Do you need to know, when is the best time to go to Illinois? Then you should take a look here, what weather you can expect there during the year.
Illinois is part of USA - United States of America. The climate of USA is very varied, as United States of America is a large country. The climate is mostly temperate, but tropical in Hawaii and Florida, and arctic in Alaska. This is not all. The climate is semiarid in the great plains west of the Mississippi River, and arid in the Great Basin of the southwest. You can expect low winter temperatures in the northwest, but they are sometimes moderated in January and February by warm winds. Winters in the south are warmer. Don? forget, that temperatures are usually significantly lower in mountains.
When speaking about the usual weather in this area, the information is relevant for example for these popular places: Adams County, Alton, Arcola, Arenzville, Arlington Heights, Atlanta, Aurora, Aviston, Barrington, Bartlett, Belleville, Berwyn, Blackhawk Waterways, Bloomington Normal, Blue Island, Bolingbrook, Bourbonnais, Bradford, Bradley, Brookfield, Buffalo Grove, Cahokia, Cahokia Mounds State Historic Site, Cairo, Carbondale, Carlinville, Central Illinois, Centralia, Chain O' Lakes, Champaign Urbana, Chenoa, Cherry, Chicago, Chicago Heights, Chicagoland, Cicero, Cobden, Collinsville, Crystal Lake, Danville, DeKalb, Decatur, Deerfield, Des Plaines, Dixon, Downers Grove, Dwight, East Peoria, East St. Louis, Edwardsville, Effingham, El Paso, Elgin, Elk Grove Village, Elmhurst, Elmwood Park, Evanston, Evergreen Park, Fairmont City, Far North Suburbs, Fithian, Forgottonia, Fox Lake, Fox River Valley, Franklin Park, Freeport, Galena, Galesburg, Gardner, Geneva, Giant City State Park, Gilman, Glen Ellyn, Glencoe, Glenview, Granite City, Grayslake, Grundy County, Gurnee, Hamel, Hartford, Harvard, Harvey, Highland Park, Hillside, Hinsdale, Illinois Driftless Area, Inner Suburbs, Itasca, Jacksonville, Joliet, Kankakee, Kankakee Area, Kishwaukee Valley, LaSalle Peru, La Grange, Lake Forest, Lansing, Lebanon, Lemont, Lexington, Libertyville, Limestone, Lincoln, Lisle, Litchfield, Lombard, Long Grove, Macomb, Makanda, Manteno, Marion, McHenry County, McLean, Medinah, Melrose Park, Metropolis, Moline, Momence, Monticello, Morris, Morton Grove, Mount Olive, Mount Prospect, Mount Vernon, Naperville, Nauvoo, New Lenox, Niles, North Chicago, North Shore, Northbrook, Northern Illinois, Northfield, Northwest Suburbs, O'Fallon, Oak Forest, Oak Park, Park Ridge, Paxton, Peoria, Petersburg, Piper City, Plymouth, Pontiac, Quincy, Rantoul, Rochelle, Rock Island, Rock River Valley, Rockford, Roselle, Rosemont, Round Lake, Salem, Sandoval, Schaumburg, Schiller Park, Shawnee National Forest, Shelbyville, Sherman, Skokie, South Suburbs, Southern Illinois, Springfield, St. Louis Metro East, Staunton, Sterling Rock Falls, Stockton, Sycamore, Tinley Park, Tuscola, Utica, Vandalia, Washington, Waukegan, West Chicago, Western Illinois, Western Suburbs, Wheaton, Williamsville, Wilmette, Winnetka, Wood Dale, Woodridge, Woodstock.
Let's look now what weather you can expect in the area around Chicago and in many other cities. The warmest month here is July, the average maximum temperature is 29.2 ℃ (84.56 ℉). The average minimum temperature in this month is 18 ℃ (64.4 ℉). The coldest month here is January, the average maximum temperature is -0.4 ℃ (31.28 ℉). The average minimum temperature in this month is -8.5 ℃ (16.7 ℉). The most rainy month here is April, the average number of rainy days is 11.9. The least rainy month here is September, the average number of rainy days is 8.1. 
When travelling around Illinois, always consider the altitude of the the place as well. We should remind you, that the highest point in USA - United States of America is Denali (Mount McKinley) (6190 m) and the lowest point is Death Valley (-86 m). So there are huge differences in altitudes in this country and the weather is very different in them therefore. Keep in mind, that the temperature usually decreases by 1.2 - 1.9 ℃ (2.2 - 3.5 ℉) per 200 m (656 ft), depending on the other conditions. (And we should add, that in the very special condition named thermal inversion even the temperature may be higher in higher altitudes.)
Typical (usual) weather in Illinois in January is this: The average highest temperature in Illinois in January is -0.4 ℃ (31.28 ℉). The average lowest temperature is -8.5 ℃ (16.7 ℉). At the beginning of January you can expect higher temperatures, the average highest temperature is around 0.7 ℃ (33.26 ℉). At the end of January you can expect higher temperatures, the average highest temperature is around 0.8 ℃ (33.44 ℉). The average number of rainy days in January is 10.3. The average rainfall is 42.9 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 9:10 (hours and minutes), in the middle of the month 9:27 and at the end of the month 9:57.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Illinois in February is this: The average highest temperature in Illinois in February is 2 ℃ (35.6 ℉). The average lowest temperature is -6.5 ℃ (20.3 ℉). At the beginning of February you can expect lower temperatures, the average highest temperature is around 0.8 ℃ (33.44 ℉). At the end of February you can expect higher temperatures, the average highest temperature is around 5.1 ℃ (41.18 ℉). The average number of rainy days in February is 8.7. The average rainfall is 46 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 9:57 (hours and minutes), in the middle of the month 10:33 and at the end of the month 11:14.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.
Typical (usual) weather in Illinois in March is this: The average highest temperature in Illinois in March is 8.2 ℃ (46.76 ℉). The average lowest temperature is -1.4 ℃ (29.48 ℉). At the beginning of March you can expect lower temperatures, the average highest temperature is around 5.1 ℃ (41.18 ℉). At the end of March you can expect higher temperatures, the average highest temperature is around 11.8 ℃ (53.24 ℉). The average number of rainy days in March is 11.2. The average rainfall is 62.8 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:14 (hours and minutes), in the middle of the month 11:56 and at the end of the month 12:39.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.
Typical (usual) weather in Illinois in April is this: The average highest temperature in Illinois in April is 15.4 ℃ (59.72 ℉). The average lowest temperature is 4.1 ℃ (39.38 ℉). At the beginning of April you can expect lower temperatures, the average highest temperature is around 11.8 ℃ (53.24 ℉). At the end of April you can expect higher temperatures, the average highest temperature is around 18.2 ℃ (64.76 ℉). The average number of rainy days in April is 11.9. The average rainfall is 86.1 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 12:39 (hours and minutes), in the middle of the month 13:21 and at the end of the month 13:59.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Illinois in May is this: The average highest temperature in Illinois in May is 21 ℃ (69.8 ℉). The average lowest temperature is 9.4 ℃ (48.92 ℉). At the beginning of May you can expect lower temperatures, the average highest temperature is around 18.2 ℃ (64.76 ℉). At the end of May you can expect higher temperatures, the average highest temperature is around 24 ℃ (75.2 ℉). The average number of rainy days in May is 11.6. The average rainfall is 92.3 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 13:59 (hours and minutes), in the middle of the month 14:33 and at the end of the month 14:59.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.
Typical (usual) weather in Illinois in June is this: The average highest temperature in Illinois in June is 27 ℃ (80.6 ℉). The average lowest temperature is 15.1 ℃ (59.18 ℉). At the beginning of June you can expect lower temperatures, the average highest temperature is around 24 ℃ (75.2 ℉). At the end of June you can expect higher temperatures, the average highest temperature is around 28.1 ℃ (82.58 ℉). The average number of rainy days in June is 10.2. The average rainfall is 89.5 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 14:59 (hours and minutes), in the middle of the month 15:12 and at the end of the month 15:12.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.
Typical (usual) weather in Illinois in July is this: The average highest temperature in Illinois in July is 29.2 ℃ (84.56 ℉). The average lowest temperature is 18 ℃ (64.4 ℉). At the beginning of July you can expect lower temperatures, the average highest temperature is around 28.1 ℃ (82.58 ℉). At the end of July you can expect lower temperatures, the average highest temperature is around 28.65 ℃ (83.57 ℉). The average number of rainy days in July is 9.6. The average rainfall is 93.4 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 15:12 (hours and minutes), in the middle of the month 14:57 and at the end of the month 14:31.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Illinois in August is this: The average highest temperature in Illinois in August is 28.1 ℃ (82.58 ℉). The average lowest temperature is 17.1 ℃ (62.78 ℉). At the beginning of August you can expect higher temperatures, the average highest temperature is around 28.65 ℃ (83.57 ℉). At the end of August you can expect lower temperatures, the average highest temperature is around 25.85 ℃ (78.53 ℉). The average number of rainy days in August is 9.6. The average rainfall is 126.6 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 14:31 (hours and minutes), in the middle of the month 13:58 and at the end of the month 13:19.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Illinois in September is this: The average highest temperature in Illinois in September is 23.6 ℃ (74.48 ℉). The average lowest temperature is 12.5 ℃ (54.5 ℉). At the beginning of September you can expect higher temperatures, the average highest temperature is around 25.85 ℃ (78.53 ℉). At the end of September you can expect lower temperatures, the average highest temperature is around 20.25 ℃ (68.45 ℉). The average number of rainy days in September is 8.1. The average rainfall is 82.9 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 13:19 (hours and minutes), in the middle of the month 12:38 and at the end of the month 11:56.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Illinois in October is this: The average highest temperature in Illinois in October is 16.9 ℃ (62.42 ℉). The average lowest temperature is 6.1 ℃ (42.98 ℉). At the beginning of October you can expect higher temperatures, the average highest temperature is around 20.25 ℃ (68.45 ℉). At the end of October you can expect lower temperatures, the average highest temperature is around 12.95 ℃ (55.31 ℉). The average number of rainy days in October is 10.4. The average rainfall is 80.1 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 11:56 (hours and minutes), in the middle of the month 11:14 and at the end of the month 10:34.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Illinois in November is this: The average highest temperature in Illinois in November is 9 ℃ (48.2 ℉). The average lowest temperature is 0.4 ℃ (32.72 ℉). At the beginning of November you can expect higher temperatures, the average highest temperature is around 12.95 ℃ (55.31 ℉). At the end of November you can expect lower temperatures, the average highest temperature is around 5.4 ℃ (41.72 ℉). The average number of rainy days in November is 10.6. The average rainfall is 79 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 10:34 (hours and minutes), in the middle of the month 9:58 and at the end of the month 9:28.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.Typical (usual) weather in Illinois in December is this: The average highest temperature in Illinois in December is 1.8 ℃ (35.24 ℉). The average lowest temperature is -6.1 ℃ (21.02 ℉). At the beginning of December you can expect higher temperatures, the average highest temperature is around 5.4 ℃ (41.72 ℉). At the end of December you can expect lower temperatures, the average highest temperature is around 0.7 ℃ (33.26 ℉). The average number of rainy days in December is 10.7. The average rainfall is 57.1 mm (look here, what this number means). When planning your travel, please, keep in mind, that actual weather may differ from these average values. The length of the day at the beginning of this month is approximately 9:28 (hours and minutes), in the middle of the month 9:10 and at the end of the month 9:07.These numbers above are valid primarily for the capital and the area around it. It is important to say, that the weather may differ significantly in different altitudes, especially in mountains.| Chicago | temperature: - | highest temperature: 1.8 ℃ (35.24 ℉) | lowest temperature: -6.1 ℃ (21.02 ℉) | rainfall: 57.1mm | rainy days: 10.7 |
| Peoria | temperature: - | highest temperature: 2.5 ℃ (36.5 ℉) | lowest temperature: -5.9 ℃ (21.38 ℉) | rainfall: 62.1mm | rainy days: 10.1 |
| Rockford | temperature: - | highest temperature: 0.9 ℃ (33.62 ℉) | lowest temperature: -7.7 ℃ (18.14 ℉) | rainfall: 51.3mm | rainy days: 10 |
| Springfield | temperature: - | highest temperature: 3.6 ℃ (38.48 ℉) | lowest temperature: -5 ℃ (23 ℉) | rainfall: 64.1mm | rainy days: 9.7 |
| Aurora | temperature: - | highest temperature: 1.2 ℃ (34.16 ℉) | lowest temperature: -6.3 ℃ (20.66 ℉) | rainfall: 55.5mm | rainy days: 10.4 |
| Joliet | temperature: - | highest temperature: 2 ℃ (35.6 ℉) | lowest temperature: -6.1 ℃ (21.02 ℉) | rainfall: 55.3mm | rainy days: 11.5 |