There would be no words to adequately describe that happened within the explosion. No one near ground zero would survive to tell their story. But what is known is that the force of the blast--equaling the detonation of 13,000 tons of TNT--and sheer heat killed at least 80,000 civilians instantly (Atomic bombings of Hiroshima and Nagasaki).
As for the witnesses on the ground far enough away from the explosion to have survived, this is one of their testimonies.
"And at the moment when I spoke to my friend, there was a flash, far brighter than one used for a camera. It exploded right in front of my eyes. There was a tremendous noise when all the buildings around me collapsed… I was so surprised to see the dark sky with all the red flames through the window because it was only a few minutes before when the sky was blue and clear. It was all quiet and the city was wrapped, enveloped in red flames" (Voices of Hibakusha: Eye-witness accounts of the bombing of Hiroshima)
--- Taeko Teramae
||VIDEO: B-29s drop an atomic bomb on Japan (World War II Database)
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"Atomic bombings of Hiroshima and Nagasaki." Atomic bombings of Hiroshima and Nagasaki - Wikipedia, the free encyclopedia. 11 Jul. 2005. Wikipedia. 11 Jul. 2005 <http://en.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_and_Nagasaki>.
Marx, Joseph L. Seven Hours to Zero. New York: G.P. Putnam’s Sons, 1967.
“Voices of Hibakusha: Eye-witness accounts of the bombing of Hiroshima.” Remembering Hiroshima and Nagasaki. 2005. LaGuardia Community College. 02 Aug. 2005. <http://www.laguardia.edu/ctl/peace/et.htm>