Notes

Fireball radius: 0.97 km (2.93 km²)
Maximum size of the nuclear fireball; relevance to damage on the ground depends on the height of detonation. If it touches the ground, the amount of radioactive fallout is significantly increased. Anything inside the fireball is effectively vaporized. Minimum burst height for negligible fallout: 0.87 km.
Heavy blast damage radius (20 psi): 2.19 km (15 km²)
At 20 psi overpressure, heavily built concrete buildings are severely damaged or demolished; fatalities approach 100%. Often used as a benchmark for heavy damage in cities. Optimal height of burst to maximize this effect is 1.82 km.
Radiation radius (500 rem): 2.5 km (19.7 km²)
500 rem ionizing radiation dose; likely fatal, in about 1 month; 15% of survivors will eventually die of cancer as a result of exposure.
Moderate blast damage radius (5 psi): 4.64 km (67.5 km²)
At 5 psi overpressure, most residential buildings collapse, injuries are universal, fatalities are widespread. The chances of a fire starting in commercial and residential damage are high, and buildings so damaged are at high risk of spreading fire. Often used as a benchmark for moderate damage in cities. Optimal height of burst to maximize this effect is 3.12 km.
Thermal radiation radius (3rd degree burns): 12.6 km (497 km²)
Third degree burns extend throughout the layers of skin, and are often painless because they destroy the pain nerves. They can cause severe scarring or disablement, and can require amputation. 100% probability for 3rd degree burns at this yield is 11.3 cal/cm2.