Wanderlust World Wanderlust World The summer season in Illinois is from June to August. During the summer in the northern part of Illinois, the temperature high averages 70 degrees Fahrenheit. In the southern part of Illinois, summer high temperatures average 77 degrees. July 14, 1954 was the hottest day ever recorded in Illinois with a temperature of 117 degrees.
The coldest months of the year in Illinois are typically December through February. During the winter, northern Illinois has an average temperature high of 22 degrees. Southern Illinois has an average winter temperature high of 37 degrees. The coldest day ever recorded in Illinois was Jan. 5, 1999 with a temperature of -36 degrees.
The average annual rainfall in Illinois is 36 inches. April through July are the months with the most rainfall in Illinois, averaging 3 1/2 inches each month.
Residents of northern Illinois see an average annual snowfall of 37 inches during the year. Their southern Illinois neighbors see an annual snowfall average of 24 inches. Most of the snowfall in Illinois occurs between December and February. The record snowfall in Illinois was during the winter of 1978 and 1979, with 90 inches.
Illinois is mostly plain state and is prone to tornadoes, especially in the central part of the state. The prime season for tornadoes to occur in Illinois is mid-March through June. In fact, Illinois is one of the most active tornado areas on the planet. The most tornadoes to occur in one year was in 1074, with 107 cyclones.