This report describes the typical weather at the O'Hare International Airport (Chicago, Illinois, United States) weather station over the course of an average September. It is based on the historical records from 1974 to 2012. Earlier records are either unavailable or unreliable.

Chicago, Illinois has a humid continental climate with hot summers and no dry season. The area within 25 mi of this station is covered by built-up areas (37%), grasslands (36%), oceans and seas (22%), forests (3%), and croplands (3%).

The month of September is characterized by rapidly falling daily high temperatures, with daily highs decreasing from 80°F to 68°F over the course of the month, exceeding 89°F or dropping below 57°F only one day in ten.

Daily High and Low Temperature in September

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The daily average low (blue) and high (red) temperature with percentile bands (inner band from 25th to 75th percentile, outer band from 10th to 90th percentile).

Fraction of Time Spent in Various Temperature Bands in September

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The average fraction of time spent in various temperature bands: frigid (below 15°F), freezing (15°F to 32°F), cold (32°F to 50°F), cool (50°F to 65°F), comfortable (65°F to 75°F), warm (75°F to 85°F), hot (85°F to 100°F) and sweltering (above 100°F).

Over the course of September, the length of the day is decreasing. From the start to the end of the month, the length of the day decreases by 1:21 hours, implying an average day-over-day decrease of 2.7 minutes.

The shortest day of the month is September 30 with 11:46 hours of daylight; the longest day is September 1 with 13:07 hours of daylight.

Daily Hours of Daylight and Twilight in September

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The number of hours during which the Sun is visible (black line), with various degrees of daylight, twilight, and night, indicated by the color bands. From bottom (most yellow) to top (most gray): full daylight, solar twilight (Sun is visible but less than 6° from the horizon), civil twilight (Sun is not visible but is less than 6° below the horizon), nautical twilight (Sun is between 6° and 12° below the horizon), astronomical twilight (Sun is between 12° and 18° below the horizon), and full night.

The earliest sunrise is at 6:18am on September 1; the latest sunset is at 7:24pm on September 1; the latest sunrise is at 6:48am on September 30; the earliest sunset is at 6:34pm on September 30.

Daylight savings time is observed in 2012 but it neither starts nor ends in September, so the entire month is in daylight time.

Daily Sunrise & Sunset with Twilight and Daylight Savings Time in September

The solar day over the course of the year 2012 . From bottom to top, the black lines are the previous solar midnight, sunrise, solar noon, sunset, and the next solar midnight. The day, twilights (solar, civil, nautical, and astronomical), and night are indicated by the color bands from yellow to gray. The transitions to and from daylight savings time are indicated by the "DST" labels.

The median cloud cover is 58% (partly cloudy) and does not vary substantially over the course of the month.

Median Cloud Cover in September

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The median daily cloud cover (black line) with percentile bands (inner band from 40th to 60th percentile, outer band from 25th to 75th percentile).

On September 15, a representative day, the sky is clear or mostly clear 42% of the time, partly cloudy 17% of the time, and mostly cloudy or overcast 41% of the time.

Cloud Cover Types in September

The fraction of time spent in each of the five sky cover categories. From top (most blue) to bottom (most gray), the categories are clear, mostly clear, partly cloudy, mostly cloudy, and overcast. Pink indicates missing data. Outside of the United States clear skies are often reported ambiguously, leading them to be lumped in with the missing data.

The average probability that some form of precipitation will be observed in a given day is 41%, with little variation over the course of the month.

Probability of Precipitation at Some Point in the Day in September

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The fraction of days in which various types of precipitation are observed. If more than one type of precipitation is reported in a given day, the more severe precipitation is counted. For example, if light rain is observed in the same day as a thunderstorm, that day counts towards the thunderstorm totals. The order of severity is from the top down in this graph, with the most severe at the bottom.

Throughout September, the most common forms of precipitation are light rain, moderate rain, and thunderstorms.

Light rain is the most severe precipitation observed during 36% of those days with precipitation. It is most likely around September 27, when it is observed during 16% of all days.

Moderate rain is the most severe precipitation observed during 32% of those days with precipitation. It is most likely around September 30, when it is observed during 14% of all days.

Thunderstorms are the most severe precipitation observed during 25% of those days with precipitation. They are most likely around September 1, when it is observed during 13% of all days.

Types of Precipitation Throughout September

Relative frequency of various types of precipitation over the course of a typical September.

Either snow is exceptionally unlikely to fall at this location in September or this station does not reliably report precipitation types.

The relative humidity typically ranges from 47% (comfortable) to 91% (very humid) over the course of a typical September, rarely dropping below 31% (comfortable) and reaching as high as 99% (very humid).

The air is driest around September 28, at which time the relative humidity drops below 57% (mildly humid) three days out of four; it is most humid around September 2, rising above 86% (very humid) three days out of four.

Relative Humidity in September

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The average daily high (blue) and low (brown) relative humidity with percentile bands (inner bands from 25th to 75th percentile, outer bands from 10th to 90th percentile).

Dew point is often a better measure of how comfortable a person will find the weather than relative humidity because it more directly relates to whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid.

Over the course of a typical September, the dew point typically varies from 42°F (dry) to 64°F (mildy humid) and is rarely below 31°F (dry) or above 71°F (very muggy).

The time of the month between September 1 and September 30 is the most comfortable, with dew points that are neither too dry nor too muggy.

Dew Point in September

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The daily average low (blue) and high (red) dew point with percentile bands (inner band from 25th to 75th percentile, outer band from 10th to 90th percentile).

Over the course of September typical wind speeds vary from 2 mph to 16 mph (light air to moderate breeze), rarely exceeding 22 mph (fresh breeze).

The highest average wind speed of 9 mph (gentle breeze) occurs around September 29, at which time the average daily maximum wind speed is 16 mph (moderate breeze).

The lowest average wind speed of 8 mph (gentle breeze) occurs around September 1, at which time the average daily maximum wind speed is 14 mph (moderate breeze).

Wind Speed in September

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The average daily minimum (red), maximum (green), and average (black) wind speed with percentile bands (inner band from 25th to 75th percentile, outer band from 10th to 90th percentile).

Wind Directions in September

The fraction of time spent with the wind blowing from the various directions over the entire year. Values do not sum to 100% because the wind direction is undefined when the wind speed is zero.

Fraction of Time Spent with Various Wind Directions in September

The fraction of time spent with the wind blowing from the various directions on a daily basis. Stacked values do not always sum to 100% because the wind direction is undefined when the wind speed is zero.