I think it’s a question we’ve all asked ourselves at one time or other. Fortunately (not least for the residents of midtown Manhattan), it’s one of those scientific queries that’s been explained theoretically, rather than anyone actually having to carry out the act of detonating an 800-kiloton nuclear warhead above midtown Manhattan and standing somewhere nearby with a pen and paper.
Spoiler: It’s not pretty.
A ball of superheated air would form, initially expanding outward at millions of miles per hour. It would act like a fast-moving piston on the surrounding air, compressing it at the edge of the fireball and creating a shockwave of vast size and power.
After one second, the fireball would be roughly a mile in diameter. It would have cooled from its initial temperature of many millions of degrees to about 16,000 degrees Fahrenheit, roughly 4,000 degrees hotter than the surface of the sun.
At the Empire State Building, Grand Central Station, the Chrysler Building, and St. Patrick’s Cathedral, about one half to three quarters of a mile from ground zero, light from the fireball would melt asphalt in the streets, burn paint off walls, and melt metal surfaces within a half second of the detonation. Roughly one second later, the blast wave and 750-mile-per-hour winds would arrive, flattening buildings and tossing burning cars into the air like leaves in a windstorm. Throughout Midtown, the interiors of vehicles and buildings in line of sight of the fireball would explode into flames.
The link above takes you to a very neat, and only moderately-dramatic (especially given the rather sensational subject matter) description of what happens bit by bit as you head away from ground zero. I quite like the thought of marble surfaces evaporating. That sounds like something I’d like to see, if not in these exact circumstances.
There’s very little good news here. It’s all rather unpleasant. Under the heading “No Survivors”, the author describes how there would be… well… no survivors:
The fire would extinguish all life and destroy almost everything else. Tens of miles downwind of the area of immediate destruction, radioactive fallout would begin to arrive within a few hours of the detonation. But that is another story.