. 36, 93 (2004).
 A. B. Pittock et al., Environmental Consequences of Nuclear War, Volume 1 (Wiley, 1986), p. 1.
 W. J. Broad, "The Hiroshima Mushroom Cloud That Wasn't," New York Times, 23 May 16.
 R. M. Rodden, F. I. John, and R. Lauring, "Exploratory Analysis of Fire Storms," Stanford Research Institute, May 1965.
 J. Malik, "Yields of the Hiroshima and Nagasaki Explosions," Los Alamos National Laboratory, LA-8819, September 1985.
 D. Cook, "An Insider's View of Nuclear Weapons Modernization," Arms Control Today 46, No. 8, 14 (2016).