I reached out to American Institute of Physics nuclear historian Alex Wellerstein to see if a more definite answer were possible. He compared the image to other photos whose timestamps we more or less know, as well as to a chart prepared by Manhattan Project scientists depicting the cloud's progression. “In looking at those,” he said, “it looks like it was taken slightly after another photo which is supposed to be the cloud at about 20,000 feet.” That elevation—20,000 feet—would have been reached about two to three minutes after the explosion, and this picture was taken slightly after that.

“There are photos taken about 10 minutes after, which show that the cloud is a really different shape at that point,” he continued. Based on that timeline, he places this picture somewhere within the two-to-five-minute window, and rules out the 20-to-30-minute window.

As you look at the photo, your mind inevitably turns to the people below. What was it like in those first few minutes after the bomb? I asked Wellerstein and he replied:

Most of the direct effects of the bomb are going to be delivered in the first 10 seconds or so—and a lot of them faster than that. You've already got the first thing that happens, which is this huge amount of radiation and heat: Anybody who's exposed to radiation might not die instantly, but they're going to die to pretty soon, within a week or so. The heat is going to give people third-degree burns within a fairly wide radius. A few seconds later there's this huge pressure blast. It's like a big puff of air. It blows in all the windows; it tips over the little stoves that they use for cooking breakfast. This happens at 8:15 or so in the morning, so it tips over all those little, tiny charcoal stoves. So by a couple of minutes, you've got the beginnings of a very large fire. It's not yet at a firestorm—that happens pretty soon afterward—but you've got all these little tiny fires, started either by the heat of the bomb itself or by the fact that all of these fire sources are knocked over and these Japanese houses are all made out of wood and paper.

And, as bad as that is, the worst was yet to come: “From a couple of minutes after and continuing for several hours, everything is on fire, everyone is confused. A lot of people are dead. A lot of people are dead but don't know it yet—they've been exposed to fatal amounts of radiation. So it's pretty bad on the ground at that time.” As the fire spreads, the winds pick up, creating tornado-like conditions which, as Father John A. Siemes—a Catholic priest living at Novitiate of the Society of Jesus in Nagatsuke, about two kilometers away—later remembered, “begins to uproot large trees, and lifts them high into the air.”

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For people trapped in that chaos, no one possibly could have understood that all of this destruction came from one single bomb. Everyone thought they themselves must have been very close to where the bomb hit. Father Siemes, who was two kilometers away, recalled seeing a flash of light, then hearing an explosion. Then, about 10 seconds after the light, “I am sprayed by fragments of glass. The entire window frame has been forced into the room. I realize now that a bomb has burst and I am under the impression that it exploded directly over our house or in the immediate vicinity.”