Nuclear winter is a hypothetical global climate condition that is predicted to be a possible outcome of a large-scale nuclear war.It is thought that severely cold weather can be caused by detonating large numbers of nuclear weapons,especially over flammable targets such as cities,where large amounts of smoke and soot would be injected into the Earth's stratosphere.
This layer of particles would significantly reduce the amount of sunlight that reached the surface,and could potentially remain in the stratosphere for weeks or even years (smoke and soot arising from the burning petroleum fuels and plastics would absorb sunlight much more effectively than would smoke from burning wood).The ash would be carried by the midlatitude west-to-east winds,forming a uniform belt of particles encircling the northern hemisphere from 30° to 60° latitude.These thick black clouds could block out much of the sun's light for a period as long as several weeks,causing surface temperatures to drop by as much as 30°C during the occlusion.
The combination of darkness and killing frosts,combined with high doses of radiation from nuclear fallout,would severely damage plant life in the region.The extreme cold,high radiation levels,and the widespread destruction of industrial,medical,and transportation infrastructures along with food supplies and crops would trigger a massive death toll from starvation,exposure,and disease.It was also thought that nitrogen oxides generated by the blasts would degrade the ozone layer,as had been observed in the first thermonuclear blasts.Secondary effects from ozone depletion (and concomitant increases in ultraviolet radiation) would be significant,with impacts on the viability of most human staple agricultural crops as well as disruption of ocean food chains by killing off phytoplankton.