Mumbai: Annual Weather Averages. May is the hottest month in Mumbai with an average temperature of 30°C (86°F) and the coldest is January at 24°C (75°F) with the most daily sunshine hours at 10 in May. The wettest month is July with an average of 640mm of rain. The best month to swim in the sea is in June when the average sea temperature is 30°C (86°F).