From wikipedia:

Trinity was the first test of technology for a nuclear weapon. It was conducted by the United States on 16 July 1945, at a location 35 miles (56 km) southeast of Socorro, New Mexico, on what is now White Sands Missile Range, headquartered near Alamogordo. Trinity was a test of an implosion-design plutonium bomb. The Fat Man bomb, using the same conceptual design, was dropped on Nagasaki, Japan, a few weeks later. The detonation was equivalent to the explosion of around 20 kilotons of TNT and is usually considered as the beginning of the Atomic Age…At the time of detonation, the surrounding mountains were illuminated “brighter than daytime” for one to two seconds, and the heat was reported as “being as hot as an oven” at the base camp. The observed colors of the illumination ranged from purple to green and eventually to white. The roar of the shock wave took 40 seconds to reach the observers. The shock wave was felt over 100 miles (160 km) away, and the mushroom cloud reached 7.5 miles (12 km) in height. As Los Alamos director J. Robert Oppenheimer watched the demonstration, he later said that a line from the Hindu scripture the Bhagavad Gita came to mind:

I am become Death, the destroyer of worlds.

“In point of fact it is possible to tell the
(”Out!”)
difference between a small nuclear explosion and
a large one by a very simple method. The calling
card of a nuclear bomb is the blinding flash that
is far more dazzling than any light on earth–brighter
even than the sun itself–and it is by the duration
of this flash that we are able to determine the size
(”What are we going to do without?”)
of the weapon. After the flash a fireball can be
seen to rise, sucking up under it the debris, dust
and living things around the area of the explosion,
and as this ascends, it soon becomes recognisable
as the familiar “mushroom cloud”. As a demonstration
of the flash duration test let’s try and count the
number of seconds for the flash emitted by a very
small bomb; then a more substantial, medium-sized
bomb; and finally, one of our very powerful,
“high-yield” bombs.

© “Breathing” lyrics KaTe Bush