August in Rio Grande is the rainiest and the hottest month of the year. In August, the weather in Rio Grande is typically hot. Average daily high temperature in August is about 30 degrees above zero. On average, Rio Grande has 23 rainy, 5 partly cloudy, and 3 sunny days in August. The rains are usually heavy. August as well as May, June, July, and September is considered the most uncomfortable time to visit Rio Grande.