Climate of Chicago

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The climate of Chicago is a humid continental climate (Koppen climate classification Dfa) typical of the Midwestern United States, but with distinctive characteristics associated with its position where the Great Plains meet Lake Michigan. Sudden changes of weather, large daily temperature ranges, and unpredictable precipitation patterns are all staples of Chicago weather. Chicago has four clearly defined seasons, although in certain years some seasons may linger into months they do not traditionally occupy, or arrive exceptionally early, preempting the current season. For example, in Chicago it has snowed in September (1942), been 90°F (32°C) in March (1982), and had a day where the high and low temperatures differed by more than 65°F (31°C) (February 8, 1900).

On a typical Chicago mid-summer day, humidity is usually moderately high and temperatures ordinarily reach anywhere between 78°F and 92°F (26°C to 33°C). Overnight temperatures in summer are usually around 62°F (17°C), but can sometimes remain well above 70°F (21°C). Yearly precipitation comes in at an average of about 36 inches (920 mm). Summer in Chicago is prone to thunderstorms, and summer rain arises from short-lived hit-or-miss storms rather than a prolonged rainfalls. In a normal summer, temperatures exceed 90°F (32°C) on 24 days. Contrary to what one might think, summer is actually the rainiest season in Chicago.^[1] In a curious shift, July was actually the wettest average month in Chicago from when records were started in 1871 until 1965. In 1965, August inexplicably overtook July as the wettest month, and it remains wetter than July to this day.

Winter in Chicago proves variable and fickle. The average Chicago winter produces 37.0 inches (94 cm) of snow. This number can prove unreliable, as Chicago winters have produced between 9.8 and 87.0 inches (25cm and 221cm) of snow. Snow tends to fall in light accumulations of around 2 inches (5 cm), but about once per year Chicago experiences a storm that can produce 10 to 14 inches (25cm to 35cm) of snow in one day. Unlike across Lake Michigan in western Michigan or in northern Indiana, Chicago rarely gets lake-effect snow because prevailing westerlies that cross the region pick up moisture from Lake Michigan after passing Chicago. Rare northeasterly winds during the winter may deposit the sort of snowfall that one associates with nearby snowbelt cities such as Grand Rapids, Michigan, Kalamazoo, Michigan, and South Bend, Indiana with the more usual westerly winds. Temperatures can vary wildly within the span of one week, but extended periods of temperatures below 32°F (0°C) are not uncommon in January and February. The temperature in January averages about 29°F (-2°C) in the afternoon, and 14°F (-10°C) at night. Temperatures can be expected to drop below 0°F (-18°C) on 15 days throughout the winter season. Although rare, temperatures in Chicago even in the middle of winter can surpass 50°F (10°C).

The highest temperature ever recorded in Chicago is an unofficial 109°F (44°C) on July 24, 1935. The highest official temperature ever recorded is 105°F (41°C) on July 17, 1995 during the Chicago Heat Wave. The coldest temperature ever recorded in Chicago is -27°F (-33°C) on January 20, 1985, with a wind chill of -83°F (-64°C). On December 24, 1983, the high temperature at Chicago reached only -11°F (-23°C), and on July 30, 1916, the low temperature sank to only 84°F (29°C), both of which are records.

Although Chicago is known as the Windy City, it is not significantly breezier than other American cities. Average wind speeds range from 8 mph (13 km/h) in late summer to 12 mph (19 km/h) in spring months. Until recently, the "windy city" moniker was believed to be a derisive reference to Chicago's politicians, coined in a New York newspaper editorial in 1890. New research by OED consultant Barry Popik has shown that the term actually appeared in print several years before the New York editorial. The phrase may have been created by Chicago tourism boosters attempting to promote the city, cooled by breezes off Lake Michigan, as an ideal summer destination.

Chicago experiences microclimatic effects because of Lake Michigan, especially during the summer. Very often during the summer a local lakeshore breeze pulls much cooler air into Chicago than the usual hot air of the Plains States, but the effect may be so local that only the immediate shoreline is much cooler than parts of the city further inland.

According to the U.S. Department of Agriculture, Chicago falls within Plant Hardiness Zone 5b, with the outlying areas falling into the sightly harsher Zone 5a.^[2]

Month [3]	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Avg high °F (°C)	29 (−1)	34 (1)	46 (7)	58 (14)	69 (21)	80 (27)	84 (29)	82 (28)	74 (24)	63 (17)	47 (8)	34 (1)	59 (15)
Avg low °F (°C)	14 (−9)	19 (−7)	28 (−1)	37 (3)	47 (8)	57 (14)	63 (17)	62 (16)	53 (12)	42 (5)	31 (0)	20 (−6)	39 (4)
Rainfall in. (mm)	1.8 (44)	1.6 (41)	2.7 (67)	3.7 (93)	3.4 (85)	3.6 (92)	3.5 (89)	4.6 (117)	3.3 (83)	2.7 (68)	3.0 (76)	2.4 (61)	36.3 (921)

[edit] References

1. ^ Chicago Summer Precipitation Rankings Summer (June-August) (11/29/2005). NOAA's National Weather Service Forecast Office.
2. ^ US National Arboretum - USDA Plant Hardiness Zone Map (2/23/2004). United States Department of Agriculture.
3. ^ Chicago, Illinois - Summary (2006). Weatherbase.

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