Most nuclear targets are in the northern hemisphere and most atmospheric circulation follows lines of latitude or regional patterns.
While a full on nuclear winter would eventually have global effects, not everywhere would be impacted equally in terms of severity or duration. The most severe effects would be felt in the north. For a more limited exchange with regional or continental wide effects, there are few regions in the north not at risk of such a conflict. With a pattern of impact driven by latitudinal or regional weather patterns, the impact on the southern hemisphere of any regional northern conflict are likely to be minimal.
