Nuclear weapons in comparison to conventional weapons
The incendiary effects of a nuclear explosion do not present any especially characteristic features. In principle, the same overall result with respect to destruction of life and property can be achieved by the use of conventional incendiary and high-explosive bombs. It has been estimated, for example, that the same fire ferocity and damage produced at Hiroshima by one 16-kiloton nuclear bomb from a single B-29 could have instead been produced by about 1,200 tons/1.2 kilotons of incendiary bombs from 220 B-29s distributed over the city.
It may seem counterintuitive that the same amount of fire damage caused by a nuclear weapon could have instead been produced by smaller total yield of thousands of incendiary bombs; however, World War II experience supports this assertion. For example, although not a perfect clone of the city of Hiroshima in 1945, in the conventional bombing of Dresden, the combined Royal Air Force (RAF) and United States Army Air Forces (USAAF) dropped a total of 3441.3 tons (approximately 3.4 kilotons) of ordnance (about half of which was incendiary bombs) on the night of 13–14 February 1945, and this resulted in "more than" 2.5 square miles (6.5 km2) of the city being destroyed by fire and firestorm effects according to one authoritative source, or approximately 8 square miles (21 km2) by another. In total about 4.5 kilotons of conventional ordnance was dropped on the city over a number of months during 1945 and this resulted in approximately 15 square miles (39 km2) of the city being destroyed by blast and fire effects. During the Operation MeetingHouse firebombing of Tokyo on 9–10 March 1945, 279 of the 334 B-29s dropped 1,665 tons of incendiary and high-explosive bombs on the city, resulting in the destruction of over 10,000 acres of buildings — 16 square miles (41 km2), a quarter of the city. In contrast to these raids, when a single 16-kiloton nuclear bomb was dropped on Hiroshima, 4.5 square miles (12 km2) of the city was destroyed by blast, fire, and firestorm effects. Similarly, Major Cortez F. Enloe, a surgeon in the USAAF who worked with the United States Strategic Bombing Survey (USSBS), said that the 22-kiloton nuclear bomb dropped on Nagasaki did not do as much fire damage as the extended conventional airstrikes on Hamburg.
Hiroshima after the bombing and firestorm. No known aerial photography of the firestorm exists.
Note the ambient wind blowing the fire's smoke plume inland. The firebombing of Tokyo on the night of 9–10 March 1945 was the single deadliest air raid of World War II, with a greater total area of fire damage and loss of life than either nuclear bombing as a single event. Due largely to the greater population density and fire conditions. 279 B-29s dropped about 1,700 tons of ordnance on target.
Hiroshima aftermath. Despite a true firestorm developing, reinforced concrete buildings, as in Tokyo, similarly remained standing. Signed by the Enola Gay pilot, Paul W. Tibbets.
This Tokyo residential section was virtually destroyed. All that remained standing were concrete buildings in this photograph.
American historian Gabriel Kolko also echoed this sentiment:
During November 1944 American B-29's began their first incendiary bomb raids on Tokyo, and on 9 March 1945, wave upon wave dropped masses of small incendiaries containing an early version of napalm on the city's population....Soon small fires spread, connected, grew into a vast firestorm that sucked the oxygen out of the lower atmosphere. The bomb raid was a 'success' for the Americans; they killed 125,000 Japanese in one attack. The Allies bombed Hamburg and Dresden in the same manner, and Nagoya, Osaka, Kobe, and Tokyo again on May 24....in fact the atomic bomb used against Hiroshima was less lethal than massive fire bombing....Only its technique was novel—nothing more....There was another difficulty posed by mass conventional bombing, and that was its very success, a success that made the two modes of human destruction qualitatively identical in fact and in the minds of the American military. "I was a little fearful", [Secretary of War] Stimson told [President] Truman, "that before we could get ready the Air Force might have Japan so thoroughly bombed out that the new weapon would not have a fair background to show its strength." To this the President "laughed and said he understood."
This break from the linear expectation of more fire damage to occur after greater explosive yield is dropped can be easily explained by two major factors. First, the order of blast and thermal events during a nuclear explosion is not ideal for the creation of fires. In an incendiary bombing raid, incendiary weapons followed after high-explosive blast weapons were dropped, in a manner designed to create the greatest probability of fires from a limited quantity of explosive and incendiary weapons. The so-called two-ton "cookies", also known as "blockbusters", were dropped first and were intended to rupture water mains, as well as to blow off roofs, doors, and windows, creating an air flow that would feed the fires caused by the incendiaries that would then follow and be dropped, ideally, into holes created by the prior blast weapons, such into attic and roof spaces. On the other hand, nuclear weapons produce effects that are in the reverse order, with thermal effects and "flash" occurring first, which are then followed by the slower blast wave. It is for this reason that conventional incendiary bombing raids are considered to be a great deal more efficient at causing mass fires than nuclear weapons of comparable yield. It is likely this led the nuclear weapon effects experts Franklin D'Olier, Samuel Glasstone and Philip J. Dolan to state that the same fire damage suffered at Hiroshima could have instead been produced by about 1 kiloton/1,000 tons of incendiary bombs.
The second factor explaining the non-intuitive break in the expected results of greater explosive yield producing greater city fire damage is that city fire damage is largely dependent not on the yield of the weapons used, but on the conditions in and around the city itself, with the fuel loading per square meter value of the city being one of the major factors. A few hundred strategically placed incendiary devices would be sufficient to start a firestorm in a city if the conditions for a firestorm, namely high fuel loading, are already inherent to the city (see Bat bomb). The Great Fire of London in 1666, although not forming a firestorm due to the single point of ignition, serves as an example that, given a densely packed and predominately wooden and thatch building construction in the urban area, a mass fire is conceivable from the mere incendiary power of no more than a domestic fireplace. On the other hand, the largest nuclear weapon conceivable will be incapable of igniting a city into a firestorm if the city's properties, namely its fuel density, are not conducive to one developing.
Despite the disadvantage of nuclear weapons when compared to conventional weapons of lower or comparable yield in terms of effectiveness at starting fires, for the reasons discussed above, nuclear weapons also do not add any fuel to a city, and fires are entirely dependent on what was contained in the city prior to bombing, in direct contrast to the incendiary device effect of conventional raids. One undeniable advantage of nuclear weapons over conventional weapons when it comes to creating fires is that nuclear weapons undoubtedly produce all their thermal and explosive effects in a very short period of time; that is, to use Arthur Harris's terminology, they are the epitome of an air raid guaranteed to be concentrated in "point in time". In contrast, early in World War II, the ability to achieve conventional air raids concentrated in "point of time" depended largely upon the skill of pilots to remain in formation, and their ability to hit the target whilst at times also being under heavy fire from anti-aircraft fire from the air defensives of the cities below. Nuclear weapons largely remove these uncertain variables. Therefore, nuclear weapons reduce the question of whether a city will firestorm or not to a smaller number of variables, to the point of becoming entirely reliant on the intrinsic properties of the city, such as fuel loading, and predictable atmospheric conditions, such as wind speed, in and around the city, and less reliant on the unpredictable possibility of hundreds of bomber crews acting together successfully as a single unit.