'Cloud gone': Two words in Hiroshima bomber's flight log reveal chilling detail 81 years later

Two words written in the flight log of the American bomber that dropped the atomic bomb on Hiroshima have helped scientists uncover new evidence about how high smoke from the devastating explosion rose into the atmosphere 81 years ago.

As per a report by ScienceAlert, the entry, "CLOUD GONE", was written by navigator Theodore Van Kirk on August 6, 1945, just 97 minutes after the bombing. Researchers now believe the observation provides rare evidence that the resulting firestorm sent soot at least 11.3 kilometres into the sky.

The American B-29 bomber Enola Gay dropped the atomic bomb, codenamed Little Boy, on Hiroshima at 8.15 am local time. As the aircraft departed, fires spread across the Japanese city, generating intense heat and a towering column of smoke.

Van Kirk recorded the words "CLOUD GONE" at 10.52 am Tinian time, according to the archived flight log.

At that moment, the aircraft was approximately 750 to 764 kilometres from Hiroshima, flying at an altitude of around 7,440 metres.

However, researchers believe the entry did not mean the smoke had disappeared. Instead, it marked the point at which the crew could no longer see the plume.

A separate military memorandum stated that the crew's "observation was then limited by haze and not curvature of the earth."

This suggested that the towering smoke plume remained above the horizon but was obscured by atmospheric haze.

Researchers led by atmospheric scientist David Romps of the University of California, Berkeley, examined the aircraft's recorded position, altitude and historical atmospheric conditions.

Their study, published in Geophysical Research Letters on October 28, used calculations accounting for how light bends through the atmosphere.

The team conducted 100,000 simulations to account for uncertainties in the aircraft's position, altitude and weather conditions.

They concluded with 95 per cent confidence that the firestorm plume exceeded 11.3 kilometres in height, reaching deep into the upper troposphere.

"The Hiroshima firestorm lofted soot well into the upper troposphere, consistent with nuclear-winter scenarios," the researchers wrote.

The findings could improve scientists' understanding of nuclear winter, a potential consequence of large-scale nuclear warfare.

If massive quantities of soot enter the upper atmosphere, they could block sunlight, lower global temperatures and threaten agricultural production.

However, the Hiroshima bombing did not cause a nuclear winter. The research instead provides historical evidence that nuclear firestorms can carry soot to altitudes relevant to such scenarios.

Researchers also clarified that the observed smoke plume was distinct from the bomb's initial mushroom cloud, which had already dispersed.