Chicago, situated in the upper Midwest, experiences a hot-summer humid continental climate, characterized by cold winters and warm, humid summers. The city’s location near Lake Michigan influences its weather patterns. The annual precipitation figures encompass all forms of moisture, including rain and snow, offering a precise look at the average water Chicago receives annually.
The Annual Precipitation Benchmark
The average annual precipitation for Chicago is 37.86 inches. This figure is calculated from the most recent 30-year climate normal period (1991 to 2020) and represents the liquid equivalent of all moisture falling in a year, including rain, melted snow, sleet, and hail. Using the 30-year average provides a stable benchmark for climate comparison, smoothing out yearly variations.
This 37.86-inch total is a moderate amount for a U.S. city, positioning Chicago as wetter than many western cities but drier than some along the East Coast. The data is officially measured at O’Hare International Airport, the city’s primary climate observation site. The measurement is standardized to ensure that all forms of water are accounted for consistently in the climate record.
Seasonal Distribution of Precipitation
Chicago’s precipitation is distributed relatively evenly throughout the year, without a distinct dry season, which is typical of its humid continental climate. A clear pattern exists, however, with the warmer months receiving the highest average totals. The wettest period generally occurs during the late spring and summer, largely due to convective thunderstorms.
May is the wettest month, averaging 4.49 inches of precipitation, followed closely by August at 4.25 inches. These summer peaks are driven by warm, humid air masses moving up from the Gulf of Mexico, resulting in heavy, short-duration rainfall events. Conversely, the winter months are the driest in liquid equivalent, with February averaging the least moisture at 1.97 inches. The slightly drier winter months are a result of colder air holding less moisture compared to the summer.
Accounting for Winter Precipitation
The annual precipitation total already incorporates all winter moisture, which is measured by converting the snowfall back into its liquid equivalent. Chicago receives an average annual snowfall of 38.4 inches. This conversion is necessary because a given depth of snow contains much less water than the same depth of rain.
Meteorologists use a snow-to-liquid ratio for this calculation, generally estimated at 10:1 (ten inches of snow melts down to one inch of water). However, the National Weather Service often uses a ratio closer to 12:1 for the Upper Midwest, as colder temperatures produce lighter, fluffier snow crystals. The actual ratio can fluctuate significantly, ranging from 5:1 for heavy, wet snow near freezing to 20:1 for very cold, dry snow.