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Why did Hiroshima experience a firestorm after the atomic bombing?

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U.S. hydrogen bomb test in the Marshall Islands — nuclear weapon A test of a U.S. thermonuclear weapon (hydrogen bomb) at Enewetak atoll in the Marshall Islands, November 1, 1952.

Hiroshima experienced a firestorm because the thermal radiation released by the atomic airburst ignited numerous individual fires across the city's level terrain. These fires were extensive enough to coalesce into a single, massive conflagration.1

This mass fire generated a powerful convective column of rising hot gases, which created a vacuum that sucked in fresh air from the periphery. The resulting inward-rushing winds and extreme temperatures consumed virtually everything combustible in the area. The city's flat geography was a critical factor in this phenomenon, allowing the firestorm to cause severe damage over 11.4 square kilometers (4.4 square miles), a significantly larger area than that affected by the bombing of the hilly terrain of Nagasaki.1