Illinois weather and outdoor projects
Illinois has a continental climate with cold winters, hot and humid summers and plenty of spring storms. Chicago is moderated by Lake Michigan: springs near the lake are cooler and fall stays milder a bit longer. Central and southern Illinois warm up earlier in spring.
Late May through June and September are the most reliable months for staining and painting. Summer can bring humid stretches and thunderstorms, so watch the overnight forecast. Winters bring many freeze-thaw cycles, which damage concrete that was not cured and sealed well, so the timing of fall pours matters.
Frost dates in Illinois
In Chicago (O’Hare), the median last spring freeze (32°F) is around April 16 and the median first fall freeze is around October 28, a frost-free season of about 193 days.
In Springfield, the median last spring freeze (32°F) is around April 14 and the median first fall freeze is around October 20, a frost-free season of about 187 days.
How Illinois compares with other states
Chicago (O’Hare) is tied for the 7th-longest deck staining season of the 50 state main stations (5 good months). Chicago (O’Hare) has the 15th-most freezing nights of the 50 state main stations (121 a year). Chicago (O’Hare) is tied for the 15th-fewest days at 90°F or hotter of the 50 state main stations (17 a year).
| Measure (Chicago (O’Hare)) | Value | Rank, highest first |
|---|---|---|
| Deck staining season | 5 good months | 7th of 50 |
| Frost-free season | 193 days | 31st of 50 |
| Freezing nights | 121 a year | 15th of 50 |
| Rainy days | 72 a year with 0.1 in or more | 28th of 50 |
| Days at 90°F or hotter | 17 a year | 35th of 50 |
| Summer afternoons | 51% humidity at 3 PM, June–August | 18th of 47 |
Best time to stain a deck in Illinois
In Chicago (O’Hare), the most reliable months are May–September. April and October can also work on the right days (average highs Apr 59°F, Oct 63°F). November–March is too cold, with average highs around 40°F. April is cool, with highs around 59°F, so choose the warmest afternoons. In October, nights average about 45°F, so finish early on mild days and check the overnight low.
In Springfield, the most reliable months are May–September. April and October can also work on the right days (average highs Apr 66°F, Oct 67°F). November–March is too cold, with average highs around 45°F. In April and October, nights average about 44°F, so finish early on mild days and check the overnight low.
Best time to paint outside in Illinois
In Chicago (O’Hare), the most reliable months are May–October. April can also work on the right days (average highs Apr 59°F). November–March is too cold, with average highs around 40°F. In April, nights average about 40°F, so finish early on mild days and check the overnight low.
In Springfield, the most reliable months are May–September. March–April and October–November can also work on the right days. December–February is too cold, with average highs around 39°F. March and November are cool, with highs around 53°F, so choose the warmest afternoons. In April and October, nights average about 44°F, so finish early on mild days and check the overnight low.
Best time to pour concrete in Illinois
In Chicago (O’Hare), the most reliable months are May–September. April and October can also work on the right days (average highs Apr 59°F, Oct 63°F). November–March averages about 22 nights a month at or below 32°F, so fresh concrete needs blankets or heated enclosures. April and October bring about 4 freezing nights a month, so pick a mild spell and keep curing blankets on hand.
In Springfield, the most reliable months are May–September. April and October can also work on the right days (average highs Apr 66°F, Oct 67°F). November–March averages about 21 nights a month at or below 32°F, so fresh concrete needs blankets or heated enclosures. April and October bring about 4 freezing nights a month, so pick a mild spell and keep curing blankets on hand.
Best time to seal a driveway in Illinois
In Chicago (O’Hare), the most reliable months are June–September. May and October can also work on the right days (average highs May 71°F, Oct 63°F). November–April is too cold, with average highs around 43°F. In May, nights average about 51°F, so finish early on mild days and check the overnight low. October is cool, with highs around 63°F, so choose the warmest afternoons.
In Springfield, the most reliable months are May–September. April and October can also work on the right days (average highs Apr 66°F, Oct 67°F). November–March is too cold, with average highs around 45°F. April and October are cool, with highs around 67°F, so choose the warmest afternoons.
Best time to fertilize or treat a lawn in Illinois
In Chicago (O’Hare), the most reliable months are May–October. April can also work on the right days (average highs Apr 59°F). November–March is too cold, with average highs around 40°F. April is cool, with highs around 59°F, so choose the warmest afternoons.
In Springfield, the most reliable months are April–June and August–October. March, July and November can also work on the right days (average highs Mar 53°F, Jul 87°F, Nov 53°F). December–February is too cold, with average highs around 39°F. March and November are cool, with highs around 53°F, so choose the warmest afternoons. July is hot, with highs near 87°F, so start early and keep out of full afternoon sun.
How the weather differs across Illinois
| Place | Hottest month avg high | Coldest month avg low | Rain per year | Last spring freeze | First fall freeze |
|---|---|---|---|---|---|
| Chicago (O’Hare) | 85°F | 19°F | 38 in | April 16 | October 28 |
| Springfield | 87°F | 20°F | 38 in | April 14 | October 20 |
Of the places above, Chicago (O’Hare) has the most good months for staining, painting and concrete combined (16 job-months), and Springfield has the fewest (15).
Monthly weather in Chicago (O’Hare)
| Month | Avg high | Avg low | Wet days* | Rain | 3 PM humidity | Freezing nights |
|---|---|---|---|---|---|---|
| January | 32°F | 19°F | 5 | 2.0 in | 67% | 28 |
| February | 36°F | 22°F | 5 | 2.0 in | 60% | 24 |
| March | 47°F | 31°F | 6 | 2.5 in | 53% | 20 |
| April | 59°F | 40°F | 7 | 3.8 in | 47% | 6 |
| May | 71°F | 51°F | 8 | 4.5 in | 49% | 0 |
| June | 80°F | 61°F | 7 | 4.1 in | 50% | 0 |
| July | 85°F | 66°F | 6 | 3.7 in | 50% | 0 |
| August | 83°F | 65°F | 7 | 4.3 in | 52% | 0 |
| September | 76°F | 57°F | 6 | 3.2 in | 51% | 0 |
| October | 63°F | 45°F | 6 | 3.4 in | 52% | 3 |
| November | 48°F | 34°F | 5 | 2.4 in | 58% | 15 |
| December | 37°F | 24°F | 5 | 2.1 in | 66% | 25 |
*Days with 0.1 inch of rain or more; column notes.
Monthly weather in Springfield
| Month | Avg high | Avg low | Wet days* | Rain | 3 PM humidity | Freezing nights |
|---|---|---|---|---|---|---|
| January | 36°F | 20°F | 5 | 2.0 in | 65% | 27 |
| February | 41°F | 24°F | 5 | 1.9 in | 59% | 23 |
| March | 53°F | 33°F | 6 | 2.8 in | 50% | 16 |
| April | 66°F | 43°F | 7 | 4.0 in | 46% | 4 |
| May | 76°F | 54°F | 8 | 4.5 in | 49% | 0 |
| June | 84°F | 63°F | 7 | 4.6 in | 52% | 0 |
| July | 87°F | 66°F | 5 | 3.9 in | 56% | 0 |
| August | 85°F | 64°F | 5 | 3.4 in | 55% | 0 |
| September | 80°F | 56°F | 5 | 2.9 in | 46% | 0 |
| October | 67°F | 45°F | 6 | 3.3 in | 46% | 4 |
| November | 53°F | 34°F | 6 | 2.7 in | 54% | 14 |
| December | 41°F | 25°F | 5 | 2.1 in | 65% | 24 |
Source: NOAA NCEI U.S. Climate Normals 1991–2020 for CHICAGO OHARE INTL AP, SPRINGFIELD CAPITAL AP. See how the ratings work and how late in fall you can stain, paint or seal.
Frequently asked questions
When is the best time to stain a deck in Illinois?
In Chicago (O’Hare), May–September. In Springfield, May–September.
When is the last frost in Illinois?
Chicago (O’Hare): around April 16; Springfield: around April 14.
Can you pour concrete in winter in Illinois?
Yes, with cold-weather practices: Chicago (O’Hare) averages about 78 freezing nights from December to February, so fresh concrete needs insulated blankets, a heated enclosure or an accelerated mix, and must not freeze in the first 48 hours. Many homeowners wait for spring.
What months are best for sealing a driveway in Illinois?
In Chicago (O’Hare), June–September. In Springfield, May–September.