From an altitude of over 40,000 feet, how immense must the destruction of hiroshima been for the enola gay's crew to have been able to see it?

The pilots of the Enola Gay were tasked with dropped the first atomic bomb on Hiroshima. After dropping the bomb and beginning to fly away the pilots report seeing the mushroom cloud rise from the target city. The 4 square miles from the impact point was complete devastation with nothing left remaining. The weeks following found radiation poisoning to be the cause of many more deaths in addition to those killed in the initial impact. 

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