It's interesting to read Wikipedia's entry on this matter (claiming it was caused by a huge firebombing raid): http://en.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_an... "The delay at the rendezvous had resulted in clouds and drifting smoke from fires started by a major firebombing raid by 224 B-29s on nearby Yahata the previous day covering 70% of the area over Kokura, obscuring the aiming point."
Yep, one of the reasons Hiroshima and Nagasaki were selected as targets was because they were one of the few major Japanese cities that /hadn't/ been firebombed in the preceding weeks. People forget that the deadliest 24 hours in the Second World War was not either of the atomic bombings (which killed c. 120,000 people), but the firebombing of Tokyo, which burned down 12 square miles of a city which had average population density of 100,000 people per square mile.