Buenos Aires: Annual Weather Averages. January is the hottest month in Buenos Aires with an average temperature of 25°C (76°F) and the coldest is July at 12°C (53°F) with the most daily sunshine hours at 9 in February.The wettest month is January with an average of 100mm of rain.The best month to swim in the sea is in January when the average sea temperature is 23°C (73°F).he wettest month is January with an average of 100mm of rain. The best month to swim in the sea is in January when the average sea temperature is 23°C (73°F).