You can also read the Switzerland section for this. Summers can be very hot and Zurich is one of the few cities in the world where you can go for a swim at the very clean lake and the also very clean Limmat river over lunch time. Best time for this is June and July until mid of August, even if in Switzerland there is no guarantee that Summers brings hot and sunny weather. Rains start at around middle of August and last until middle to end of September. Sunny days with nich temperatures come back in October until the end of the month. November and December can be foggy and rainy. The first snow can be expected around beginning of December. More snow,  which never stay for more than a couple of days, come back in January, normally the coldest month in Zurich (temperatures can reach -15°C or 5°F) and stay that way for a couple of weeks. This fortunately brings sunny weather. The first signs of Spring can appear in February and March. April always brings back snow with lots of rain continuing through May. So, the best months to visit Zurich are January (for the winter fans without garantee of snow and nice weather), June to mid August (again without guarantee of a hot and sunny Summer) and October (for fans of hiking and long walks). December brings a magic Christmas feeling through the city with Christmas markets in many places of the city and the special Christmas lighting of that period.

Meteo Swiss Weather - 5 Day Forecast