7.6. The highest average temperature in Salem is July at 69.1 degrees. The coldest average temperature in Salem is December at 38.8 degrees. The most monthly precipitation in Salem occurs in December with 7.6 inches. The Salem weather information is based on the average of the previous 3-7 years of data.he Salem, OR air quality index is 3.7% less than the Oregon average and 37.6% less than the national average.