December in Mariehamn is the rainiest and the windiest month of the year. In December, the weather in Mariehamn is typically cold. Average daily high temperature in December is about 3 degrees above zero. On average, Mariehamn has 18 rainy, 10 cloudy, and 3 sunny days in December. The rains are usually moderate. December as well as January, March, October, and November is considered the most uncomfortable time to visit Mariehamn.