Illinois is the 21st U.S. state and is located in the Midwest region of the United States of America. The northeastern border of Illinois is Lake Michigan. Its eastern border with Indiana is all of the land west of the Wabash River, and a north-south line above Post Vincennes, or 87° 30' west longitude. Its northern border with Wisconsin is fixed at 42° 30' latitude. Its western border with Missouri and Iowa is the Mississippi River. Its southern border with Kentucky is the Ohio River. Illinois also borders Michigan, but only via a water boundary in Lake Michigan.
Though Illinois lies entirely in the Interior Plains, it has three major geographical divisions. The first is Chicagoland, including the city of Chicago, its suburbs, and the adjoining exurban area into which the metropolis is expanding. Southward and westward, the second major division is central Illinois, an area of mostly flat prairie. The third division is southern Illinois, comprising the area south of U.S. Route 50, and including Little Egypt, near the juncture of the Mississippi River and Ohio River. This region can be distinguished from the other two by its warmer climate, different mix of crops, more rugged topography, as well as small-scale oil deposits and coal mining. In extreme northwestern Illinois, the Driftless Zone, a region of unglaciated and therefore higher and more rugged topography, occupies a small part of the state.
Because of its nearly 400 mile length, Illinois has a widely varying climate. Monthly average temperatures range from a high of 88°F in the south during the month of August to a low of 10°F in the northwest during February. Average yearly precipitation for Illinois varies from just over 48 inches at the southern tip to around 35 inches in the northern portion of the state. Normal annual snowfall exceeds 38 inches in Chicagoland due to lake effect snow, while the south normally receives less than 14 inches.