When a nuclear weapon explodes in the air, the surrounding air is subjected to great heat, followed by relatively rapid cooling. These conditions are ideal for the production of tremendous amounts of nitric oxides. These oxides are carried into the upper atmosphere, where they reduce the concentration of protective ozone. Ozone is necessary to block harmful ultraviolet radiation from reaching the Earth's surface.
The nitric oxides produced by the weapons could reduce the ozone levels in the Northern Hemisphere by as much as 30 to 70 percent. Such a depletion might produce changes in the Earth's climate, and would allow more ultraviolet radiation from the sun through the atmosphere to the surface of the Earth, where it could produce dangerous burns and a variety of potentially dangerous ecological effects.
It has been estimated that as much as 5,000 tons of nitric oxide is produced for each megaton of nuclear explosive power.