MACH STEM – the Nagasaki bombing intensified

In Nagasaki stood Shiroyama middle school, a sturdy building with a splendid view of the city. On the day of the bombing, some children had stayed home, and some were helping in the Mitsubishi plant next door. Their teachers were having meetings and working on bomb sheltering procedures.

The shock wave passed through the building in one-tenth of a second. Afterward, survivors tried to get out and understand what had happened.

The building withstood the blast, but almost everyone died instantly. More died within hours or days, and a very few lived to old age. 138 teachers and students lost their lives there.

One survivor was Hideo Arakawa, the assistant principal, who was in a meeting at the moment of the blast. He was the only survivor in the room, which had windows into the courtyard of the school. In the years that followed, he recorded everything he could about the event, interviewing other survivors and contributing to the historic archive.

After the blast, US military researchers also took careful measurements of the school building, its location and fatalities, comparing the results with the calculations they had made ahead of the drop. In particular they were looking for the results of the Mach Stem – a phenomenon that amplifies the destructive power shock waves that originate from a height.

This NHK documentary explores both the physics of Mach Stem, which accounts for the far greater destructive power of the Nagasaki bomb over its Hiroshima counterpart*, and the human story that resolves to such details as interviews and a great many US declassified documents. It also makes use of investigation by the famed Japanese meteorologist Tetsuya Fujita*, whose documents on the blast surfaced in 2013. The documentarians also commissioned a university project to carry out a blast simulation to tie together the science from Fujita and the Pentagon.

The Mach Stem effect was well known to makers of large bombs, and had been a key subject of the targeting committee. Nagasaki was a triumph, of sorts, of the scientific art of destruction.


  • This edition of the documentary has English narration in place of the original Japanese. There are a few blank spots in the audio where longer Japanese narratives had been.
  • It may seem strange to have school children helping in a wartime factory. Perhaps, it is not so strange. In the US, children helped with recycling drives, victory gardens, and war preparedness. Children on both sides learned writing and literature, mathematics and science, art and music, history and of course, patriotism.
  • I had this in my “Watch Later” queue for quite a long time; it is fascinating both scientifically and historically. But the imagery of learned men coldly calculating destruction is not easy to digest. What is the best height and location for an airburst? Let’s get rooms full of mathematicians working on it. (In those days such mathematicians were known as ‘computers’. Their electronic counterparts had not yet taken over that role.)
  • The computers’ labor bore fruit: the Nagasaki Bomb was 30% more powerful, but leveled ten times the area of reinforced buildings, as the Hiroshima bomb.
  • Can you imagine the Japanese reaction to Americans climbing over wreckage, taking photographs and make tape measurements? Recording the position of corpses and their condition? Mapping trees and how they were broken. It must have seemed ghoulish.
  • We refer to Fujita every time we designate a tornado as an F-3, for example.

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Older technology guy with photography and history background