4.6. The highest average temperature in Indianapolis is August at 75.5 degrees. The coldest average temperature in Indianapolis is February at 30.3 degrees. The most monthly precipitation in Indianapolis occurs in June with 5.2 inches.The Indianapolis weather information is based on the average of the previous 3-7 years of data. Loading...he Indianapolis, IN air quality index is 5.7% greater than the Indiana average and 18.7% greater than the national average.