Chicago records about 940mm of rain across roughly 115 wet days a year, which is a 32% chance on an average day. May is the wettest month at 106mm (11% of the annual total) and February the driest at 49mm. Local rain type: thunderstorms. Chicago summers bring powerful thunderstorms off Lake Michigan. Always check weather alerts from May through September — storms can be severe.
Chance of rain on an average day: 32%
Rainfall climatology for Chicago. The live hour-by-hour answer for today loads at the top of this page from Open-Meteo and refreshes through the day — the figures below describe a typical year, not today.
Last updated: · Live forecast data refreshes hourly.
| Annual rainfall | 940mm |
| Rain days per year | 115 |
| Odds on an average day | 32% |
| Wettest month | May — 106mm over 12 days |
| Driest month | February — 49mm over 6 days |
| Rain type | thunderstorms |
| Best dry window | September and early October offer warm, increasingly dry weather — "Indian summer" at its finest. |
| Worst rain window | May through July brings the heaviest rainfall, often as severe thunderstorms with dramatic lightning over the lake. |
May delivers 116% more rain than February (106mm vs 49mm). That gap is the difference between planning around showers and ignoring them.
No single month holds more than 11% of the annual total, so rain in Chicago is spread through the year — there is no season you can rely on being dry.
Chicago rain arrives in short, heavy bursts — shelter and timing matter more than gear.
Rain falls on about 12 days in May against 6 in February — the frequency swing matters more for planning than the millimetre totals.
Climate normals. Use them to judge whether today's live forecast at the top of the page is typical or unusual for the season.
| Month | Rainfall | Rain days | Chance on a given day | Highs |
|---|---|---|---|---|
| January | 52mm | 6 | 20% | 0°C |
| February | 49mm | 6 | 20% | 0°C |
| March | 68mm | 8 | 27% | 3°C |
| April | 92mm | 11 | 37% | 10°C |
| May | 106mm | 12 | 40% | 18°C |
| June | 105mm | 12 | 40% | 25°C |
| July | 101mm | 12 | 40% | 28°C |
| August | 101mm | 12 | 40% | 28°C |
| September | 84mm | 10 | 33% | 25°C |
| October | 82mm | 10 | 33% | 18°C |
| November | 79mm | 9 | 30% | 10°C |
| December | 65mm | 8 | 27% | 3°C |
Rain probability is the chance that at least 0.2mm falls at your location — not how long it rains. For Chicago we combine the hourly probability curve, expected volume in millimetres, and the city's own rainfall record into a single umbrella verdict.
| Factor | Weight | What it measures |
|---|---|---|
| Hourly probability | 40% | Peak and sustained chance of measurable rain across the next 24 hours. |
| Expected volume | 30% | Total millimetres, which separates a passing shower from a genuine soaking. |
| Timing & duration | 20% | When rain starts, when it eases, and the longest continuous dry window. |
| Local rain type | 10% | Drizzle, brief showers, thunderstorms or monsoon — each needs different gear. |
Live hourly values come from Open-Meteo and refresh through the day. Monthly rainfall figures are climate normals, so they describe a typical year rather than a forecast.
Chicago records rain on roughly 115 days a year — about a 32% chance on any given day — totalling around 940mm.
May, with about 106mm over 12 rain days. That is 11% of the entire year's rainfall in a single month.
February is the driest month at 49mm across roughly 6 days. September and early October offer warm, increasingly dry weather — "Indian summer" at its finest.
Chicago summers bring powerful thunderstorms off Lake Michigan. Always check weather alerts from May through September — storms can be severe.
No. Probability is the chance that at least 0.2mm of rain falls at your location at some point in the period. Check the expected millimetres alongside it: 40% with 0.4mm is a passing shower, 40% with 12mm is a soaking.