Climate of Illinois

Köppen climate types of Illinois (with counties overlaid), using 1991-2020 climate normals.

The Climate of Illinois describes the weather conditions, and extremes, noted within the state of Illinois, United States, over time.

Because of its nearly 400-mile (640 km) length and mid-continental location, Illinois has a widely varying climate. Most of Illinois has a humid continental climate (Köppen climate classification Dfa) with hot, humid summers and cool to cold winters. However, the southern half of the state, from about Springfield southward, has a humid subtropical climate (Köppen Cfa) with winters becoming more moderate as one travels south. Average yearly precipitation for Illinois varies from 48 inches or 1,220 mm at the southern tip to 35 inches or 890 mm in the northern portion of the state. Normal annual snowfall exceeds 38 inches or 0.97 m in Chicago, while the southern portion of the state normally receives less than 14 inches or 0.36 m.[1] The highest temperature recorded in Illinois was 117 °F (47.2 °C), recorded on July 14, 1954, at East St. Louis, while the lowest temperature was −38 °F (−38.9 °C), recorded on January 31, 2019, at Mount Carroll.

Illinois averages around 50 days of thunderstorm activity a year which put it somewhat above average for number of thunderstorm days for the United States. Illinois is vulnerable to tornadoes with an average of 54 occurring annually, which puts much of the state at around 9.7 tornadoes per 10,000 square miles (30,000 km2) annually.[2][3] The deadliest tornado on record in the nation occurred largely in Illinois. The Tri-State Tornado of 1925 killed 695 people in three states; 613 of the victims lived in Illinois.[4]

  1. ^ Illinois State Climatologist Office Archived 2006-02-07 at the Wayback Machine. Climate Maps for Illinois Archived February 7, 2006, at the Wayback Machine. Accessed April 22, 2006.
  2. ^ "Annual average number of tornadoes, 1991-2010", NOAA National Climatic Data Center. Retrieved on December 12, 2018.
  3. ^ "Average Annual Number of EF0-EF5 Tornadoes 1991-2010". National Climatic Data Center. October 26, 2013. Archived from the original on October 29, 2013. Retrieved December 4, 2018.
  4. ^ PAH Webmaster (2005-11-02). "NWS Paducah, KY: NOAA/NWS 1925 Tri-State Tornado Web Site -- General Information". Retrieved 2006-11-16.