Other facts from our historical weather data: Which is the warmest month in Mariehamn? August has an average maximum temperature of 20°C (68°F) and is the warmest month of the year. Which is the coldest month in Mariehamn? The coldest month is February with an average maximum temperature of 1°C (34°F). Which is the wettest month in Mariehamn? October tops the wettest month list with 67mm (2.6 in) of rainfall. Which is the driest month in Mariehamn? March is the driest month with 33mm (1.3 in) of precipitation. No idea where to travel to this year? We have a tool that recommends destinations based on your ideal conditions. Find out where to go with our weather planner.