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Average Weather in September at Chicago O'Hare International Airport Illinois, United States

At Chicago O'Hare International Airport, the month of September is characterized by rapidly falling daily high temperatures, with daily highs decreasing by 11°F, from 79°F to 68°F over the course of the month, and rarely exceeding 88°F or dropping below 57°F.

Daily low temperatures decrease by 11°F, from 62°F to 51°F, rarely falling below 40°F or exceeding 71°F.

For reference, on July 19, the hottest day of the year, temperatures at Chicago O'Hare International Airport typically range from 66°F to 84°F, while on January 29, the coldest day of the year, they range from 18°F to 31°F.

Average High and Low Temperature in September

The daily average high (red line) and low (blue line) temperature, with 25th to 75th and 10th to 90th percentile bands. The thin dotted lines are the corresponding average perceived temperatures.

The figure below shows you a compact characterization of the hourly average temperatures for the quarter of the year centered on September. The horizontal axis is the day, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.

Average Hourly Temperature in September

Average Hourly Temperature in September at Chicago O'Hare International Airport12 AM4 AM8 AM12 PM4 PM8 PM12 AM18152229112233445566778899101011111212131314141515161617171818191920202121222223232424252526262727282829293030AugAugOctOctcoldcoldcoolcoolcomfortablecomfortablewarmwarmchillychilly
The average hourly temperature, color coded into bands: frigid < 15°F < freezing < 32°F < chilly < 45°F < cold < 55°F < cool < 65°F < comfortable < 75°F < warm < 85°F < hot < 95°F < sweltering. The shaded overlays indicate night and civil twilight.

Clouds

The month of September at Chicago O'Hare International Airport experiences essentially constant cloud cover, with the percentage of time that the sky is overcast or mostly cloudy remaining about 35% throughout the month.

The clearest day of the month is September 15, with clear, mostly clear, or partly cloudy conditions 67% of the time.

For reference, on December 29, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 58%, while on August 26, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 68%.

Cloud Cover in September

Cloud Cover in September at Chicago O'Hare International Airport0%10%20%30%40%50%60%70%80%90%100%18152229112233445566778899101011111212131314141515161617171818191920202121222223232424252526262727282829293030AugAugOctOctSep 133%Sep 133%Sep 3036%Sep 3036%Sep 1134%Sep 1134%Sep 2134%Sep 2134%overcastovercastmostly cloudymostly cloudymostly clearmostly clearclearclear
The percentage of time spent in each cloud cover band, categorized by the percentage of the sky covered by clouds: clear < 20% < mostly clear < 40% < partly cloudy < 60% < mostly cloudy < 80% < overcast.

Precipitation

A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. At Chicago O'Hare International Airport, the chance of a wet day over the course of September is decreasing, starting the month at 30% and ending it at 26%.

For reference, the year's highest daily chance of a wet day is 37% on June 13, and its lowest chance is 15% on January 30.

Probability of Precipitation in September

The percentage of days in which various types of precipitation are observed, excluding trace quantities: rain alone, snow alone, and mixed (both rain and snow fell in the same day).

Rainfall

To show variation within the month and not just the monthly total, we show the rainfall accumulated over a sliding 31-day period centered around each day.

The average sliding 31-day rainfall during September at Chicago O'Hare International Airport is gradually decreasing, starting the month at 3.3 inches, when it rarely exceeds 5.6 inches or falls below 1.3 inches, and ending the month at 2.8 inches, when it rarely exceeds 4.7 inches or falls below 1.1 inches.

Average Monthly Rainfall in September

The average rainfall (solid line) accumulated over the course of a sliding 31-day period centered on the day in question, with 25th to 75th and 10th to 90th percentile bands. The thin dotted line is the corresponding average liquid-equivalent snowfall.

Sun

Over the course of September at Chicago O'Hare International Airport, the length of the day is rapidly decreasing. From the start to the end of the month, the length of the day decreases by 1 hour, 21 minutes, implying an average daily decrease of 2 minutes, 47 seconds, and weekly decrease of 19 minutes, 27 seconds.

The shortest day of the month is September 30, with 11 hours, 47 minutes of daylight and the longest day is September 1, with 13 hours, 7 minutes of daylight.

Hours of Daylight and Twilight in September

Hours of Daylight and Twilight in September at Chicago O'Hare International Airport0 hr4 hr8 hr12 hr16 hr20 hr24 hr18152229112233445566778899101011111212131314141515161617171818191920202121222223232424252526262727282829293030AugAugOctOctSep 2212 hr, 9 minSep 2212 hr, 9 minnightnightdaydaySep 113 hr, 7 minSep 113 hr, 7 min
The number of hours during which the Sun is visible (black line). From bottom (most yellow) to top (most gray), the color bands indicate: full daylight, twilight (civil, nautical, and astronomical), and full night.

The earliest sunrise of the month at Chicago O'Hare International Airport is 6:17 AM on September 1 and the latest sunrise is 30 minutes later at 6:47 AM on September 30.

The latest sunset is 7:24 PM on September 1 and the earliest sunset is 50 minutes earlier at 6:34 PM on September 30.

Daylight saving time is observed at Chicago O'Hare International Airport during 2017, but it neither starts nor ends during September, so the entire month is in daylight saving time.

For reference, on June 21, the longest day of the year, the Sun rises at 5:16 AM and sets 15 hours, 14 minutes later, at 8:30 PM, while on December 21, the shortest day of the year, it rises at 7:16 AM and sets 9 hours, 7 minutes later, at 4:23 PM.

Sunrise & Sunset with Twilight in September

The solar day over the course of September. From bottom to top, the black lines are the previous solar midnight, sunrise, solar noon, sunset, and the next solar midnight. The day, twilights (civil, nautical, and astronomical), and night are indicated by the color bands from yellow to gray.

Humidity

We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.

The chance that a given day will be muggy at Chicago O'Hare International Airport is very rapidly decreasing during September, falling from 28% to 4% over the course of the month.

For reference, on August 1, the muggiest day of the year, there are muggy conditions 42% of the time, while on November 16, the leasy muggy day of the year, there are muggy conditions 0% of the time.

Humidity Comfort Levels in September

Humidity Comfort Levels in September at Chicago O'Hare International Airport0%10%20%30%40%50%60%70%80%90%100%18152229112233445566778899101011111212131314141515161617171818191920202121222223232424252526262727282829293030AugAugOctOctSep 128%Sep 128%Sep 304%Sep 304%Sep 1119%Sep 1119%Sep 2110%Sep 2110%oppressiveoppressivemuggymuggycomfortablecomfortabledrydrymiserablemiserable
The percentage of time spent at various humidity comfort levels, categorized by dew point: dry < 55°F < comfortable < 60°F < humid < 65°F < muggy < 70°F < oppressive < 75°F < miserable.

Wind

This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.

The average hourly wind speed at Chicago O'Hare International Airport is gradually increasing during September, increasing from 4.9 miles per hour to 5.9 miles per hour over the course of the month.

For reference, on January 15, the windiest day of the year, the daily average wind speed is 7.4 miles per hour, while on August 3, the calmest day of the year, the daily average wind speed is 4.5 miles per hour.

Average Wind Speed in September

The average of mean hourly wind speeds (dark gray line), with 25th to 75th and 10th to 90th percentile bands.
The hourly average wind direction at Chicago O'Hare International Airport throughout September is predominantly from the calm, with a peak proportion of 36% on September 7.

Wind Direction in September

Wind Direction in September at Chicago O'Hare International Airport0%20%40%60%80%100%18152229112233445566778899101011111212131314141515161617171818191920202121222223232424252526262727282829293030AugAugOctOctwestwestsouthsoutheasteastnorthnorth
The percentage of hours in which the mean wind direction is from each of the four cardinal wind directions (north, east, south, and west), excluding hours in which the mean wind speed is less than 1 mph. The lightly tinted areas at the boundaries are the percentage of hours spent in the implied intermediate directions (northeast, southeast, southwest, and northwest).

Water Temperature

Chicago O'Hare International Airport is located near a large body of water (e.g., ocean, sea, or large lake). This section reports on the wide-area average surface temperature of that water.

The average surface water temperature at Chicago O'Hare International Airport is rapidly decreasing during September, falling by 8°F, from 71°F to 63°F, over the course of the month.

Average Water Temperature in September

The daily average water temperature (purple line), with 25th to 75th and 10th to 90th percentile bands.

Solar Energy

This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.

The average daily incident shortwave solar energy at Chicago O'Hare International Airport is decreasing during September, falling by 1.2 kWh, from 5.5 kWh to 4.3 kWh, over the course of the month.

Average Daily Incident Shortwave Solar Energy in September

The average daily shortwave solar energy reaching the ground per square meter (orange line), with 25th to 75th and 10th to 90th percentile bands.

Topography

For the purposes of this report, the geographical coordinates of Chicago O'Hare International Airport are 41.980 deg latitude, -87.904 deg longitude, and 650 ft elevation.

The topography within 2 miles of Chicago O'Hare International Airport is essentially flat, with a maximum elevation change of 95 feet and an average elevation above sea level of 651 feet. Within 10 miles is essentially flat (354 feet). Within 50 miles contains only modest variations in elevation (768 feet).

The area within 2 miles of Chicago O'Hare International Airport is covered by artificial surfaces (100%), within 10 miles by artificial surfaces (97%), and within 50 miles by cropland (34%) and water (31%).

Data Sources

This report illustrates the typical weather at Chicago O'Hare International Airport year round, based on a statistical analysis of historical hourly weather reports and model reconstructions from January 1, 1980 to December 31, 2016.

Temperature and Dew Point

Chicago O'Hare International Airport has a weather station that reported reliably enough during the analysis period that we have included it in our network. When available, historical temperature and dew point measurements are taken directly from this weather station. These records are obtained from NOAA's Integrated Surface Hourly data set, falling back on ICAO METAR records as required.

In the case of missing or erroneous measurements from this station, we fall back on records from nearby stations, adjusted according to typical seasonal and diurnal intra-station differences. For a given day of the year and hour of the day, the fallback station is selected to minimize the prediction error over the years for which there are measurements for both stations.

The stations on which we may fall back include but are not limited to Chicago Executive Airport, Naval Air Station Glenview (historical), Chicago Midway International Airport, DuPage County Airport, Lewis University Airport, Waukegan Regional Airport, Joliet Regional Airport, and Chicago/Aurora Municipal Airport.

Other Data

All data relating to the Sun's position (e.g., sunrise and sunset) are computed using astronomical formulas from the book, Astronomical Tables of the Sun, Moon and Planets , by Jean Meeus.

All other weather data, including cloud cover, precipitation, wind speed and direction, and solar flux, come from NASA's MERRA-2 Modern-Era Retrospective Analysis . This reanalysis combines a variety of wide-area measurements in a state-of-the-art global meteorological model to reconstruct the hourly history of weather throughout the world on a 50-kilometer grid.

Land Use data comes from the Global Land Cover SHARE database , published by the Food and Agriculture Organization of the United Nations.

Elevation data comes from the Shuttle Radar Topography Mission (SRTM) , published by NASA's Jet Propulsion Laboratory.

Names, locations, and time zones of places and some airports come from the GeoNames Geographical Database .

Time zones for aiports and weather stations are provided by AskGeo.com .

Maps are © Esri, with data from National Geographic, Esri, DeLorme, NAVTEQ, UNEP-WCMC, USGS, NASA, ESA, METI, NRCAN, GEBCO, NOAA, and iPC.