Other facts from our historical weather data: Which is the warmest month in Heddon on the Wall? July has an average maximum temperature of 20°C (68°F) and is the warmest month of the year. Which is the coldest month in Heddon on the Wall? The coldest month is January with an average maximum temperature of 8°C (46°F). Which is the wettest month in Heddon on the Wall? August tops the wettest month list with 83mm (3.3 in) of rainfall. Which is the driest month in Heddon on the Wall? March is the driest month with 56mm (2.2 in) of precipitation. Which is the sunniest month in Heddon on the Wall? May is the sunniest month with an average of 183 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.