This report describes the typical weather at the O'Hare International Airport (Chicago, Illinois, United States) weather station over the course of an average October 12. 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 temperature typically varies from 47°F to 61°F and is rarely below 36°F or above 75°F.
The coolest hours of the day are from 2am to 9am with the coldest at 7am, at which time the temperature is below 53°F three days out of four.
The warmest hours of the day are from noon to 6pm with the hottest at 3pm, at which time the temperature is above 54°F three days out of four.
The day has gained half its heat by 10am and lost it again by 9pm.
On October 12 the Sun rises at 7:01am and sets at 6:13pm.
Solar noon is at 12:37pm.
For reference, on the longest day of the year the Sun rises at 5:16am and sets at 8:30pm, while on the shortest day of the year it rises at 7:16am and sets at 4:23pm.
Astronomical twilight begins and ends 92 minutes before sunrise and after sunset at 5:28am and 7:46pm, respectively.
Nautical twilight begins and ends 60 minutes before sunrise and after sunset at 6:00am and 7:14pm, respectively.
Civil twilight begins and ends 28 minutes before sunrise and after sunset at 6:33am and 6:42pm, respectively.
Solar twilight begins and ends 37 minutes after sunrise and before sunset at 7:39am and 5:35pm, respectively.
The median cloud cover ranges from 60% (partly cloudy) to 80% (mostly cloudy).
At 2am, the clearest time of the day, the sky is clear, mostly clear, or partly cloudy 56% of the time, and overcast or mostly cloudy 44% of the time.
At 2pm, the cloudiest time of the day, the sky is overcast, mostly cloudy, or partly cloudy 69% of the time, and clear or mostly clear 31% of the time.
The sky is typically clearing in earnest by 9pm.
The clouds have typically started forming in earnest by 8am.
There is a 45% chance that precipitation will be observed at some point during the day.
The average measured liquid-equivalent quantity of precipitation is 0.094". Among only those days for which there is at least some measured precipitation, the average is 0.302" and the total rarely exceeds 0.648".
When precipitation does occur it is most often in the form of slight rain (40% of days with precipitation have at worst slight rain), moderate rain (32%), thunderstorms (12%), and slight snow (7%).
Precipitation is most likely around 11pm and least likely around 1pm.
Either snow is exceptionally unlikely to fall at this location on October 12 or this station does not reliably report precipitation types.
The relative humidity typically ranges from 56% (mildly humid) to 81% (humid), rarely dropping below 36% (comfortable), or exceeding 95% (very humid).
The air is driest at 3pm, at which time the relative humidity is less than 67% (mildly humid) three days out of four; it is most humid at 5am, exceeding 74% (humid) three days out of four.
The dew point ranges from 41°F to 43°F with an average of 42°F.
For reference, comfortable indoor conditions should have a relative humidity between 30% and 50% with a dew point of less than 65°F.
Sustained wind speeds typically vary from 8 mph to 12 mph (gentle breeze to moderate breeze) over the course of the day.
The peak sustained wind speed typically occurs around 4pm and is rarely outside the range 6 mph to 18 mph (light breeze to fresh breeze).
The wind is most calm around 5am, rarely exceeding 14 mph (moderate breeze).
The wind is most often out of the south (21% of the time), west (17% of the time), south west (16% of the time), north west (11% of the time), and north (10% of the time).