The exact moment fall arrives in Chicago depends entirely on the definition used to mark the seasonal shift. The start date of the season is not a single, universally defined day, but rather a concept interpreted differently by astronomers, weather experts, and local residents. The transition from summer heat to cooler temperatures occurs gradually across the region, making the academic definition and the lived experience of the season quite distinct.
The Astronomical Start Date
The astronomical definition of fall is based on the Earth’s position in its orbit around the sun. This moment is known as the Autumnal Equinox in the Northern Hemisphere, and it typically occurs on September 22nd or 23rd. The equinox is a precise moment in time when the center of the sun passes directly over the Earth’s equator. This alignment results in the duration of daytime and nighttime being nearly equal across the entire globe. Following the equinox, the Northern Hemisphere begins to tilt away from the sun, causing day length to progressively shorten and temperatures to decrease. Since the Earth’s orbit is elliptical and its axis tilt is constant, the exact time and date of the equinox shifts slightly each year, which prevents the start of astronomical fall from being a fixed calendar day.
The Meteorological Start Date
Meteorologists and climatologists use a fixed calendar approach to define the seasons, which provides consistent three-month periods for climate record-keeping. Meteorological fall is defined as the three-month span of September, October, and November. This fixed date system is used by weather services for statistical consistency and to align with the annual temperature cycle. The calendar-based definition begins on September 1st and ends on November 30th. This allows scientists to group weather observations and calculate seasonal averages more accurately, simplifying the comparison of climatological statistics from one year to the next and aiding in the study of long-term climate trends.
Local Markers of Fall in Chicago
For many Chicagoans, the true beginning of fall is marked by noticeable shifts in local weather and seasonal traditions rather than a specific date on the calendar. The city’s average daily high temperature typically falls below the 70°F mark during the first half of October. This drop signals a real change in daily comfort levels, moving away from the summer heat and humidity.
The cooling trend continues as the season progresses, with the average high temperature consistently dropping below 60°F by the end of October or early November. This temperature range is necessary for the development of vibrant fall foliage in the region. Peak fall colors in the greater Chicago area generally occur during the mid-to-late October period.
The local experience of the season is also tied to seasonal activities that begin well before the equinox. Many local apple orchards and pumpkin patches in the surrounding Chicagoland area open for the season in late August or early September. These openings provide a cultural marker for the start of autumn. The annual Chicago Marathon, held in early October, also serves as a public event that occurs squarely within the cooler, vibrant part of the season.