None of us (well, very few of us) have a clue as to what really happens when a military-grade nuclear device (say 300 megaton [Hiroshima was only 15]) explodes. The article linked to above assumes just one of these is exploded 1500 feet above the ground, centered on the Pentagon. It's terrifying on the one hand, comforting on the other (you'll never feel a thing). Read the whole thing (taking breaks to sip water, hug your children, cry), but here's the initial score:
The detonation of a 300-kiloton nuclear bomb would release an extraordinary amount of energy in an instant—about 300 trillion calories within about a millionth of a second. More than 95 percent of the energy initially released would be in the form of intense light. This light would be absorbed by the air around the weapon, superheating the air to very high temperatures and creating a ball of intense heat—a fireball.Forget your quaint notion of a simple mushroom cloud.
Because this fireball would be so hot, it would expand rapidly. Almost all of the air that originally occupied the volume within and around the fireball would be compressed into a thin shell of superheated, glowing, high-pressure gas. This shell of gas would compress the surrounding air, forming a steeply fronted, luminous shockwave of enormous extent and power—the blast wave.
By the time the fireball approached its maximum size, it would be more than a mile in diameter. It would very briefly produce temperatures at its center of more than 200 million degrees Fahrenheit (about 100 million degrees Celsius)—about four to five times the temperature at the center of the sun.
This enormous release of light and heat would create an environment of almost unimaginable lethality. Vast amounts of thermal energy would ignite extensive fires over urban and suburban areas. In addition, the blast wave and high-speed winds would crush many structures and tear them apart. The blast wave would also boost the incidence and rate of fire-spread by exposing ignitable surfaces, releasing flammable materials, and dispersing burning materials.
Within minutes of a detonation, fire would be everywhere. Numerous fires and firebrands—burning materials that set more fires—would coalesce into a mass fire. (Scientists prefer this term to "firestorm," but I will use them interchangeably here.) This fire would engulf tens of square miles and begin to heat enormous volumes of air that would rise, while cool air from the fire's periphery would be pulled in. Within tens of minutes after the detonation, the pumping action from rising hot air would generate superheated ground winds of hurricane force, further intensifying the fire. 
Virtually no one in an area of about 40–65 square miles would survive.