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