Other facts from our historical weather data: Which is the warmest month in Castlederg? July has an average maximum temperature of 19°C (66°F) and is the warmest month of the year. Which is the coldest month in Castlederg? The coldest month is February with an average maximum temperature of 8°C (46°F). Which is the wettest month in Castlederg? August tops the wettest month list with 108mm (4.3 in) of rainfall. Which is the driest month in Castlederg? April is the driest month with 87mm (3.4 in) of precipitation. Which is the sunniest month in Castlederg? May is the sunniest month with an average of 203 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.