Geography

Illinois is located in the Midwest region of the United States and is one of the eight states and Ontario, Canada, in the Great Lakes region of North America.

Boundaries

Illinois's eastern border with Indiana consists of a north–south line at 87° 31′ 30″ west longitude in Lake Michigan at the north, to the Wabash River in the south 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's southern border with Kentucky, which runs along the northern shoreline of the Ohio River. Most of the western border with Missouri and Iowa is the Mississippi River; Kaskaskia is an exclave of Illinois, lying west of the Mississippi and reachable only from Missouri. The state's northern border with Wisconsin is fixed at 42° 30′ north latitude. The northeastern border of Illinois lies in Lake Michigan, within which Illinois shares a water boundary with the state of Michigan, as well as Wisconsin and Indiana.

Topography

Though Illinois lies entirely in the Interior Plains, it does have some minor 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. Southern Illinois includes the hilly areas around the Shawnee National Forest.

Charles Mound, located in the Driftless region, has the state's highest natural elevation above sea level at 1,235 feet (376 m). Other highlands include the Shawnee Hills in the south, and there is varying topography along its rivers; the Illinois River bisects the state northeast to southwest. The floodplain on the Mississippi River from Alton to the Kaskaskia River is known as the American Bottom.

Divisions

Illinois has three major geographical divisions. Northern Illinois is dominated by Chicago metropolitan area, or Chicagoland, which is the city of Chicago and its suburbs, and the adjoining exurban area into which the metropolis is expanding. As defined by the federal government, the Chicago metro area includes several counties in Illinois, Indiana, and Wisconsin, and has a population of over 9.8 million. Chicago itself is a cosmopolitan city, densely populated, industrialized, the transportation hub of the nation, and settled by a wide variety of ethnic groups. The city of Rockford, Illinois's third-largest city and center of the state's fourth largest metropolitan area, sits along Interstates 39 and 90 some 75 miles (121 km) northwest of Chicago. The Quad Cities region, located along the Mississippi River in northern Illinois, had a population of 381,342 in 2011.

The midsection of Illinois is the second major division, called Central Illinois. It is an area of mainly prairie and known as the Heart of Illinois. It is characterized by small towns and medium–small cities. The western section (west of the Illinois River) was originally part of the Military Tract of 1812 and forms the conspicuous western bulge of the state. Agriculture, particularly corn and soybeans, as well as educational institutions and manufacturing centers, figure prominently in Central Illinois. Cities include Peoria; 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, 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 has a somewhat warmer winter 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, such as East St. Louis, are located in this region, and collectively, they are known as the Metro-East. The other somewhat 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, locally 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 the Chicago Metropolitan area is often called "downstate" Illinois. This term is flexible, but is generally meant to mean everything outside the influence of the Chicago area. Thus, some cities in Northern Illinois, such as DeKalb, which is west of Chicago, and Rockford—which is actually north of Chicago—are sometimes incorrectly considered to be 'downstate'.

Climate

Illinois has a climate that varies widely throughout the year. Because of its nearly 400-mile distance between its northernmost and southernmost extremes, as well as its mid-continental situation, most of Illinois has a humid continental climate (Köppen climate classification Dfa), with hot, humid summers and cold winters. The southern part of the state, from about Carbondale southward, has 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, and the all-time low temperature was −38 °F (−39 °C), recorded on January 31, 2019, during the January 2019 North American cold wave at a weather station near Mount Carroll, and confirmed on March 5, 2019. This followed the previous record of −36 °F (−38 °C) recorded on January 5, 1999, near Congerville. Prior to the Mount Carroll record, a temperature of −37 °F (−38 °C) was recorded on January 15, 2009, at Rochelle, but at a weather station not subjected to the same quality control as official records.

Illinois averages approximately 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. While tornadoes are no more powerful in Illinois than other states, some of Tornado Alley's deadliest tornadoes on record have occurred in the state. The Tri-State Tornado of 1925 killed 695 people in three states; 613 of the victims died in Illinois.

Monthly Normal High and Low Temperatures For Various Illinois Cities

City	January	February	March	April	May	June	July	August	September	October	November	December
Cairo	43/25	48/29	59/37	70/46	78/57	86/67	90/71	88/69	81/61	71/49	57/39	46/30
Chicago	31/16	36/21	47/31	59/42	70/52	81/61	85/65	83/65	75/57	64/45	48/34	36/22
Edwardsville	36/19	42/24	52/34	64/45	75/55	84/64	89/69	86/66	79/58	68/46	53/35	41/25
Moline	30/12	36/18	48/29	62/39	73/50	83/60	86/64	84/62	76/53	64/42	48/30	34/18
Peoria	31/14	37/20	49/30	62/40	73/51	82/60	86/65	84/63	77/54	64/42	49/31	36/20
Rockford	27/11	33/16	46/27	59/37	71/48	80/58	83/63	81/61	74/52	62/40	46/29	32/17
Springfield	33/17	39/22	51/32	63/42	74/53	83/62	86/66	84/64	78/55	67/44	51/34	38/23