Petaluma has a mild Mediterranean climate. Its dry summer is characterized by typically warm days and cool nights with a large degree of diurnal temperature variation. Summer mornings often start out foggy and chilly, but the fog usually clears by midday or so, giving way to clear skies and warmth for the remainder of the day. August is usually the warmest month, with average daily temperatures ranging from 82 °F (28 °C) to 53 °F (12 °C). December is usually the coldest month, with average daily temperatures ranging from 57 °F (14 °C) to 39 °F (4 °C). Winter is cool and rainy, with frost occasionally occurring on clear nights.
Weather Underground’s reporting station in Petaluma had a record high temperature of 111 °F (44 °C) on September 6, 2020. The record low temperature of 16 °F (−9 °C) was recorded on November 14, 1916, and December 14, 1932. The wettest year was 1998 with 45.93 inches (1,167 mm) and the driest year was 1976 with 8.29 inches (211 mm). The wettest month was February 1998 with 19.59 inches (498 mm). The most precipitation in 24 hours was 4.29 inches (109 mm) on December 27, 2004. Although snow is rare in Petaluma, 1.5 inches (38 mm) fell in January 1916, as well as about 3 inches (76 mm) in January 2002.