Prague: Annual Weather Averages. July is the hottest month in Prague with an average temperature of 18°C (64°F) and the coldest is January at -1°C (30°F) with the most daily sunshine hours at 11 in July.The wettest month is May with an average of 69mm of rain.Loading weather data.rague: Annual Weather Averages. July is the hottest month in Prague with an average temperature of 18°C (64°F) and the coldest is January at -1°C (30°F) with the most daily sunshine hours at 11 in July.