In recent years, average temperatures in the county seat of Belvidere have ranged from a low of 11 °F (−12 °C) in January to a high of 85 °F (29 °C) in July, although a record low of −29 °F (−34 °C) was recorded in January 1979 and a record high of 109 °F (43 °C) was recorded in July 1936. Average monthly precipitation ranged from 1.29 inches (33 mm) in February to 4.56 inches (116 mm) in June.
There were 14,597 households out of which 40.10% had children under the age of 18 living with them, 64.20% were married couples living together, 8.70% had a female householder with no husband present, and 22.90% were non-families. 19.00% of all households were made up of individuals and 8.00% had someone living alone who was 65 years of age or older. The average household size was 2.84 and the average family size was 3.24.
In the county the population was spread out with 29.80% under the age of 18, 7.70% from 18 to 24, 29.90% from 25 to 44, 22.00% from 45 to 64, and 10.70% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 100.20 males. For every 100 females age 18 and over, there were 97.60 males.
The median income for a household in the county was $52,397, and the median income for a family was $59,305. Males had a median income of $41,992 versus $25,695 for females. The per capita income for the county was $21,590. About 5.10% of families and 7.00% of the population were below the poverty line, including 8.70% of those under age 18 and 5.80% of those age 65 or over.