Illinois has a humid continental climate (Köppen climate classification Dfa, Dfb) in the majority of the state with hot summers and cold winters. The southernmost part experiences a humid subtropical climate (Köppen Cfa) with hot summers and cold winters. Illinois lies in the American Midwest and is part of the Great Lakes Region of North America. Illinois borders with Indiana in the east, Kentucky in the southeast, Missouri in the southwest, Iowa in the northeast, and Wisconsin in the north. Latitude and weather systems influence the climate statewide, with the local influence of Lake Michigan.
Illinois, the Prairie State, mainly lies in the Interior Plains of central North America, with hilly areas in the south. The elevation varies from 85 meters at the confluence of the Mississippi and Ohio rivers to 376 meters at the highest point of Charles Mound. The Illinois River bisects the state, and a vast prairie region lies at the center of the state. Savanna type grasslands, forests, wetlands, agriculture lands, and pastures form a significant part of the topography. The absence of high mountain ranges and remoteness from seas and oceans contribute to extreme seasonal variations in the weather.
Summers tend to be humid and warm to hot in Illinois, with occasional extreme temperatures during a heatwave. The average high temperatures are above 80°F (26.7°C) at the peak of July, while nights are in the range of 60°F (15.6°C) to 70°F (21.1°C) during the hot season. The urban heat effect is dominant as major cities register up to 2°F (-16.7°C) higher temperatures than their remote suburbs. Winters are cold, with ice storms and invasion of cold waves. The average high temperatures are near freezing in the north, while the south is slightly warmer. Low temperatures in winter often drop below 10°F (-12.2°C). Spring has moderate temperatures but is wet in the latter part of the season. Fall has mild temperatures and covers the landscape with beautiful colors. Temperature variations are extreme throughout the year.
Illinois registers an annual average of 32" (812.8mm) of rainfall in the northern part, which gradually increases to 48" (1219.2mm) in the southernmost tip of the state. Late spring and early summer tend to be the wettest period. The south shores of Lake Michigan see an increase in precipitation due to lake effect snow in the winter that reduces in the summer. Snowfall ranges from an annual 14" (355.6mm) in the south to 38" (965.2mm) in the north. Snow covers the ground in the winter, while blizzards amass large quantities. Illinois receives an average of 2500 hours of sunlight over the year.
The record high temperature for Illinois is 117°F (47.2°C), set in East Saint Louis on July 14, 1954, while the lowest recorded temperature is -36°F (-37.8°C), set in Congerville on January 5, 1999.
The best time to visit Illinois
The best time to visit Illinois is from April to mid-May in the spring and September to October in the fall. The atmosphere is pleasant compared to the scorching heat of summer and the cold of winter. Autumn is colorful, with pleasant mornings and breezy evenings. Late spring and early summer tend to be wet with moderate temperatures. Late May and June are subject to thunderstorms, with severe flooding many a time. Late fall is cold and windy as the temperatures drop rapidly. Summers are hot and humid, with crowds everywhere. Spring and fall are light on the wallet than the summer.
The worst time to visit Illinois
The worst time to visit Illinois is during the winter from December to February when the state lies in the grip of severe cold weather. Night temperatures drop below 0°F (-17.8°C) in the coldest parts in the extreme north. Cold waves are frequent in the winter, and the temperatures can drop to extreme levels. The lack of natural barriers is a reason for strong winds to sweep the state. The Chicago Area, in particular, is windy and cold. Blizzards are frequent in the winter, and snow cover is universal by January. A massive snowstorm can produce as much as 20" (508mm) to 25" (635mm) of snow in a single day. Snow depth can easily reach up to 1 meter in many parts.
Illinois is vulnerable to thunderstorms, lightning, floods, and tornadoes. Fifty days of thunderstorm activity occurs in the year, and an average of 54 tornadoes sweep the state annually. The deadliest tornado in the nation occurred in Illinois in 1925, with severe damage to life and property. Floods routinely cause extensive damage. Heat and cold waves are climate hazards with far more annual deaths than lightning, floods, and tornadoes. The 1995 and 1999 heatwaves were responsible for 753 human casualties and major power outages. Ice storms deposit large amounts of snow, and the strong gale is a threat in the winter.
Weather in January
January is brutally cold in Illinois, as even the day temperatures register below freezing in the northern regions. Freeport, in northern Illinois, registers average temperatures between 11°F (-11.7°C) to 29°F (-1.7°C), while Chicago is between 13°F (-10.6°C) to 34°F (1.1°C). The average temperatures statewide are in the 11°F (-11.7°C) to 43°F (6.1°C) range, with remarkable mildness in the southern regions. Lake effect snow is dominant in the Lake Michigan region, which leads to increased precipitation on its southern shores. The skies are partly cloudy and occasionally gloomy as strong winds scourge the landscape. Ice skating rinks and snow-covered hills provide enjoyment in the winter to enthusiastic snow lovers. Ice fishing is popular with a large number of lakes present in Illinois. Be sure to carry warm coats and dress in layers to counter the icy weather. Hot chocolate is a quick energizer during cold days. January is the snowiest month of the year in Illinois during the season of snowstorms.
Weather in February
February lies in the firm grip of the winter season in Illinois, with the average temperatures in the 15°F (-9.4°C) to 48°F (8.9°C) range. Springfield, the capital in the central region, is cold between 23°F (-5°C) to 40°F (4.4°C), while Dixon, north of Interstate 88 registers in the 16°F (-8.9°C) to 34°F (1.1°C) range. Snowfall registers between 6" (152.4mm) to 10" (254mm) in the northern and central regions. Southern Illinois receives light snowfall between 1" (25.4mm) to 3" (76.2mm). Snowstorms and blizzards frequently occur to accumulate large deposits of snow, poor visibility on the roads, and power outages. Slippery roads are slick with ice and hazardous, even for experienced drivers. Four-wheel drives and latest safety technologies are prudent things to consider while buying an automobile suitable for year-round weather in Illinois. At least half of the month has overcast skies, and the sunshine is weak at best. Expect snow cover and cold to last even at the end of February in Illinois.
Weather in March
March is the beginning of the spring season in Illinois, which starts with cold and icy conditions. The average low temperatures are in the cold 26°F (-3.3°C) to 36°F (2.2°C), while the average high temperatures are in the mild 44°F (6.7°C) to 59°F (15°C) zone. The southernmost region of Illinois registers the warmest temperatures, with the rapid advance of spring. Cairo in the south is one of the mildest places in March, with temperatures in the 37°F (2.8°C) to 59°F (15°C) range. The rainfall sees an uptick to register in the 2" (50.8mm) to 4" (101.6mm) range, while the Chicago Area and the northern regions receive up to 6" (152.4mm) of snow. Snowstorms occur occasionally, and cold waves are common during the early days of spring. The weather demands warm clothing in the form of coats and jackets to counter the icy precipitation and snow. The moderate sunlight lasts for 6 hours a day, but the nights are cold, windy, and soggy in many regions in March in Illinois.
Weather in April
April is wet and mild in Illinois, with the unabated advance of the spring season. Light snowfall occurs in the northern regions, but the ground is devoid of snow in the south. The thawing of snow is an excellent time to cruise on the lakes and rivers. Boating tours are readily available, and a tour down the river in the beautiful sunlight is an experience to cherish. The average high temperatures are in the warm 57°F (13.9°C) to 65°F (18.3°C) range, while the average low temperatures register in the mild 37°F (2.8°C) to 47°F (8.3°C) range. The minimum 7 hours of sunlight is sufficient to shade off the bulky winter coats, but 10 to 12 wet days in the month ask for at least a rain jacket to combat the moistness. The vast prairie is green to the hilt, as the rain and melting of snow fill the rivers, lakes, and streams with abundant freshwater. April is one of the best times to visit Illinois.
Weather in May
May is warm in late spring in Illinois but prone to occasional bouts of heavy rain. The average high temperatures are in the beautiful 68°F (20°C) to 78°F (25.6°C) range, while the nights are comfortable between 46°F (7.8°C) to 57°F (13.9°C). Marion in the south averages in the warm 54°F (12.2°C) to 74°F (23.3°C) range, while Antioch in the north is between 47°F (8.3°C) to 68°F (20°C) and Quincy in the center ranges between 53°F (11.7°C) to 73°F (22.8°C). The last remnants of snow disappear from the ground as substantial rainfall to the tune of 5" (127mm) to 6" (152.4mm) occurs in many parts, especially in the south. Tourist numbers swarm in the state parks as the conditions are warm in the absence of cold waves. The skies quickly transition from sunny to cloudy, and heavy rain catches tourists by surprise. The season of thunderstorms and tornadoes begins in May and often brings severe bouts of weather. Expect occasional spells of heavy rain along with occasional hail. Spring is the time for scenic drives in the breezy atmosphere of May in Illinois.
Weather in June
June is the beginning of the warm to the hot summer season in Illinois as the landscape bathes in bright sunshine. The average high temperatures are in the warm 75°F (23.9°C) to hot 85°F (29.4°C) range, while average low temperatures hover in the mild 55°F (12.8°C) to 65°F (18.3°C). Alton and Cahokia in central Illinois, register average temperatures in the 65°F (18.3°C) to 85°F (29.4°C) range. The longest days of the year provide ample daylight for hiking, biking, and camping. Forests and state parks see the heaviest footfalls even though June brings heavy rain. No matter the location, there is a state park, forest, or woody area nearby to explore and hike. Occasional hailstorms accompany heavy rainfall to damage crops in many parts of the state. Sweatshirts and t-shirts provide comfort during the day when the heat and humidity are high. Carry adequate rain protection on outdoor trips, especially when camping for the night. Keep an eye on the weather for tornadoes in June in Illinois.
Weather in July
July is by far the hottest month of the year in Illinois with the highest number of sunny days. The average high temperatures in the state are in the hot 82°F (27.8°C) to 89°F (31.7°C), while the nights are comparatively mild between 62°F (16.7°C) to 69°F (20.6°C). Day temperatures reach 90°F (32.2°C) to 100°F (37.8°C) in the event of a heatwave at the peak of summer. Peoria in central Illinois registers temperatures between 66°F (18.9°C) to 86°F (30°C), while Cairo in the south is between 66°F (18.9°C) to 89°F (31.7°C). The 4" (101.6mm) to 5" (127mm) of precipitation is mainly due to thunderstorms that are prominent during summer. Expect humidity to rise rapidly and increase the overall heat index. Fishing, swimming, and boating are popular with many creeks, lakes, and rivers in the state. It is better to stay in the shade during afternoons when the heat is at its peak. Wear sunglasses and large-brimmed hats on outdoor adventures such as hiking and trailing in the hills and mountains. Beware of extreme temperatures in July in Illinois.
Weather in August
August is a hot and humid month in Illinois with long daylight hours and many sunny days. The average high temperatures reach up to 88°F (31.1°C), with the highest temperatures in central and southern Illinois. The northern regions have relatively mild temperatures by a few degrees and comfortable nights. Moline in the north registers temperatures in the 63°F (17.2°C) to 84°F (28.9°C) range. The sunny days and clear night skies offer the best time for camping and stargazing in the state parks. Summer rainfall is to the tune of 3" (76.2mm) to 4" (101.6mm) statewide. The peak of the tornado season is over, but thunderstorms and occasional tropical cyclones are still at work in August. Light summer clothing is comfortable during hot days, as the perspiring body cools faster in a sweatshirt. Lakes are hot spots during the summer as the proximity to the water brings a refreshing coolness to the atmosphere. August is an excellent time to visit Illinois for people who can bear the summer heat.
Weather in September
September brings the beautiful fall season to Illinois with comfortable conditions and dazzling colors. The daily sunlight of 8 hours peaks in the afternoon, while the humidity is at the highest in the mornings. The average temperatures are in the pleasant 50°F (10°C) to 80°F (26.7°C) range across the state, with 3" (76.2mm) to 4" (101.6mm) of rainfall. The maples, elms, and walnuts change colors by mid-September as the landscape assumes a fairyland look. By the end of September, the hues are prominent with brilliant shades of red, orange, yellow, purple, and magenta. An autumn weekend is ripe to experience a fall color festival. Colors run amok from rural farming towns to urban metropolitan areas and their suburbs. Fall evenings are perfect for romantic settings in a natural environment untouched by time. Light jackets provide comfort during the breezy evenings, while t-shirts are suitable during the day when the temperatures are high. September is one of the best times to visit Illinois.
Weather in October
October is mild and colorful in Illinois as fall foliage is at its peak in the entire state. The average high temperatures are in the beautiful range of 60°F (15.6°C) to 70°F (21.1°C), while the average low temperatures are in the mild 40°F (4.4°C) to 50°F (10°C) zone. Rockford in the north averages between 41°F (5°C) to 63°F (17.2°C), while Mount Vernon in the south is between 44°F (6.7°C) to 67°F (19.4°C). The fall foliage is at its peak throughout Illinois by the third week of October. The atmosphere is perfect for scenic hikes in the Shawnee National Forest in southern Illinois with breathtaking nature views along the trails. Fall is the time to harvest the abundant supply of fresh apples with their aroma filling the orchards. Pumpkin patches and corn mazes rule the roost. Great River Road offers terrific views of the Mississippi River during the autumn season as it runs through miles of woodlands. Expect the conditions to steadily become colder with each passing day in October in Illinois.
Weather in November
November is mild to cold in the last month of the fall season in Illinois. The daylight recedes rapidly, and the night temperatures drop drastically. The average temperatures are in the range of 28°F (-2.2°C) to 51°F (10.6°C), with 8 to 10 wet days that bring 2" (50.8mm) to 4" (101.6mm) of rainfall. The daily sunshine reduces to about 4 to 5 hours and is moderate at best, as cloudy skies dominate the late fall season. The first snowflakes of the season arrive in November, which registers 1" (25.4mm) to 2" (50.8mm) of snow. The Chicago Area registers temperatures in the 35°F (1.7°C) to 51°F (10.6°C) range, while Urbana in central Illinois is between 32°F (0°C) to 51°F (10.6°C). The fall foliage is past its peak in the northern regions as trees begin to shed leaves. Expect frost in late fall to go along with snow and ice. Prepare for the arrival of the winter by late November. Tourist numbers drop drastically along with temperatures by the mid of November in Illinois.
Weather in December
December begins the winter season cold in Illinois with a steep decline in temperatures. The average high temperatures are in the cold to the mild zone of 32°F (0°C) to 46°F (7.8°C), while the average low temperatures are in the freezing range of 16°F (-8.9°C) to 29°F (-1.7°C). The precipitation is mainly in the form of snow and ice, as rainfall hardly crosses 2" (50.8mm) to 3" (76.2mm) statewide. Snow covers the ground and registers in the 6" (152.4mm) to 12" (304.8mm) in the central to northern regions. The south receives light snowfall to the tune of 1" (25.4mm) to 3" (76.2mm). Illinois turns into a winter wonderland as a white Christmas dawns in the upper half of the state. Strong winds add to the chilliness as the daily sunshine limits to 4 hours, and the days become short. Warm winter coats, gloves, and caps drop out of the closets as the partly cloudy skies provide little chance of warm sunshine. December has rare periods of favorable weather for a visit to Illinois.