I remember hearing/reading something somewhere a long time ago about how the weather patterns on earth mostly cycle away between the equator and the poles, and that this means if there was a northern hemisphere nuclear war (US/Europe/Russia/EU/China/India/Pakistan)the radioactive isotopes would mostly remain in the northern hemisphere, for quite a long time. This could mean the southern hemisphere is either spared entirely, or has a grace period, before it too becomes uninhabitable.
I'm working on something set in the second scenario, based around the idea that the survivors would be building space station arcs, and vying for inclusion. Assuming the violence was enough to effectively eliminate the entire northern hemisphere, how long could the southern hemisphere remain viable?
UPDATE: I figured some more specificity would help so lets say that atleast 50% of the worlds nuclear arsenal was fired, but this was exclusively in the northern hemisphere, perhaps not even near the equator in case that makes a difference, the lowest latitude hit could could be Mumbai.