Brennilis has an oceanic climate (Köppen climate classificationCfb). The average annual temperature in Brennilis is 10.6 °C (51.1 °F). The average annual rainfall is 1,508.2 mm (59.38 in) with January as the wettest month. The temperatures are highest on average in July, at around 16.7 °C (62.1 °F), and lowest in February, at around 5.3 °C (41.5 °F). The highest temperature ever recorded in Brennilis was 36.9 °C (98.4 °F) on 8 August 2003; the coldest temperature ever recorded was −15.0 °C (5.0 °F) on 12 January 1987.
Climate data for Brennilis (1981–2010 averages, extremes 1977−2020)