Illinois' eastern border with Indiana consists of a north-south line at 87° 31′ 30″ west longitude, from Lake Michigan to theWabash River, above Post Vincennes. The Wabash River continues as the eastern/southeastern border with Indiana, until the Wabash enters the Ohio River. This marks the beginning of Illinois' southern border with Kentucky, which runs along the northern shoreline of the Ohio River. Its western border with Missouri and Iowa is the Mississippi River. Its northern border with Wisconsin is fixed at 42° 30' north latitude. The northeastern border of Illinois actually lies within Lake Michigan, within which Illinois shares a water boundary with the state of Michigan.
Though Illinois lies entirely in the Interior Plains, it does have some variation in its elevation. In extreme northwestern Illinois, the Driftless Area, a region of unglaciated and therefore higher and more rugged topography, occupies a small part of the state. Charles Mound, located in this region, has the state's highest elevation above sea level at 1,235 feet (376 m). The highest structure in Illinois is Willis Tower with a roof elevation of approximately 2,034 feet (620 m) above sea level. [Chicago elevation (580 ft) + tower height (1454 ft) = 2034.] The floodplain on the Mississippi River from Alton to theKaskaskia River is known as the American Bottom.
Illinois has three major geographical divisions. The first is Northern Illinois, dominated by the Chicago metropolitan area, including the city of Chicago, its suburbs, and the adjoining exurban area into which the metropolis is expanding. As defined by the federal government, the Chicago metro area includes a few counties in Indiana and Wisconsin and stretches across much of northeastern Illinois. It is a cosmopolitan city, densely populated, industrialized, and settled by a wide variety of ethnic groups. The city of Rockford, the fourth largest metropolitan area and the state's third largest city sits along Interstates 39 and 90 some 75 miles (121 km) northwest of Chicago. The Quad Cities, located at the western extreme of the area, had a population in 2009 of 379,066. This population makes it the third largest metropolitan area to include area in Illinois, but its area spans two states, as two of the Quad Cities lie in Iowa.
Southward and westward, the second major division is Central Illinois, an area of mostly prairie. Known as the Heart of Illinois, it is characterized by small towns and mid-sized cities. The western section (west of the Illinois River) was originally part of the Military Tract of 1812 and forms the distinctive western bulge of the state. Agriculture, particularly corn and soybeans, as well as educational institutions and manufacturing centers, figure prominently. Cities include Peoria, the third largest metropolitan area in Illinois at 370,000; Springfield, the state capital; Quincy; Decatur; Bloomington-Normal; and Champaign-Urbana.
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. Southern Illinois is the site of the ancient city of Cahokia, as well as the site of the first state capital, at Kaskaskia which today is separated from the rest of the state by the Mississippi River. This region can be distinguished from the other two by its warmer climate, different variety of crops (including some cotton farming in the past), more rugged topography (due to the area remaining unglaciated during the Illinoian Stage, unlike most of the rest of the state), as well as small-scale oil deposits and coal mining. The Illinois suburbs of St. Louis comprise the second most populous metropolitan area in Illinois with over 700,000 inhabitants, and are known collectively as theMetro-East. The other significant concentration of population in Southern Illinois is the Carbondale-Marion-Herrin, Illinois Combined Statistical Area centered on Carbondale and Marion, a two-county area that is home to 123,272 residents. A portion of southeastern Illinois is part of the extended Evansville, Indiana Metro Area, commonly referred to as the Tri-State with Indiana and Kentucky. Seven Illinois counties are in the area.
In addition to these three, largely latitudinally defined divisions, all of the region outside of the Chicago Metropolitan area is often called "downstate" Illinois. This term is flexible, but is generally meant to mean everything outside the Chicago-area. Thus, some cities in Northern Illinois, such as Rockford – which is actually north of Chicago – and DeKalb, which is west of Chicago, are considered to be "downstate".
Because its nearly 400 miles (644 km) distance from its northernmost and southernmost extremes, as well as its mid-continental situation, Illinois has a widely varying climate. Most of Illinois has a humid continental climate (Köppen climate classification Dfa), with hot, humid summers and cold winters. The southernmost part of the state, from about Carbondale southward, borders on a humid subtropical climate(Koppen Cfa), with more moderate winters. Average yearly precipitation for Illinois varies from just over 48 inches (1,219 mm) at the southern tip to around 35 inches (889 mm) in the northern portion of the state. Normal annual snowfall exceeds 38 inches (965 mm) in the Chicago area, while the southern portion of the state normally receives less than 14 inches (356 mm). The all time high temperature was 117 °F(47 °C), recorded on July 14, 1954, at East St. Louis, Illinois, while the all time low temperature was −36 °F (−38 °C), recorded on January 5, 1999, at Congerville, Illinois.
Illinois averages around 51 days of thunderstorm activity a year, which ranks somewhat above average in the number of thunderstorm days for the United States. Illinois is vulnerable to tornadoes with an average of 35 occurring annually, which puts much of the state at around five tornadoes per 10,000 square miles (30,000 km2) annually. The deadliest tornadoes on record in the nation have occurred largely in Illinois, not because the tornadoes are more common or frequent in Illinois, but rather, because Illinois is simply the most populous state in Tornado Alley. The Tri-State Tornado of 1925 killed 695 people in three states; 613 of the victims lived in Illinois. Modern developments in storm tracking have caused death tolls from tornadoes to dramatically decline since the 1960s, with no major losses of life since the 1967 tornado storm in northern Illinois.