The climate of Illinois is characterized by warm to hot summers and cool to cold winters. In winter polar air masses move south or southeast across the state from Canada, bringing cold and crisp weather. In summer warm air masses move up from the Gulf of Mexico, and the weather is often hot and muggy. Lake Michigan tempers the summer heat somewhat for Chicago and other cities along its shores and also delays the date of the first fall frosts nearby.
Average July temperatures increase from about 24°C (about 75°F) in northeastern Illinois to more than 26°C (79°F) in the south, which is the hottest part of the state. During July, daytime highs average 29°C (84°F) at Chicago and about 32°C (about 90°F) at East Saint Louis, where a temperature of 47°C (117°F) has been recorded. Summer nights are usually warm throughout the state, ranging from about 19°C (about 66°F) in the north to about 21°C (about 69°F) in the south.
January averages range from less than -4°C (24°F) in the northwest to more than 1°C (34°F) in the south. Chicago has January low temperatures averaging -11°C (13°F) and highs -2°C (29°F). In the north freezing temperatures occur 140 to 145 days a year. Precipitation (rainfall and snowfall) generally increases from north to south. Average precipitation for the state as a whole is about 940 mm (about 37 in) a year. The south is the wettest part of the state, with about 1,220 mm (about 48 in) of precipitation a year in places. The driest sections are in the north, where a few places average about 810 mm (about 32 in).
Most precipitation falls in the form of rain, especially thundershowers, in late spring and summer, when it is most needed for crops. Damaging hailstorms sometimes occur in summer, and violent windstorms occasionally sweep across the state during the early spring months. Tornadoes may occur in any time of the year. Snowfall is often heavy in the north but is usually light in the south.
The growing season, or period between the last killing frost in the spring and the first killing frost in the fall, increases from less than 155 days in extreme northern Illinois to more than 205 days in the extreme south. Over much of the state the growing season is 190 days. The last killing spring frost occurs in early April in the far south and a month later in the north. The first fall frosts usually occur in early October in the north and in late October in the south. © Emmanuel Buchot and Encarta
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