Continuing the subject matter of nuclear winter following a nuclear exchange. The firestorms generated by the nuclear exchange and the initial blast would release vast quantities of greenhouse gases into the atmosphere which would spread over the entire planet, not just the area of the atmosphere covered by the dust cloud. This would create a significant difference in the atmospheres between the area under the dust cloud and the rest of the planet.
If we imagine the dust cloud covering most of the northern hemisphere then under that cloud their would be little sunlight and temperatures would be significantly reduced, because of the reduced temperatures atmospheric pressures would be slightly reduced and so would atmospheric moisture levels, less evaporation would occur from bodies of standing water due to the lack of sunlight and reduced temperatures.
Over the rest of the earth temperatures would be rising due to the greenhouse gases in the atmosphere, the pressure in the atmosphere would be rising due to the increased atmosphere, evaporation from bodies of water would be rising and the atmosphere would be able to hold more water in vapor form than previously so levels of moisture in the atmosphere would increase.
Meanwhile along the edge of the dust cloud their would be a marked difference between the the atmosphere below the cloud and that outside of the dust cloud, the growing difference in temperatures, pressures and moisture levels. This would create storm systems were the two atmospheric systems mix potentially massive storm systems as the relatively warm, moist air outside of the cloud is sucked into the colder, drier zone beneath the cloud were it would be cooled and could potentially fall as snow. This is a very similar situation to that which you get over glacial regions, such as Antarctica, know as a glacial anti-cyclone, anti because it has a high pressure zone in its center rather than a low pressure zone as in most cyclonic storm systems.
That is the process by which glaciers grow and that is how it could theoretically result in an ice age, as 'theoretically' a glacier could be deposited by multiple layers of snow over a continent or continents. As the dust cloud faded away after 1 or 2 years the snow and ice left behind would reflect sunlight back into space so the temperature would not immediately return to normal and a new ice age would begin.
However the main greenhouse gas released would be carbon dioxide which i think has a half life in the atmosphere of about 30 years. Because of this the global warming produced by the co2 over a thirty year period would almost certainly cancel out the effect of the snow and ice deposit and the net effect would be an increase in global temperatures.
Sorry sci-fi writers!