First of all we must say that the term ‘Nuclear Winter’ refers to a situation that is purely theoretical and that cannot be tested on the field entirely.
It is a short description for an hypothetical global climate change caused in a very short period of time by a global thermonuclear war or by some analogous phenomenon like an asteroid collision with Earth.
It has been theorized that a large number of nuclear warhead detonated against surface targets could lift into the atmosphere several hundreds millions of tons of fine particle dust by three mechanisms: ejection of solid particles from ground, vaporization (i.e. sublimation) of rocks and blow off of surface dust. In addition, a nuclear explosion is enough to set on fire any flammable materials both by impact and by simple exposure at extremely high temperatures. This secondary firestorms are enough to create another gigantic emission of smoke and dust.
Since a nulcear war is most likely to take palce on a medium latitude of the northern emishere, the wind circualtion act as a concentrators of the dust that can encopass the temperate zone of the entire emisphere.
The dust act cooling the earth surface and the lower troposphere, blocking solar radiation that, instead, warm up the higher part of the clouds along with the upper troposphere and the lower stratosphere. No greenhouse effect will take place in this case since the infrared is immediately radiate back to space at this altitudes.
The results is a loss of insulation at the ground level of 10% at the minimum: this would affect both temperature, winds, oceans heat circulation and finally plants grow rate.
The intense energy delivered by such nuclear strike would also produce a large quantities of NOx compounds that directly depleted the Ozone layer that could be reduced even by 50% of its average thickness leading to an increase of UVB penetration.
Because of the continental-scale artificial cloud and the signification change in solar radiation perceived in the lower atmosphere, the climate would be greatly affected, but it is uncertain if our model can correctly predict the results of a such large modification. All considered, anyway, it is estimated that this nuclear winter would last for 1 or 2 years, so it would probably not enough to overcome the whole Earth climatic inertia thus eliminating the possibility of a prolonged ice age. At the same time it is largely unknown if, how and when a large quantity of debris could eventually migrate from northern to southern hemisphere thus rendering the climate change a global phenomenon and not a regional one.
Finally, in case of nuclear war, we also must take into account the large radiation fallout that will encompass two or more whole continents. The total radiation exposure cannot be calculated exactly because of the different types of nuclear warheads that could be used in a conflict, but the so-called salted bomb (a thermonuclear device ‘salted’ with cobalt isotope) could eventually wipe all the life from earth, given the proper amount of raw materials.
Again, we must point out that this whole scenario is modeled after simulation and approximation and that science is still not able to replicate something so complex as the climate of the entire world, nor the effect of a conflict of such scale. Even though, it is irrefutable that an event of this kind will lead to great devastation both to the planets an to our civilization for months and years.