Well, from what was immediately observable to anyone who wasn't in the city at the time, the result really wasn't uniquely dramatic. The radiation and the fact

Well, from what was immediately observable to anyone who wasn't in the city at the time, the result really wasn't uniquely dramatic. The radiation and the fact that a single plane with a single bomb did it were game-changers but to anyone outside the city looking in it would appear to be another fire-storm bombing, albeit larger than Dresden and in a far shorter time-frame than Tokyo. That is why I think it really would take more time for things to settle in after a single bomb. The second bomb drives home the "yes, this really is what it looks like; this isn't like before." factor. I think it would have taken more time for a complete appreciation of what happened at Hiroshima to sink in. Whether or not a complete appreciation of that single bombing would have lead to surrender is debatable, I'm inclined to think not.