St. Catharines’ climate is humid continental . It has a unique micro-climate because of the moderating influence of Lake Ontario/Lake Erie and the sheltering effect of the Niagara Escarpment to the south. This climate allows wineries to flourish. As a result, the city records numerous frost-free days and frequent thaws in the winter, although it sometimes receives heavy lake-effect snow during certain wind conditions, and micro-cooling lakeside on some spring afternoons. The summer season is predominantly warm, sometimes hot and humid, with an average high temperature of 27 °C (81 °F) in July. Summer thunderstorms are commonplace but generally less prevalent and less severe than further west in southern Ontario due to the diminishing effect of the surrounding lakes.
The highest temperature ever recorded in St. Catharines was 40.0 °C (104 °F) on 10 July 1936. The coldest temperature ever recorded was −25.7 °C (−14.3 °F) on 18 February 1979.