Santa Rosa has a warm-summer Mediterranean climate with cool, wet winters and warm, dry summers. In the summer, fog and low overcast often move in from the Pacific Ocean during the evenings and mornings. They usually clear up to warm, sunny weather by late morning or noon before returning in the later evening but will occasionally linger all day. Average annual rainfall is 32.20 inches (818 mm), falling on 74 days annually. The wettest year was 1983 with 63.07 inches (1,602 mm) and the driest year was 1976 with 11.38 inches (289 mm). The most rainfall in one month was 19.42 inches (493 mm) in February 1998 and the most rainfall in 24 hours was 5.23 inches (133 mm) on December 19, 1981. Measurable snowfall is rare in the lowlands, but light amounts sometimes fall in the nearby mountains.
There are an average of 28.9 days with highs of 90 °F (32 °C) or more and an average of 30.2 days with lows reaching the freezing mark. The record high was 113 °F (45 °C) on July 11, 1913, and the record low was 9 °F (−13 °C) on December 25, 1924.