Other facts from our historical weather data: Which is the warmest month in Saint Merryn? 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 Saint Merryn? The coldest month is February with an average maximum temperature of 10°C (50°F). Which is the wettest month in Saint Merryn? December tops the wettest month list with 108mm (4.3 in) of rainfall. Which is the driest month in Saint Merryn? May is the driest month with 71mm (2.8 in) of precipitation. Which is the sunniest month in Saint Merryn? May is the sunniest month with an average of 222 hours of sunshine. 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.