Las Vegas: Annual Weather Averages. July is the hottest month in Las Vegas with an average temperature of 34°C (92°F) and the coldest is December at 8°C (46°F) with the most daily sunshine hours at 13 in August.The wettest month is August with an average of 14mm of rain.Loading weather data.uly is the hottest month in Las Vegas with an average temperature of 34°C (92°F) and the coldest is December at 8°C (46°F) with the most daily sunshine hours at 13 in August. The wettest month is August with an average of 14mm of rain. Loading weather data.