Last updated: January 18, 2025
Illinois experiences a diverse climate characterized by the Köppen-Geiger classifications of Cfa, Dfa, and Dfb, which denote humid subtropical and humid continental zones. The state endures a wide temperature range, experiencing minimums plunging to -34°C (-30°F) during the peak of February, while summer highs can soar to 39°C (102°F) in July. The annual average temperature hovers around 12°C (53°F), reflecting the state's significant seasonal variations and providing a compelling backdrop for both ecological diversity and agricultural practices.
Illinois showcases a dramatic temperature range throughout the year, characterized by distinct seasonal patterns. Winter months (January and February) experience severe cold, with minimum temperatures plummeting to as low as -34°C (-30°F), while average temperatures struggle to exceed -1°C (31°F). As spring arrives in March and April, a gradual warming trend is observed, culminating in more temperate conditions with averages reaching 10°C (51°F) by April. The transition into summer brings notable warmth, with July peaking at 39°C (102°F), alongside consistently high averages, highlighting the state's hot season. In August and September, temperatures remain elevated before a notable decline in the fall, where October's averages drop to 13°C (56°F) and prelude the colder months again. This cycle underscores Illinois' continental climate, marked by harsh winters, mild springs, and hot summers, leading to significant variability that influences both natural ecosystems and human activity.
Illinois showcases a range of temperatures across its cities, with notable variations between Cahokia, Springfield, and Chicago. In Cahokia, temperatures reach a chilly minimum of -21°C (-5°F) in January, warming to a maximum of 38°C (102°F) in July, reflecting its diverse climate profile as seen in the monthly temperature in Cahokia. Meanwhile, Springfield experiences slightly cooler winters with a minimum of -26°C (-15°F) in January, while peaking at 36°C (97°F) in July. More about this can be observed in the monthly temperature in Springfield. Chicago endures the harshest winter weather, with temperatures dropping to -30°C (-22°F) in February, and summers reaching 33°C (92°F) by August. For a deeper insight, refer to the monthly temperature in Chicago.
Illinois exhibits a varied precipitation pattern throughout the year, with a noticeable increase during the spring and early summer months. January and February typically receive about 70 mm (2.7 in) and 73 mm (2.9 in), respectively, while precipitation escalates in March and April to 106 mm (4.2 in) and 121 mm (4.7 in). The climax of rainfall occurs in May, where an average of 156 mm (6.1 in) is recorded, corresponding with the region's peak growing season. This winter-spring transition highlights a rising trend in both rainfall totals and frequency, with May averaging approximately 14 rainy days.
As the summer unfolds, Illinois maintains substantial precipitation levels, with June contributing 123 mm (4.8 in) and July slightly reducing to 98 mm (3.8 in). The latter part of the year sees a gradual decline, with precipitation continuing to fall to 92 mm (3.6 in) in August and 81 mm (3.2 in) in September. October mirrors early spring moistness with 122 mm (4.8 in), while the late fall and winter months see lower totals of 66 mm (2.6 in) in November and 59 mm (2.3 in) in December, resulting in about 7-10 rainy days during these cooler months. This seasonal variation in precipitation is critical for the state's agriculture and ecosystem dynamics.
In Illinois, sunshine duration exhibits a clear seasonal pattern, with a marked increase from the winter months to the peak of summer. January offers just 121 hours of sunshine, but as February arrives, this figure rises to 177 hours, signaling the gradual return of longer days. The trend continues through March and April, where hours of sunshine escalate to 221 and 244 hours, respectively, as spring ushers in milder weather and blooming landscapes, encouraging increased outdoor activity and agricultural preparations.
The summer months shine brightly in Illinois, with July receiving the highest sunshine duration at an impressive 355 hours. June and August also contribute significantly, with 331 and 333 hours respectively, creating a warm and sunlit environment ideal for recreation. As the year progresses into fall, sunshine hours begin to decline, with September showing 286 hours and October dropping to 226 hours. The decline continues into November and December, where the lack of sunlight is pronounced at 198 and 187 hours respectively. These seasonal variations illustrate the impact of Illinois' climate on daily life, emphasizing the importance of the sun for both residents and the natural ecosystem throughout the year.
The UV Index in Illinois reveals a significant seasonal variation, peaking during the summer months when sun exposure is at its highest. Starting from a moderate level of 3 in January and December, the index gradually increases in February to 6 before approaching 7 in March. The arrival of spring is marked by a notable rise, with April reaching 10 and May escalating to a peak of 11 in both June and July. This period poses a heightened risk of harm from unprotected sun exposure, suggesting that residents should take precautions during these sun-drenched days.
As summer transitions to fall, the UV Index begins its gradual decline, with August maintaining a slightly reduced level of 10, followed by September at 9. October and November experience a further dip to 7 and 5, respectively, highlighting the lowering intensity of solar radiation as the year progresses toward winter. This downward trend reflects a critical awareness for individuals to adapt their sun protection strategies depending on the month, particularly in the high UV conditions prevalent during late spring and summer.
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Daylight duration in Illinois demonstrates a clear seasonal pattern, with significant fluctuations throughout the year. In January, residents experience only 9 hours of daylight, gradually increasing to 10 hours by February. As spring approaches, March brings an average of 11 hours, while April further extends the daylight to 13 hours, heralding the arrival of longer, sunnier days. May continues this trend, offering 14 hours of daylight, peaking in June with a robust 15 hours, creating the ideal conditions for outdoor activities and community events.
Following the summer solstice, daylight hours begin to wane, with July still maintaining 14 hours before tapering off to 13 hours in August. The transition into fall results in a gradual decrease, bringing September down to 12 hours and October to 11 hours. As daylight diminishes further, November and December see a return to 9 hours, marking a return to the shorter days typical of winter. These variations in daylight not only affect the natural rhythm of life in Illinois but also influence seasonal activities, agricultural practices, and overall well-being throughout the year.
Cahokia, Illinois experiences a diverse climate with rather extreme temperatures throughout the year. Winters can be quite cold, with January lows plummeting to -21°C (-5°F) and average temperatures hovering around freezing. The summer months bring considerably warmer conditions, with July seeing average highs of up to 38°C (102°F). Precipitation is fairly spread out, peaking in May at 146 mm (5.7 in), while September marks a drier period with only 58 mm (2.3 in) of rainfall. Sunshine hours increase from winter through summer, reaching a peak of 349 hours in July. For more detailed information, visit the Cahokia climate page.
Chicago experiences a diverse climate characterized by cold winters and warm summers, marked by significant seasonal variations. In winter, temperatures can drop as low as -30°C (-22°F) in February, with average temperatures hovering just below freezing, while snowfall and precipitation peak in April at 125 mm (4.9 in). As spring progresses into summer, temperatures rise considerably, with July averaging 24°C (75°F) and experiencing up to 362 sunshine hours. Rainfall is highest in May at 156 mm (6.1 in), contributing to the vibrant spring growth leading into the warmer months. Chicago climate is influenced by its geographical position, adding a dynamic range to its weather patterns.
Galena, United States experiences a diverse climate with cold winters and warm summers, highlighted by a wide range of temperatures throughout the year. January features temperatures as low as -32°C (-26°F) and up to 10°C (50°F), while July warms significantly with a range from 13°C (56°F) to 35°C (95°F). Precipitation varies seasonally, with May being the wettest month at 146 mm (5.8 in) and December the driest at 43 mm (1.7 in). Sunshine hours peak in July at 361 hours, providing long, bright days in summer, compared to 133 hours in January. Galena climate details more about these weather patterns.
Grant Park, United States, experiences a diverse climate with significant seasonal variations. Winters are cold, with average temperatures in January reaching -3°C (27°F) and plunging to as low as -31°C (-24°F). As spring unfolds, temperatures climb, with March averaging 5°C (41°F) and April reaching up to 29°C (85°F). Summers are warmer, with June and July averaging around 23-24°C (73-76°F) and occasional highs of 35°C (95°F). Precipitation varies throughout the year, peaking in May at 161 mm (6.3 in), while sunshine is most abundant during July, reaching 353 hours. Learn more about the Grant Park climate.
Nauvoo, located in the United States, experiences a diverse climate with cold winters and warm summers. January is particularly frigid, averaging -3°C (27°F), with temperatures dipping as low as -29°C (-20°F). In contrast, July is the warmest month, averaging 26°C (79°F) and reaching highs of up to 37°C (100°F). The area receives variable precipitation throughout the year, notably in May with 169 mm (6.7 in) of rainfall. For more detailed information on Nauvoo's climate, visit Nauvoo climate.
Springfield, United States experiences a diverse climate with significant seasonal temperature fluctuations. Winters are cold, with January's average temperature at -2°C (29°F) and lows reaching -26°C (-15°F), while summers are warm, with July averaging 25°C (78°F) and highs hitting 36°C (97°F). Precipitation is moderate throughout the year, peaking in May with 144 mm (5.7 in), and the city enjoys the most sunshine in July with 351 hours. For more detailed information, visit the Springfield climate.
The climate in The Galena Territory, United States, is characterized by cold winters and warm summers. In January, temperatures range from a minimum of -32°C (-26°F) to a maximum of 11°C (52°F), while July sees temperatures between 12°C (55°F) and 35°C (95°F). Precipitation peaks during May with 144 mm (5.7 in), making it the wettest month, while December receives the least at 44 mm (1.7 in). Sunshine hours increase significantly from 133 hours in January to 361 hours in July, offering abundant daylight during the summer. For more details on weather patterns, visit The Galena Territory climate.
In summary, Illinois experiences distinct climate characteristics marked by varying temperature, precipitation, sunshine duration, UV Index, and daylight hours throughout the year. The state’s winters are notable for their cold temperatures and limited sunlight, while the summer months bring warmth, abundant sunshine, and higher UV levels, necessitating precautions for outdoor activities. Seasonal transitions also play a crucial role in influencing agriculture and ecosystem dynamics, as seen in the patterns of precipitation and daylight duration. Understanding these climatic and environmental factors is essential for residents and promotes informed decisions related to health, outdoor engagement, and agricultural practices in the diverse Illinois landscape.