IORC Regions of Illinois

IORC uses regional labels Northern, Central, and Southern following their original defintion by Gault (1922). Gault based the division of the state on county borders closest to isothermal lines at the time of 51 degrees mean annual temperature (separating the Northern and Central regions) and of 55 degrees mean annual temperature (separating the Central and Southern regions). This division of the state, as shown on a map in Gault (1922), has Northern Illinois corresponding roughtly to the Transition Zone, Central Illinois to the Upper Austral Zone, and Southern Illinois to the Lower Austral Zone.

County Map of Illinois with Northern Illinois in orange, Central Illinois in green, and Southern Illinois in blue.

Map from Gault (1922) showing the division into regions and the life zones of the state.

References

Gault, Benjamin T. 1922. Check List of the Birds of Illinois. Illinois Audubon Society, Chicago IL.

This page was last updated on 22 March 2022