In recent years, average temperatures in the county seat of Petersburg have ranged from a low of 17 °F (−8 °C) in January to a high of 87 °F (31 °C) in July, although a record low of −24 °F (−31 °C) was recorded in February 1905 and a record high of 112 °F (44 °C) was recorded in July 1954. Average monthly precipitation ranged from 1.62 inches (41 mm) in January to 4.06 inches (103 mm) in May.
There were 4,873 households out of which 36.10% had children under the age of 18 living with them, 60.70% were married couples living together, 9.10% had a female householder with no husband present, and 27.10% were non-families. 23.80% of all households were made up of individuals and 10.30% had someone living alone who was 65 years of age or older. The average household size was 2.52 and the average family size was 2.99.
In the county the population was spread out with 26.50% under the age of 18, 6.80% from 18 to 24, 28.90% from 25 to 44, 24.60% from 45 to 64, and 13.20% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 96.20 males. For every 100 females age 18 and over, there were 92.00 males.
The median income for a household in the county was $46,596, and the median income for a family was $52,995. Males had a median income of $36,870 versus $27,010 for females. The per capita income for the county was $21,584. About 6.10% of families and 8.20% of the population were below the poverty line, including 12.50% of those under age 18 and 6.00% of those age 65 or over.