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 miles (640 km) length and mid-continental placement, 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. The southernmost part of the state, from about Carbondale southward, and the eastern suburbs of St. Louis, has a humid subtropical climate (Köppen Cfa) with more moderate winters. 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. 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 −37 °F (−38.3 °C), recorded on January 15, 2009, at Rochelle.
Illinois averages around 47 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 35 occurring annually, which puts much of the state at around 5 tornadoes per 10,000 square miles (30,000 km2) annually. 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.
The highest temperature recorded in Illinois was 118 °F (47.8 °C), recorded on July 14, 1954, at East St. Louis. The 1995 Chicago heat wave was one of the worst weather-related disasters in state history, with 525 dead within a five-day period as overnight lows rose as high as 84 °F (28.9 °C) and daytime highs rose to 106 °F (41.1 °C). The most deadly heatwave in the history of the United States struck Illinois during July 1936, which killed 2,656 people.
Average yearly precipitation for Illinois varies from just over 48 inches (1,200 mm) at the southern tip to just under 32 inches (810 mm) in the northern portion of the state. May and May are the wettest months of the year. Flooding is the most damaging weather hazard within the state. Increased warming within urban heat islands leads to an increase in rainfall downwind of cities. Lake Michigan leads to an increase in winter precipitation along its south shore due to lake effect snow forming over the relatively warm lakes. In summer, the relatively cooler lake leads to a more stable atmosphere near the lake shore, reducing rainfall potential.
While normal annual snowfall exceeds 38 inches or 0.97 metres in Chicago, the southern portion of the state normally receives less than 14 inches or 0.36 metres. The snowiest winter on record for Chicago was 89.7 inches (2.28 m) during the winter of 1978/1979. During the winter of 1830/1831, southern sections of the state were covered with 3 feet (0.91 m) of snow, with drifts up to 6 feet (1.8 m) tall. Storms exceeding the normal winter value are possible within one day. The heaviest snowfall recorded in recent history for Chicago was 23 inches (0.58 m) in ten hours during January 26, 1967.
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. Thunderstorms contribute over half of the annual precipitation statewide. Illinois is vulnerable to tornadoes with an average of 35 occurring annually, which puts much of the state at around 5 tornadoes per 10,000 square miles (30,000 km2) annually. Peak tornado activity occurs between April and May. 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. Illinois also receives the remnants of tropical cyclones which have struck the Gulf coast. One of the wetter systems, Tropical Storm Claudette, impacted the state in July 1979 bringing rainfall amounts up to 7.64 inches (194 mm).