Rome: Annual Weather Averages. July is the hottest month in Rome with an average temperature of 26°C (78°F) and the coldest is January at 8°C (46°F) with the most daily sunshine hours at 14 in July.The wettest month is December with an average of 96mm of rain.ome: Annual Weather Averages. July is the hottest month in Rome with an average temperature of 26°C (78°F) and the coldest is January at 8°C (46°F) with the most daily sunshine hours at 14 in July.