1 second after detonation
At the end of the first second, the shock wave will have an overpressure of 20 psi. at a distance of four tenths of a mile from ground zero. Even the most heavily reinforced steel and concrete buildings will be destroyed. These buildings include the Empire State Building, Madison Square Gardens, Penn Central Railroad Station and the New York Public Library. Most of the material that comprises these buildings will remain and pile up to a depth of hundreds of feet in places, but nothing inside this ring will be recognizable.
This circle contains a daytime population of roughly 75,000. There will be no survivors. Those caught outside will be exposed to the full effects of the blast, including severe lung and ear drum damage and exposure to flying debris. Those in the direct line of sight of the blast will be exposed to a thermal pulse in excess of 500 cal/sq.cm., causing instant death. Those inside, though shielded from some of the blast and thermal effects, will be killed as buildings collapse.
The fireball will have a maximum radius of 1,023 feet (0.2 miles). However, the blast effects will greatly outweigh any direct thermal effects due to the fireball.