View source for Climate of Chicago

{{Short description|none}} <!-- "none" is preferred when the title is sufficiently descriptive; see [[WP:SDNONE]] -->
{{Use mdy dates|date=May 2018}}
[[File:Chicago Aerial Tour (5458300997).jpg|thumb|Downtown Chicago, showing Lake Michigan in the foreground]]

The '''climate of Chicago''' is classified as [[humid continental climate#Hot summer subtype|hot-summer humid continental]] ([[Köppen climate classification#Group D: Continental/microthermal climates|Köppen: ''Dfa'']], [[Trewartha climate classification|Trewartha]]: ''Dca'') with hot humid summers and cold, occasionally snowy winters. Although lakefront areas such as [[Northerly Island]] have a ''Cfa'' ([[humid subtropical climate|humid subtropical]]) climate using Köppen's -3 °C (27 °F) winter isotherm, even those areas are continental (''Dca'') under Trewartha due to winters averaging below 0 °C (32 °F),<ref>{{cite web | url=https://open.oregonstate.education/permaculturedesign/chapter/climate-classification-systems/ | title=Climate Classification Systems | date=August 2019 | last1=Millison | first1=Andrew }}</ref> and inland areas such as [[Midway International Airport|Midway]] and [[O'Hare International Airport|O'Hare International Airports]] are continental even under Köppen.<ref>{{cite web | url=https://www.britannica.com/science/Koppen-climate-classification/World-distribution-of-major-climatic-types | title=Köppen climate classification - World Distribution, Major Types, Climate Zones &#124; Britannica }}</ref> All four seasons are distinctly represented: Winters are cold and often see snow with below 0 Celsius temperatures and windchills, while summers are warm and humid with temperatures being hotter inland, spring and fall bring bouts of both cool and warm weather and fairly sunny skies. Annual precipitation in [[Chicago]] is moderate and relatively evenly distributed, the driest months being January<ref>{{cite web |title=Precipitation Rankings for January for Chicago, IL|url=https://www.weather.gov/lot/January_Precip_Rankings_Chicago |website=NWS Chicago |access-date=3 December 2019}}</ref> and February<ref>{{cite web |title=February Precipitation Rankings for Chicago, IL |url=https://www.weather.gov/lot/February_Precip_Rankings_Chicago |website=NWS Chicago |access-date=3 December 2019}}</ref> and the wettest July<ref>{{cite web |title=July Precipitation Amounts for Chicago, IL |url=https://www.weather.gov/lot/July_Precip_Rankings_Chicago |website=NWS Chicago |access-date=3 December 2019}}</ref> and August.<ref>{{cite web |title=August Precipitation Amounts for Chicago, IL |url=https://www.weather.gov/lot/August_Precip_Rankings_Chicago |website=NWS Chicago |access-date=3 December 2019}}</ref> Chicago's weather is influenced during all four seasons by the nearby presence of [[Lake Michigan]].<ref>{{Cite web |date=13 Dec 2025 |title=How does Chicago's proximity to Lake Michigan affect our weather? |url=https://wgntv.com/weather/weather-blog/ask-tom-why/how-does-chicagos-proximity-to-lake-michigan-affect-our-weather/}}</ref><ref>{{Cite web |title=Climate of Chicago - Description, Illinois State Climatologist Office, Illinois State Water Survey, U of I |url=https://www.isws.illinois.edu/statecli/general/chicago-climate-narrative.htm |access-date=2025-01-13 |website=www.isws.illinois.edu}}</ref>

== Official locations ==
The [[National Weather Service Chicago, Illinois|National Weather Service office of Chicago]] has one of the longest periods of official weather records, dating back to 1870, though all the 1870 and 1871 weather records taken at 181 West Washington Street were lost in the [[Great Chicago Fire]]. Of the two major airports located in Chicago, Midway Airport began observations in 1928, and O'Hare Airport began them in 1958. Both sites have served in the past as the official observation location, the latter being the current official station. Weather data from Midway Airport before July 1, 1942, and after January 16, 1980, and data from O'Hare Airport before January 17, 1980, are not part of the official climate record of Chicago.

Here is a list of official weather observation locations for the Chicago office:<ref>{{cite web|url=http://www.weather.gov/lot/observation_history|title=History of the Chicago and Rockford weather observation sites}}</ref>
{| class="wikitable"
|-
! Dates !! Location
|'''Remarks'''
|-
| October 15, 1870 – October 8, 1871 || 181 W. Washington St. ||All records lost due to [[Great Chicago Fire]]
|-
| October 15, 1871 – June 11, 1872 || 427 W. Randolph St.
|-
| June 11, 1872 – June 8, 1873 || 20 N. Wacker Dr.
|-
| June 8, 1873 – January 1, 1887 || [[Roanoke Building]]
|-
| January 1, 1887 – February 1, 1890 || [[Chicago Opera House]]
|-
| February 1, 1890 – July 1, 1905 || [[Auditorium Tower, Chicago|Auditorium Tower]]
|Automatic rain gauge installed in 1897
|-
| July 1, 1905 – December 31, 1925 || U.S. Court House
|Supplemental observations through 1970
|-
| January 1, 1926 – June 30, 1942 || [[University of Chicago]]
|Supplemental observations through 1962
|-
| July 1, 1942 – January 16, 1980 || [[Midway International Airport|Midway Airport]]
|Continues to provide observations
|-
| January 17, 1980 – present || [[O'Hare International Airport|O'Hare Airport]]
|}
''Note: Some of the addresses prior to 1909 are different from the post-1909 addresses''

== Classifications ==
{{Schemebox
|city = Chicago

| source_k =<ref>{{Cite web|url=http://www.weatherbase.com/weather/weather-summary.php3?s=3527&cityname=Chicago,+Illinois,+United+States+of+America|title=Chicago, Illinois Köppen Climate Classification (Weatherbase)|website=Weatherbase|access-date=2019-07-01}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=I2rrBgAAQBAJ&q=chicago+koppen+dfa&pg=PA197|title=Sustainable Renovation: Strategies for Commercial Building Systems and Envelope|last1=Gelfand|first1=Lisa|last2=Duncan|first2=Chris|date=2011-11-08|publisher=John Wiley & Sons|isbn=9780470872611|language=en}}</ref>
| initial_k ={{KoppenClimate|Dfa}}
| description_k =Hot-summer [[humid continental climate]]

| source_t =<ref>{{Citation|last=Peterson|first=Adam|title=English: Trewartha climate types for the contiguous United States|date=2016-09-22|url=https://commons.wikimedia.org/wiki/File:US_trewartha.svg|access-date=2019-07-01}}</ref><ref>{{Cite web|url=http://www.fao.org/3/ad652e/ad652e07.htm|title=Global Ecological Zoning for the Global Forest Resources Management Assessment 2000|website=www.fao.org|access-date=2019-07-01}}</ref>
| initial_t ={{TrewarthaClimate|Dca}}
| description_t =[[Continental climate]]

| source_a =<ref>{{Citation|last=Cmapm|first=Kliimavöötmed svg: Urmasderivative work|title=Русский: Климатические пояса Земли по Б. П. Алисову.|date=2011-06-16|url=https://commons.wikimedia.org/wiki/File:Alisov%27s_classification_of_climate_ru.jpg|access-date=2019-07-01}}</ref>
| initial_a ={{n/a}} 
| description_a =[[Temperate climate]]{{Efn|Mid-latitude temperate interior climate|name=|group=}}

| source_s =<ref>{{Cite web|url=https://i.pinimg.com/originals/c5/16/b8/c516b8c463fa99df0da778f768c85965.jpg|title=World Strahler Climate Map}}</ref>
| initial_s ={{n/a}}
| description_s =[[Humid continental climate|Moist continental climate]]

| source_tw =<ref>{{Cite journal|last=Feddema|first=Johannes J.|date=January 2005|title=A Revised Thornthwaite-Type Global Climate Classification|journal=Physical Geography|language=en|volume=26|issue=6|pages=442–466|doi=10.2747/0272-3646.26.6.442|bibcode=2005PhGeo..26..442F |s2cid=128745497|issn=0272-3646}}</ref>
| initial_tw ={{center|''B1 B'1''}}
| description_tw =Humid and [[mesothermal]]

| source_n =<ref>{{Cite web|url=https://www.spektrum.de/lexikon/geographie/klimaklassifikation/4159|title=Klimaklassifikation|website=www.spektrum.de|language=de|access-date=2019-07-01}}</ref><ref>{{Cite web|url=https://www.klett.de/sixcms/media.php/klett72.a.427.de/upload/klimazonen_neef.jpg|title=Klimazonen nach E. Neef (1989)|publisher=Klett-Perthes}}</ref>
| initial_n ={{n/a}} 
| description_n =Cool [[continental climate]]
}}

== Data==

{{Chicago weatherbox}}
{{weatherbox |single line =yes |collapsed = yes |width = auto
|location = [[Northerly Island]], 2005-2023 Temperature normals
|precipitation colour = green

| Jan record high F  =
| Feb record high F  =
| Mar record high F  =
| Apr record high F  =
| May record high F  =
| Jun record high F  =
| Jul record high F  =
| Aug record high F  =
| Sep record high F  =
| Oct record high F  =
| Nov record high F  =
| Dec record high F  =
| year record high F =
| Jan avg record high F  = 51
| Feb avg record high F  = 55
| Mar avg record high F  = 70
| Apr avg record high F  = 80
| May avg record high F  = 87
| Jun avg record high F  = 92
| Jul avg record high F  = 94
| Aug avg record high F  = 93
| Sep avg record high F  = 90
| Oct avg record high F  = 82
| Nov avg record high F  = 68
| Dec avg record high F  = 58
| year avg record high F =
| Jan high F  =33.2
| Feb high F  =34.8
| Mar high F  =46.4
| Apr high F  =55.7
| May high F  =67.0
| Jun high F  =76.9
| Jul high F  =81.8
| Aug high F  =81.1
| Sep high F  =74.9
| Oct high F  =62.4
| Nov high F  =49.8
| Dec high F  =38.0
| year high F =
| Jan mean F  =27.2
| Feb mean F  =28.3
| Mar mean F  =39.9
| Apr mean F  =48.6
| May mean F  =59.3
| Jun mean F  =69.4
| Jul mean F  =75.2
| Aug mean F  =74.9
| Sep mean F  =68.5
| Oct mean F  =55.9
| Nov mean F  =43.5
| Dec mean F  =32.3
| year mean F =
| Jan low F  =21.1
| Feb low F  =21.9
| Mar low F  =33.3
| Apr low F  =41.4
| May low F  =51.6
| Jun low F  =62.0
| Jul low F  =68.7
| Aug low F  =68.7
| Sep low F  =62.0
| Oct low F  =49.5
| Nov low F  =37.3
| Dec low F  =26.7
| year low F =
| Jan avg record low F  = 0
| Feb avg record low F  = 3
| Mar avg record low F  = 20
| Apr avg record low F  = 31
| May avg record low F  = 41
| Jun avg record low F  = 52
| Jul avg record low F  = 62
| Aug avg record low F  = 61
| Sep avg record low F  = 50
| Oct avg record low F  = 37
| Nov avg record low F  = 21
| Dec avg record low F  = 8
| year avg record low F =
| Jan record low F  =
| Feb record low F  =
| Mar record low F  =
| Apr record low F  =
| May record low F  =
| Jun record low F  =
| Jul record low F  =
| Aug record low F  =
| Sep record low F  =
| Oct record low F  =
| Nov record low F  =
| Dec record low F  =
| year record low F =

|source = National Weather Service<ref>{{Cite web |url=https://www.weather.gov/wrh/Climate?wfo=lot |title=NOAA Online Weather Data: Chicago Northerly Island |access-date=26 February 2024 |publisher = [[National Weather Service|NWS]]}}</ref>
}}
{{Graph:Weather monthly history
| table=ncei.noaa.gov/weather/Chicago.tab
| title=Chicago official monthly weather statistics
}}

== Seasons ==
{{climate chart
| Chicago
| 18.2 | 31.5 | 2.06
| 21.7 | 35.8 | 1.94
| 30.9 | 46.8 | 2.72
| 41.7 | 59.2 | 3.64
| 51.6 | 70.2 | 4.13
| 62.1 | 79.9 | 4.06
| 67.5 | 84.2 | 4.01
| 66.2 | 82.1 | 3.99
| 57.5 | 75.3 | 3.31
| 45.7 | 62.8 | 3.24
| 34.5 | 48.6 | 3.42
| 22.7 | 35.3 | 2.57
|units=imperial
|float=right
|clear=none
|source = NOAA<ref name = "NOAA MDW txt"/>
}}

=== Winter ===
Winter in Chicago is generally cold and snowy. The city typically sees less snow in winter than other major cities near the [[Great Lakes]], such as [[Cleveland]] or [[Buffalo, New York|Buffalo]], but more snow than on the East Coast in cities like [[Washington DC]] or [[New York City]]. Seasonal snowfall in the city has ranged from {{convert|9.8|in|cm|1}} (in 1920–21) up to {{convert|89.7|in|cm|0|abbr=on}} (in 1978–79), and the average annual snowfall in Chicago is {{convert|36|in|cm}}.<ref>{{cite web|title=Chicagos Seasonal Snowfall Amounts|url=http://www.crh.noaa.gov/lot/?n=chi_seasonal_snow|publisher=National Weather Service, Chicago, IL}}</ref> Most winters produce many snow falls during the season in light accumulations of around {{convert|2|in|cm|abbr=on}}. Cities on the other side of Lake Michigan usually receive more snow than Chicago because of the [[lake-effect snow]] that falls on these communities, even though northeasterly winds can sometimes bring lake-effect snow to Chicago area too. However, every three years or so during the winter Chicago experiences a heavier snowstorm that can produce over {{convert|10|in|cm|0|abbr=on}} of snow over a 1- to 3-day period, a level of snowfall very often seen in cities on the "[[snowbelt]]" on other side of the lake such as [[Grand Rapids, Michigan]], [[Kalamazoo, Michigan]], and [[South Bend, Indiana]].

Winter temperatures are generally cold. However, like much of the northern USA, can vary tremendously within the span of one week. The daily average high temperature in January at O'Hare is {{convert|31.0|F}} with the average daily low of {{convert|16.5|F}} and the daily mean of {{convert|23.6|F}}. Temperatures drop to or below {{convert|0|°F|0}} on 5.5 nights annually at Midway and 8.2 nights at O'Hare and up to 10–14 nights in some far western and northern suburbs,<ref>{{cite web|url=http://www.ncdc.noaa.gov/cdo-web/datatools/normals|title=1981–2010 Normals – Data Tools – Climate Data Online (CDO) – National Climatic Data Center (NCDC)|first=National Centers for Environmental Information|last=(NCEI)|website=www.ncdc.noaa.gov|access-date=April 7, 2018}}</ref> although subzero (°F) readings in the absence of snow cover are rare.<ref>{{cite web
 |url = http://www.myfoxphilly.com/story/20650138/tuesday-mornings-sub-zero-temperatures-end-711-day-streak
 |title = Sub-zero temperatures end 711-day streak Tuesday – Philadelphia News, Weather and Sports from WTFX FOX 29
 |publisher = Fox Philadelphia
 |access-date = 2013-03-18
 |url-status = dead
 |archive-url = https://web.archive.org/web/20140714122249/http://www.myfoxphilly.com/story/20650138/tuesday-mornings-sub-zero-temperatures-end-711-day-streak
 |archive-date = July 14, 2014
 }}</ref> There have been streaks of multiple winter seasons without a single subzero reading, yet there have also been winters with 20 or more subzero readings. The highest temperature recorded during the meteorological winter months of December, January, and February is {{convert|75|F}}, set on February 27, 1976. The lowest temperature recorded during meteorological winter is {{convert|-27|F}}, set on January 20, 1985. In addition, the all-time record low maximum temperature of {{convert|-11|F}} was set on December 24, 1983, and tied on January 18, 1994. However, in late [[January–February 2019 North American cold wave|January 2019]], a violent polar vortex drifted southward, enveloping the city in new record-breaking temperatures as low as {{convert|-23|F}} on January 30, though the city just missed out on tying the all-time record low maximum temperature, recording a high of {{convert|-10|F}} one day during the outbreak. Wind speeds reached at least 20 miles per hour, exacerbating the [[wind chill]] effect.<ref>{{Cite web|url=https://abc7chicago.com/weather/chicago-weather-polar-vortex-brings-record-cold-wind-chill-warning-in-effect/5114090/|title=Chicago Weather: Snow falls on heels of record cold temperatures|date=February 2019}}</ref><ref>{{Cite web|url=https://abc7chicago.com/weather/steam-rises-off-lake-michigan-in-chicago-as-temps-plunge/5113047/|title = Steam rises off Lake Michigan in Chicago on 2nd day of record-breaking cold|date = January 31, 2019}}</ref><ref>{{Cite news|url=https://www.nytimes.com/2019/01/30/us/extreme-cold-weather.html|title=A Merciless Cold Lingers in the Midwest|newspaper=The New York Times|date=January 30, 2019|last1=Bosman|first1=Julie|last2=Davey|first2=Monica}}</ref>

The warming effect of Lake Michigan during the winter makes subzero temperatures somewhat less common on the lakefront than in the more inland parts of the city. Highs reach {{convert|50|°F}} an average of 8.8 days each winter from December to February at Midway.

Based on 30-year averages obtained from [[NOAA]]'s [[National Climatic Data Center]] for the months of December, January and February, [[The Weather Channel]] ranked Chicago the sixth-coldest major U.S. city as of 2014.<ref>{{cite web |url=https://weather.com/sports-recreation/ski/news/20-coldest-large-cities-america-20140107|title=America's 20 coldest major cities|author=Jon Erdman|publisher=The Weather Channel|date=January 27, 2014 |access-date=April 25, 2025}}</ref>

Although it is extremely rare, temperatures during late (astronomical) winter (generally March 1–20) can reach up to and well over {{convert|80|°F|0}}. In 2012, there were eight days in the month of March with temperatures {{convert|80|°F|0}}+ (with a ninth day occurring in many suburban areas) during the record-breaking [[March 2012 North American heat wave]]. The last couple of {{convert|80|°F|0}} days in this record-breaking stretch of warmth occurred after the vernal equinox.

=== Spring ===
Spring in Chicago is perhaps the city's most unpredictable season: Winter weather can persist until well into April or even into early May, with the 1953-54 winter in Chicago lasting from November until May, as measured by timespan between the first and last measurable snows of the season.<ref>{{cite news| url=http://wgntv.com/2019/04/16/longest-time-span-between-chicagos-first-and-last-measurable-snows/ | work=WGN | first=Randall | last=Mchenry | title=Longest time span between Chicago's first and last measurable snow | date=April 16, 2019}}</ref> Thunderstorms can occur any time of the year but are most prevalent in the springtime as the city's central location within the United States, as well as its lakeside location, makes it a center of conflicts between large volumes of warm and cold air, which can trigger a wide variety of severe weather. The most severe storms can contain large hail, damaging straight-line winds, flooding, and tornadoes. Since 1850, 23 tornadoes have struck the Chicago city limits. During thunderstorms, lightning strikes are seen to frequently hit Chicago's skyscrapers.

Large snowfalls have occurred in late March and in early April. In 1970, over {{convert|10|in|cm|0|abbr=on}} of snow fell in a storm that occurred on April 1–2.<ref>{{cite news| url=https://www.chicagotribune.com/2005/03/27/late-march-madness-the-unprecedented-spring-snowstorm-of-1930/ | work=Chicago Tribune | first=Tom | last=Skilling | title=Late-March madness: The unprecedented spring snowstorm of 1930 | date=March 27, 2005}}</ref> Twelve years later, Opening Day for the [[Chicago White Sox]] was postponed due to another {{convert|9|in|cm|adj=mid}} snowfall that had occurred on April 5.<ref>{{cite web|url=http://www.baseball-almanac.com/teams/white_sox_04.shtml|title=Chicago White Sox Timeline (April)|website=Baseball Almanac|access-date=April 7, 2018}}</ref> Even more extraordinary, over {{convert|18|in|cm|0|abbr=on}} of snow fell on March 25–26, 1930, which remains one of the city's five biggest recorded snowstorms despite it occurring past the [[March equinox|vernal equinox]].<ref>{{cite news| url=https://www.chicagotribune.com/2005/03/28/double-trouble-the-week-apart-spring-snowstorms-of-1970/ | work=Chicago Tribune | first=Tom | last=Skilling | title=Double trouble: The week-apart spring snowstorms of 1970 | date=March 28, 2005}}</ref> The average date for last measurable snowfall (≥{{convert|0.1|in|cm|abbr=on|disp=or}}) is April 1.

Temperatures vary tremendously in the springtime; at {{convert|100|°F|0}}, March is the month with the greatest span between the record high and low. At O'Hare, temperatures as low as {{convert|7|and|31|°F|0}} have been recorded as late as April 7 and May 21, respectively. Conversely, in official records, the earliest {{convert|100|F}} high occurred on June 1, 1934, when official readings were taken closer to [[Lake Michigan]]. Though rare, triple-digit heat has occurred in late May at [[Midway Airport]] and in outlying suburban locations. Typically, the last freezing low of the season on average occurs on April 13 at Midway and ten days later at O'Hare. The highest temperature recorded during the meteorological spring months of March, April, and May is, officially, {{convert|98|F}} on May 31, 1934, when weather records were still taken near Lake Michigan. The lowest temperature recorded in meteorological spring is {{convert|-12|F}}, set on March 4, 1873.

During the springtime, the effects of Lake Michigan are most prevalent. During this season, the lake is still quite cold, as the effects of much warmer temperatures are slow to affect the large body of water of Lake Michigan. It is common for Lake Michigan shoreline and water temperatures to remain in the 40s even into May. If the winds blow from the east, or from Lake Michigan into the city, a wide discrepancy in temperatures in a matter of miles can be found, especially on particularly warm days. It is not uncommon for high temperatures to be officially recorded in the 80s or lower 90s&nbsp;°F (27&ndash;34&nbsp;°C), particularly in early June, at O'Hare, Midway, and in suburban locations but to have temperatures be {{convert|20|to|30|F-change|C-change}} cooler along the immediate lakeshore.

=== Summer ===
On a typical summer day, humidity is usually moderately high, and temperatures ordinarily reach anywhere between {{convert|78|and|92|F|C}}. In July, it is not uncommon for the temperature to go around {{convert|90|and|93|F|C}}. Overnight temperatures in summer usually drop to around {{convert|65|–|70|°F|°C|0}}, although even in July and August there can be several nights where the temperature drops below {{convert|60|°F|0}}, particularly during the cooler summers, and it is not uncommon to see temperatures plummet lower than {{convert|50|F|C}}. Conversely, on the other extreme, temperatures can on a rare basis remain above {{convert|80|°F|0}} overnight, though this level of overnight warmth is generally limited to the city proper with its [[urban heat island]] effects along with Lake Michigan nearby. On such warm nights, especially during strong [[heat wave]]s, most suburban locations drop down to between {{convert|75|and|79|F|C}} but quickly rebound in the early morning hours. During such strong heat waves, the outlying suburban areas can record temperatures more than {{convert|5|F-change}} above city and lakeshore locations.

A perfect example of such an occurrence happening, although considered "unofficial", occurred during the [[Dust Bowl]] years. [[Midway Airport]] recorded a record eight consecutive 100&nbsp;°F-plus days in July 1936. In that heat wave, temperatures at the lake remained in the middle and upper 90s Fahrenheit (middle 30s Celsius), whereas Midway Airport recorded temperatures over {{convert|100|°F|C}} for nearly two weeks, peaking at {{convert|107|F}} on July 11. The official record high for Chicago for July 11 is also from 1936—but is recorded as just {{convert|97|°F|°C|abbr=on}}. Further west in what would today be the near and far suburbs (e.g. [[DuPage County, Illinois|DuPage County]] and westward), temperatures reached a blistering {{convert|110|F}} or still higher at points during this massive heat wave. These extreme temperatures are not shown in the official records because daily weather observations were taken at the University of Chicago until 1942. The University of Chicago is close to the lake, which can and does reduce temperatures in the immediate shoreline area in the summer.

The highest temperature recorded in Chicago during the meteorological summer months of June, July, and August, which is also additionally the all-time record high in the city, is {{convert|105|F}}, set on July 24, 1934, though at Midway Airport, a future observation site, the temperature reached {{convert|109|F}}. The lowest temperature recorded in the meteorological summer months is {{convert|35|F}}, set on June 4, 1945. In addition, the all-time record high minimum temperature of {{convert|85|F}} was set on July 29, 1916.

Chicago's yearly precipitation comes in at an average of about {{convert|36|in|mm}}, but during the summer, rain arises from short-lived hit-or-miss rain rather than actual prolonged rainfalls, and [[thunderstorm]]s also occur with regularity at night. [[Derecho]]s are also common during the summertime, the most notable being, the [[Tornadoes of 2008|August 4, 2008]] derecho, which produced five tornadoes across the Chicago area and killed one person. In a normal summer, temperatures exceed {{convert|90|°F}} on 23 days. Summer is both the rainiest and sunniest season in Chicago;<ref>[http://www.crh.noaa.gov/lot/?n=CHI_summer_precip Chicago Summer Precipitation Rankings Summer (June–August) (November 29, 2005)]. ''NOAA's National Weather Service Forecast Office''.</ref> only the three months of June through August experience more than 65% of possible sunshine.<ref name="NOAA sun" />

In July 2012, during the [[2012 North American heat wave]], Chicago reached and exceeded {{convert|100|F}} for three consecutive days at [[O'Hare Airport]] with highs reaching {{convert|103|F}} in the city and many suburban areas recording temperatures between {{convert|105|and|110|F|C}}. It was the first time in 65 years that Chicago had ever seen a triad of {{convert|100|°F|°C|abbr=on}} days.<ref>{{cite news |url=http://www.huffingtonpost.com/2012/07/06/chicago-heat-wave-friday-_n_1653812.html |title=Chicago Heat Wave: Friday Completes City's First Triad Of 100-Degree Days Since 1947 |access-date=July 11, 2012 |work=Huffington Post |date=July 6, 2012 |first=Joseph |last=Erbentraut}}</ref> Chicago nearly recorded a fourth consecutive {{convert|100|°F|°C|abbr=on}} day, but the temperature reached 98 degrees at O'Hare in late morning before a weak cold front came through the area and cooled temperatures off slightly in the city area, holding in the lower 90s. The effects of the cold front did not affect many suburban areas, as temperatures reached or exceeded {{convert|100|°F|°C|abbr=on}} for a fourth consecutive day throughout much of the region.

During the summer, Lake Michigan continues to have an effect on Chicago weather, but it is not as common or as strong as it is during the spring months. On very hot days, temperatures can still be cooler along the immediate shoreline and slightly inland of the lake if winds blow from the east. Temperatures can be held in the 70s or 80s in these areas while outlying and suburban areas temperatures are rising well into the 90s. Temperatures can also reach extreme levels of heat on the immediate shoreline, such as when the air temperature reached {{convert|105|°F}} at [[Northerly Island]] during the aforementioned July 2012 heat wave.

=== Autumn ===
The extreme heat that Chicago is capable of experiencing during the height of the summer season can persist into the autumn season. Temperatures have reached {{convert|100|°F|°C|abbr=on}} as late as September 7 (with {{convert|99|F}} occurring as late as September 29), and temperatures have reached {{convert|90|F}} as late as October 6, which occurred in 1963, with a temperature of {{convert|94|F}} on that day. Conversely, temperatures have dropped to freezing overnight as early as September 23, and temperatures below {{convert|0|F}} have arrived as early as November 23. The first freeze of the season on average occurs on October 24 at Midway and 11 days earlier at O'Hare. Although extremely rare, temperatures at or above {{convert|70|F}} have been recorded into early December, most recently in 2012 when {{convert|70|°F|°C|abbr=on}} was recorded on December 3, some surrounding areas reaching temperatures as high as {{convert|72|F}} to {{convert|75|F}}.

Autumn, in some ways, is a calmer season than any of the other three in Chicago. However, wild weather can and does occur in the region during this season. In most years, a period of warm weather, known as an [[Indian summer]], occurs well after the autumnal equinox has occurred. In these so-called Indian summers, unusually warm weather can persist for several days well into October, and in particularly warm autumns, into November. For example, during the [[2005 American League Division Series]], for both home games between the [[Chicago White Sox]] and the [[Boston Red Sox]], temperatures soared to near {{convert|90|F}}, despite the fact that it was already well into October. On top of that, during the same warm stretch in October 2005, for two consecutive days the overnight temperature failed to drop below {{convert|70|F}}, a rare occurrence for Chicago in October. Another example occurred in November 2020, when a week-long stretch of temperatures in the low to mid 70s occurred, including two of the eleven warmest high temperatures in November records from November 4 through 10.<ref>{{cite web | url=https://www.weather.gov/lot/2020NovemberWarmth | title=The Warmth of Early November 2020 in Perspective }}</ref>

The highest temperature recorded during the meteorological autumn months of September, October, and November is {{convert|101|F}}, set on back to back days, September 1–2, 1953, closely followed by Chicago's latest {{convert|100|F}} on September 7, 1960. The lowest temperature recorded during the meteorological autumn months is {{convert|-2|F}}, set on November 29, 1872, and tied on November 24, 1950.

Autumn can bring heavy rain showers, some of which are capable of producing serious flooding. As the winter solstice nears, the threat of a major winter or snowstorm grows, and there have been major winter storms around the Thanksgiving holiday, causing major delays at the city's two major airports. The first measurable ≥{{convert|0.1|in|cm|abbr=on}} snow on average falls on November 19. However, in the 2012–13 winter season, the first measurable snow did not fall until December 20, 2012, eclipsing the previous record of December 17, 1899. The 2012–13 autumn/winter season would fail to produce a daily maximum temperature below freezing {{convert|32|F}} until January 1, 2013, the first such time that has happened in Chicago weather records. The entire calendar year of 2012 did not record a temperature lower than {{convert|5|F}}. The largest snowstorm before the winter solstice dropped {{convert|14.8|in}} at Midway Airport in December 1929.

During the autumn, the effects of Lake Michigan are usually reversed from the spring or summer, particularly in the late autumn. Temperatures near the immediate lakeshore can be a few degrees warmer than in outlying areas, especially during nighttime due to the delayed effects of cooler temperatures on the large body of water. It is rare, though possible, during the Indian summer when unusually warm temperatures are occurring in the inland areas of the city and suburbs for temperatures to be somewhat cooler along the lake as often happens during the spring season.

== Extremes ==
The highest temperature ever recorded in the Chicago city limits is an unofficial {{convert|109|°F}} on July 24, 1934, at [[Midway International Airport|Midway Airport]]. The official reading of {{convert|105|°F}} for that day was taken at the [[University of Chicago]] campus near the shoreline off [[Lake Michigan]]. The 105° high that day is the highest ''official'' temperature ever recorded in the city.<ref name="temperature records">[http://www.crh.noaa.gov/lot/?n=chi_records Chicago's Official Records]. National Weather Service. Retrieved November 25, 2012.</ref> On July 29, 1916, the low temperature sank to only {{convert|85|°F|0}}. Many suburban, exurban and rural locations have all-time records that have surpassed {{convert|110|°F}}, many of which were set during a heat wave in [[July 1936]], when a massive heat wave engulfed the entire Chicago and northern Illinois region, resulting in eight consecutive days at or above {{convert|100|°F}} at Midway Airport, peaking at {{convert|107|°F}} on July 11, 1936. Around the region, many sites recorded at least 10 days at or above 100°, including 15 consecutive days at or above 100 in south rural [[LaSalle, Illinois|LaSalle]], which recorded an average high temperature of {{convert|100.8|F|C}} for the month of July, 1936. Temperatures peaked as high as {{convert|112|°F}} in Rockford.<ref>{{cite web | url=https://wgntv.com/weather/weather-blog/uncharted-heat-july-1936-chicagos-hidden-heat-wave/ | title=Uncharted heat—July 1936—Chicago's "hidden heat wave" | date=July 18, 2021 }}</ref> The official observations were taken at the University of Chicago, resulting in the official records remaining mostly in the mid-90s, with one day peaking at {{convert|102|°F}}, masking the true nature of the heat wave and illustrating the power of [[Lake Michigan]]'s cooling effects in immediate shoreline areas. Official observations were not moved to Midway Airport in 1942.

Summertime readings near the lake can be several degrees cooler than inland locations, especially if lake breezes are present, which suggests that the higher unofficial reading may also be accurate. During the [[Chicago Heat Wave of 1995]], which killed 739 people, official temperatures reached {{convert|104|°F}} at [[O'Hare Airport]] and {{convert|106|°F}} at Midway<!--verify by checking "Chicago Midway AP 3SW" station-->;<ref name= NOAA/> high humidity pushed the [[heat index]] to {{convert|15|–|20|F-change|C-change|0}} above actual air temperatures, resulting in heat indexes well over {{convert|120|°F}}.

The coldest temperature ever recorded in Chicago city limits is {{convert|-27|°F|0}} at O'Hare on [[January 1985 Arctic outbreak|January 20, 1985]],<ref name= NOAA/> though unofficial temperatures as low as {{convert|-33|°F|0}} have been recorded at Chicago Aurora Airport in far western suburbs and in the rural areas to the west of Chicago.<ref>{{Cite web |title= |url=https://www.glenallenweather.com/historylinks/2019/jan/Chicago.pdf}}</ref> On December 24, 1983, and [[1994 North American cold wave|January 18, 1994]], the high temperature reached only {{convert|-11|°F|0}}.

The greatest 24-hour precipitation, in a single calendar day, was {{convert|6.86|in|1|abbr=on}} at O'Hare on July 23, 2011.<ref name= NOAA/> The greatest amount of precipitation that fell in a 24-hour period was {{convert|9.35|in|1|abbr=on}} on August 13–14, 1987. That event also set the records for heaviest 6 and 12 hour rainfall amounts and contributed to August 1987's record rainfall. The heaviest calendar day snowfall was {{convert|18.6|in|cm|1|abbr=on}}, again at O'Hare, on January 2, 1999.<ref name= NOAA/> Daily snow depth has amassed as high as {{convert|29|in|cm|0|abbr=on}} on January 14, 1979; {{As of|2019|lc=y}}, January 1979 alone holds the first 8 of the top 10 daily snow-depth measurements.<ref name= NOAA/>

Official monthly mean temperatures have ranged from {{convert|10.1|°F|1}} in January 1977 to {{convert|81.3|°F|1}} in July 1955, both recorded at Midway,<ref name= NOAA/> while the annual mean temperature has ranged from {{convert|45.1|°F|1}} in 1875, as recorded at the [[Roanoke Building]], to {{convert|54.5|°F|1}}, as recorded at O'Hare.<ref name= NOAA/> The driest month on record has been September 1979 with a mere {{convert|0.01|in|mm|abbr=on}} of rainfall, while the wettest is August 1987 with {{convert|17.10|in|mm|abbr=on}} of rainfall; annual precipitation has ranged from {{convert|22.22|in|mm|abbr=on}} in 1962 to {{convert|50.86|in|mm|abbr=on}} in 2008.<ref name= NOAA/>

== Windy City ==
Chicago is known as the [[Windy City, Origin of Name (Chicago)|Windy City]]. The "Windy City" moniker did not originally refer to Chicago's climate. It is believed to have been created by a New York newspaper writer deriding Chicagoans' bluster as they promoted their city as the site of the 1893 Columbian Exposition. It is also believed to be called the "Windy City" because of politicians in the area blowing hot air. In terms of climate, Chicago is slightly windier than the average American city. Average wind speeds range from {{convert|8|mph}} in summer to {{convert|12|mph}} in spring.

== Lake breeze ==
{{further|Lake-effect snow}}
Chicago can be cooler and moister than other parts of Illinois because of its proximity to the relatively cooler waters of [[Lake Michigan]], effects which are most pronounced during spring and early summer. A frequent lakeshore breeze pushes much cooler, moister air into Chicago than the usual hot air of the Plains States (usually a moist air mass depending on upper-level circulation), but the effect can be so localized that only the immediate waterfront neighborhoods (both north and southside lake adjacent communities) are cooler than inland parts of the city. (This is where the oft-repeated weather-forecast phrase “cooler by the lake" comes from.) South, west and southwestern suburbs can be more than 20°F (11°C) warmer than the lakefront at some times of year.{{citation needed|date=April 2015}} The lake breeze also has other effects, including dense [[fog]] spilling into the city. Because of the closed-loop circulation pattern with a lake breeze that moves back and forth across the city, it is thought to significantly increase low-level [[ozone]] counts.<ref>{{cite web | title= Chicago Lake Breeze Effect Could Increase Asthma Risk | publisher= Science Daily | url= https://www.sciencedaily.com/releases/2003/06/030603083134.htm}}</ref> Differing wind direction on either side of the thermal dividing line allows for sharp [[updraft]]s under certain conditions, favorable to [[thunderstorm]] development. Offshore or land breezes shutdown the lake breeze and can have the opposite effect, with over lake convection occurring, usually in mid- to late summer and autumn. As a general rule, the opposite trend occurs during the winter—winter temperatures are warmer along the lakeshore and downtown than inland.

== See also ==
* [[Tornadoes in Chicago]]

== Notes ==
{{notelist }}

== References ==
{{reflist|30em}}

{{Chicago|state=expanded}}
{{ClimateUS}}

[[Category:Geography of Chicago]]
[[Category:Climate in the United States by populated place|Chicago]]
[[Category:Climate of Illinois]]