 According to the U.S. Census Bureau, the county has a total area of , of which is land and (2.2%) is water.In recent years, average temperatures in the county seat of Mount Vernon have ranged from a low of in January to a high of in July, although a record low of was recorded in January 1994 and a record high of was recorded in July 1936.Average monthly precipitation ranged from in January to in May.