Sizun has an oceanic climate (Köppen climate classificationCfb). The average annual temperature in Sizun is 10.4 °C (50.7 °F). The average annual rainfall is 1,324.0 mm (52.13 in) with January as the wettest month. The temperatures are highest on average in August, at around 16.3 °C (61.3 °F), and lowest in February, at around 5.4 °C (41.7 °F). The highest temperature ever recorded in Sizun was 37.7 °C (99.9 °F) on 9 August 2003; the coldest temperature ever recorded was −11.3 °C (11.7 °F) on 17 January 1985.
Climate data for Sizun (1981–2010 averages, extremes 1983−present)