December in Petaluma is the coldest month of the year. In December, the weather in Petaluma is typically warm. Average daily high temperature in December is about 12 degrees above zero. On average, Petaluma has 12 sunny, 12 partly cloudy, and 7 rainy days in December. The rains are usually light. December as well as January and February is considered the most uncomfortable time to visit Petaluma.