Every number below comes from one place: the National Weather Service's official 1991–2020 climate normals for Chicago, the thirty-year averages the forecasters themselves work from.
Two of those numbers surprise people. The wet season runs late spring through summer — May, June and August are the three rainiest months of the year, not April. And the biggest single warmup on the calendar is March into April, when the normal high jumps a full twelve degrees.
The Year in One Table
| Month | High °F | Low °F | Rain (in) | Snow (in) |
|---|---|---|---|---|
| January | 31.6 | 18.8 | 1.99 | 11.3 |
| February | 35.7 | 21.8 | 1.97 | 10.7 |
| March | 47.0 | 31.0 | 2.45 | 5.5 |
| April | 59.0 | 40.3 | 3.75 | 1.3 |
| May | 70.5 | 50.6 | 4.49 | trace |
| June | 80.4 | 60.8 | 4.10 | 0 |
| July | 84.5 | 66.4 | 3.71 | 0 |
| August | 82.5 | 65.1 | 4.25 | 0 |
| September | 75.5 | 57.1 | 3.19 | 0 |
| October | 62.7 | 45.4 | 3.43 | 0.2 |
| November | 48.4 | 34.1 | 2.42 | 1.8 |
| December | 36.6 | 24.4 | 2.11 | 7.6 |
Annual: 37.86 inches of precipitation, 38.4 inches of snow, and a mean temperature of 51.2°F. The numbers are recorded at O'Hare; close to the lake, summer afternoons run cooler and winter lake-effect bands can run snowier.
Winter: December Through February
December
36.6 / 24.4 °F · 2.11 in precip · 7.6 in snow
The mildest of the three on paper — December's snow normal is barely two-thirds of January's, and the cold usually builds through the month rather than arriving with it. December also means buying a real tree in DuPage, where there is no cut-your-own farm in the county at all. The nearest one is about 25 minutes west.
January
31.6 / 18.8 °F · 1.99 in precip · 11.3 in snow
The coldest and snowiest month, and the normals understate the range — a January week can sit below zero or above forty. Plan around the snow total: eleven inches doesn't arrive evenly, it arrives in two or three storms.
February
35.7 / 21.8 °F · 1.97 in precip · 10.7 in snow
Statistically January with slightly better marketing. The snow machine is still fully on; the four-degree improvement in the normal high is not something you can feel.
Spring: March Through May
March
47.0 / 31.0 °F · 2.45 in precip · 5.5 in snow
Up 11.3 degrees on the normal high — the year's second-biggest jump, and the first one anyone feels. It still snows (five and a half inches in the normals), but the snow stops staying.
April
59.0 / 40.3 °F · 3.75 in precip · 1.3 in snow
The month everyone asks about, usually while planning something outdoors. This is the year's biggest month-over-month warmup, twelve full degrees on the normal high. The normals still say a real chance of rain and a little snow that melts on landing. Pack layers, not faith.
May
70.5 / 50.6 °F · 4.49 in precip · trace snow
The wettest month of the year, and also the month the weather becomes reliably pleasant between the rains. Patio season opens on the first warm Saturday regardless of what the calendar says.
Summer: June Through August
June
80.4 / 60.8 °F · 4.10 in precip
Summer arrives fully formed, with thunderstorms attached — June's four-plus inches of rain mostly fall in bursts you can wait out.
July
84.5 / 66.4 °F · 3.71 in precip
The hottest month. The normal high of 84.5 undersells the humid stretches when heat indices go well past ninety; the lake is the natural air conditioner, which is why the beaches earn their crowds.
August
82.5 / 65.1 °F · 4.25 in precip
Barely distinguishable from July, and the second-wettest month of the year. The rain normal is the one to respect when planning an outdoor event: an August afternoon storm is routine, not bad luck.
Fall: September Through November
September
75.5 / 57.1 °F · 3.19 in precip
The best weather month in Chicago. Mid-seventies, the driest month since March, and the humidity finally breaks — no other month does all of that at once. It's also when the fall festival calendar starts filling weekends.
October
62.7 / 45.4 °F · 3.43 in precip · 0.2 in snow
Crisp and orange, and wetter than people remember. This is pumpkin patch weather, and the first trace of snow shows up in the normals right at the end.
November
48.4 / 34.1 °F · 2.42 in precip · 1.8 in snow
The year's steepest drop — the normal high falls more than fourteen degrees from October. Real snow returns in small amounts, and daylight goes early. The outdoor season is over; nobody sends a memo.
The Best Time to Visit
September, then early October, then late May into June.
September wins on every measure at once — warm and comparatively dry, with the festival calendar filling. June trades a few degrees of comfort for thunderstorm risk. July and August work if the heat doesn't bother you, but they're the months where the lake breeze is the difference between pleasant and punishing.
The months to plan around rather than for: January through mid-March if you dislike winter, and April if your plans can't survive rain.
Takeaway: September is the payoff month, and May is secretly the wettest. The year's biggest weather shift is March turning into April, a twelve-degree jump that explains the local mood in early spring.
All figures are the National Weather Service's 1991–2020 climate normals for Chicago (O'Hare station), the standard averaging period in use until the next update in 2031. Normals are averages, not forecasts — any given week can and will ignore them.
This guide is part of The Car-Free Renter's Suburban Starter Pack.