They only had to obscure the target zone, which was a very small area (a single factory complex, the Kokura Arsenal), and by chance it happened to be obscured during the 30 minutes or so that the plane was overhead.
This wasn't a normal bombing run. It had elements of an experiment - they wanted to understand how the bomb worked, so they wanted to control the drop precisely and be able to observe the effects.
The aiming points were very precise (a bridge in the case of Hiroshima), and they were chosen well in advance, and the mission orders were quite rigid. If the crew couldn't see the target clearly after several attempts, they didn't have the authority to randomly drop the bomb somewhere else - they had to go on to the next city.
Ironically, after all that, I think they eventually missed the aiming point in Nagasaki by 2 km or so, because it was also obscured. By then, the plane was low on fuel though, so I guess they were forced to do the best they could. I've read that they weren't supposed to do that, according to the mission rules. The plane came very close to running out of fuel, which I guess might have made them drop the bomb to reduce weight.
edit: This is Field Order 13 - for the attack on Hiroshima. It includes Kokura and Nagasaki as secondary targets, so you can see how precise they were about the aiming points. I'm pretty sure the target details would be exactly the same for the Nagasaki attack (Field Order 17, which I can't find online).
That diary is a fantastic read. The language, the drama, the bird's eye view of one of the most pivotal moments in world history. Beats fiction any day.
Just thought I'd share some of my puzzlement here; but it sounded as if they'd dropped a third atomic bomb. I though this unlikely, unless the diary is revealing a secret kept (the drop sounds as if it'd failed), but then I came across this:
http://en.wikipedia.org/wiki/Pumpkin_bomb
So, in addition to the nuclear models, they dropped identical conventional bombs of that type as well. Never knew that.
This wasn't a normal bombing run. It had elements of an experiment - they wanted to understand how the bomb worked, so they wanted to control the drop precisely and be able to observe the effects.
The aiming points were very precise (a bridge in the case of Hiroshima), and they were chosen well in advance, and the mission orders were quite rigid. If the crew couldn't see the target clearly after several attempts, they didn't have the authority to randomly drop the bomb somewhere else - they had to go on to the next city.
Ironically, after all that, I think they eventually missed the aiming point in Nagasaki by 2 km or so, because it was also obscured. By then, the plane was low on fuel though, so I guess they were forced to do the best they could. I've read that they weren't supposed to do that, according to the mission rules. The plane came very close to running out of fuel, which I guess might have made them drop the bomb to reduce weight.
edit: This is Field Order 13 - for the attack on Hiroshima. It includes Kokura and Nagasaki as secondary targets, so you can see how precise they were about the aiming points. I'm pretty sure the target details would be exactly the same for the Nagasaki attack (Field Order 17, which I can't find online).
https://www.flickr.com/photos/kafaikan/6693719785/sizes/l
edit2: Bomber crew member's diary which explains what happened over Kokura - http://www.mphpa.org/classic/COLLECTIONS/CG-ASPI/01/Pages/CG...