Weather in Vienna is subject to sudden changes. Winter highs can reach 55 F/13 C, and the low temperatures can plunge below freezing. Expect chilly winds. There is often heavy snow beginning in late December. Summer highs can reach 90 F/32 C but can fall to 50 F/10 C after dark.
Heavy rain may occur at any time of the year, although rainfall is most abundant March-May. September and October might be the best months to visit: It is cooler and mostly dry, and there are fewer tourists (so the lines are shorter for museums and attractions). Skiers should wait until midwinter, however.