WEATHER · CLIMATE NORMALS · 1991–2020
Chicago Climate — Averages by Month
What the weather in Chicago, USaverages across a standard 30-year period. Each month's figure is the 1991–2020 mean temperature and total rainfall from the ERA5 reanalysis — the long-run normal a place settles around, not a forecast for any particular day.
Month-by-month normals
| Month | Avg temperature | Avg rainfall |
|---|---|---|
| January | -3.7 °C · coldest | 64.0 mm |
| February | -2.3 °C | 56.3 mm |
| March | 2.4 °C | 69.4 mm |
| April | 8.2 °C | 110.8 mm · wettest |
| May | 14.6 °C | 107.9 mm |
| June | 20.8 °C | 96.1 mm |
| July | 23.7 °C · warmest | 90.0 mm |
| August | 22.9 °C | 96.3 mm |
| September | 18.9 °C | 77.6 mm |
| October | 12.2 °C | 85.6 mm |
| November | 5.3 °C | 70.7 mm |
| December | -0.5 °C | 63.9 mm |
How to read these numbers
These are reanalysis normals, not readings from a single weather station. They come from the ERA5 0.25° (~25 km) global reanalysis grid, sampled at the grid cell nearest Chicago — centred at 42.00°N, 87.75°W. A reanalysis blends observations with a physics model onto an even grid, so a coastal or mountain city's cell can read a little cooler, warmer, wetter or drier than the city centre itself. The month-to-month shape — when it's hot, when it rains — is what these normals capture best.
“Normal” means the 1991–2020 average, the current World Meteorological Organization standard period. A normal is what a place tends toward over decades — any single day, month or year can and will depart from it.