[Include Graphic Table] Chicago Weather Story | U.S. Climate At-a-Glance (NOAA)

Chicago Weather Story
Chicago short term weather forecasts are probably less reliable than many other cities in the United States due to the proximity to Lake Michigan and the latitude which lies between Canadian air and Gulf Air. The Chicago Metro Area is located along the southwest shore of Lake Michigan on a plain which only rises from 600 to 800 feet above sea level. Lake Michigan averages 579 feet above sea level. The plain extends to about the Fox River Valley and does not significantly affect air flow in or near the city. The suburb of Crystal Lake, just west of the Fox River, is the highest point in the area at 951 feet above sea level. Hills east of the Fox River in the Barrington area run about 800 to 900 feet above sea level. The lesser frictional drag over Lake Michigan can cause winds to be stronger along the lakeshore, and often permits air masses moving from the north to reach shore areas an hour or more before affecting western parts of the city.

Seasons
Chicago experiences changeable weather from day to day and season to season. The climate is predominantly continental, ranging from relatively warm in summer to relatively cold in winter. However, the continental climate is partially modified by Lake Michigan, and to a lesser extent by other Great Lakes. In late autumn and winter, air masses that are initially very cold often reach the city only after being tempered by passage over one or more of the lakes. Similarly, in late spring and summer, air masses reaching the city from the north, northeast, or east are cooler because of movement over the Great Lakes. Very low winter temperatures most often occur in air that flows southward to the west of Lake Superior before reaching the Chicago area. Clear nights allow more heat to escape. In some years, cold, damp springs are prolonged from March until about June 1. Other years, relatively warm springs begin in March. Fall weather tends to stay warm from the heat lag of the warm water of the lake -- especially with warm sunny days -- but rainy Septembers can make a dreary, colder fall. Even warm days, yield to cool nights by the end of September. Normal highs are near 80 degrees Fahrenheit at the end of August, drop to around 70 degrees by the end of September, and are in the middle to upper 50's by the end of October.

In summer the higher temperatures result from south or southwest flow and are therefore not influenced by the lake, the only modifying effect being a local lake breeze. Bike riders experiencing 90 degree temperatures west of Skokie Boulevard or Interstate 94 will notice a drop in temperature as they approach the lake, even when prevailing winds are from the West. However, an especially strong south or southwest flow may overcome the lake breeze and cause high temperature to extend over the entire city.

During the warm season, when the lake is cold relative to land, there is frequently a lake breeze (from the lake, toward land) that reduces daytime temperature near the shore -- sometimes by 10 degrees or more below temperatures reached farther inland. When the breeze off the lake is light, this effect usually reaches inland only a mile or two, but with stronger on-shore winds the whole city and west suburbs are cooled. The larger effect is common in April and May. Frequently, spring days in these months start out sunny, but end up cool and cloudy by noon. On the other hand, temperatures at night are warmer near the lake so that 24-hour averages are only slightly different in various parts of the city and suburbs.

Extremes, Heating & Cooling (see also Weather Data Center and Chicago Weather Extremes)
At O'Hare International Airport temperatures of 96 degrees or higher occur in about half the summers, while about half the winters have a minimum as low as -15 degrees. The average occurrence of the first temperature as low as 32 degrees in the fall is mid-October. The average occurrence of the last temperature as low as 32 degrees in the spring is in late April. May 15 is usually the last day gardeners worry about a late frost (Note: May 25, 1992 recorded a low of 32 degrees. October 1 is usually the first day to be alert for early frosts (Note: September 22, 1995 recorded a low of 32 degrees).

The normal heating season lasts from mid-September until late May. Indoor relative humidity usually drops to dry levels by October 1. Indoor humidified air is recommended from about October 1 until late March or April. Indoor air in December and January, especially, gets dry enough to dry out nasal passages, wood furniture and indoor house plants. The dry, unhumidified air frequently causes sore throats; burning nasal passages; dry, itchy skin, and electrostatic shocks. Inflamed throats and nasal passages may increase the chance of catching the common cold or flu. The shocks can disrupt computer systems. People who live in upper floors of high rises may require a longer humidification period because the higher altitudes carry even drier air.

The normal air conditioning season lasts from June until early September. June is the first month of the year to offer consistent balmy weather. October brings clear, crisp days with beautiful sunsets. However, many residents complain about the loss of daylight and the impending Winter season.

Precipitation & Storms (see also Weather Data Center and Chicago Weather Extremes)
Precipitation falls mostly from air that has passed over the Gulf of Mexico. In winter there is some snowfall that is light inland, but locally heavy near the lakeshore, with Lake Michigan providing the moisture. Heavy lakeshore snow occurs when initially cold air moves from the north with a long trajectory over Lake Michigan. When the air crosses the Chicago lakeshore, the air mass is warmed and its moisture content increases up to a height of several thousand feet. Snowfall is produced by upward currents that become stronger when the air moves from the lake onto land and experiences frictional effects. This type of snowfall therefore tends to be heavier and to extend farther inland in south-shore areas of the city and in Indiana suburbs, where the angle between wind-flow and shoreline is greatest. The effect of Lake Michigan, both on winter temperatures and lake-produced snowfall, is enhanced by the lack of ice cover over much of the lake during the winter, even though areas and harbors are often ice-choked. (Some years the southern tip of Lake Michigan is totally ice covered during winter.) During extreme cold outbreaks over lake water that has not yet frozen, great walls of fog/clouds rise out of Lake Michigan like a giant cloud factory, which is quite a site to see -- especially just after sunrise. A departing low pressure system associated with its departing snow storm often prolongs snowfall (a snow backlash) as the low pressure system -- spinning counter clockwise east of the lake -- sends strong northeast winds over the lake toward Northern Illinois. The highest recorded snowfall rate in Chicago is 3 inches per hour, as in the Big Snow of 1967.

Summer thunderstorms are often locally heavy and variable. Some suburbs of the city may receive substantial rainfall -- even hail -- while other suburbs experience sunshine. While the tornado season begins in March, the peak tornado season for the Chicago area is July through September. June is usually the month with the most thunderstorms. The summer thunderstorms often bring relief after several days of a hot spell. Many of the late summer and early fall thunderstorms move in from the north-northwest. These storms often produce turbulent severe-looking squall lines with sudden drops in temperatures from 90 to 60 degrees. Tall thunderheads that are passing to the south of the city or storms that have recently departed -- leaving sunny skies -- provide spectacular sights. The upward growth of the thunderheads are clearly visible and the onset of dusk brings spectacular cloud-to-cloud lightning displays. Using the Flash-to-Bang System you can judge the distance of a lightning bolt. Every 5 seconds between the flash of lightning and the sound of thunder equals 1 mile. Many thunderstorms, which are detected on radar as strengthening as they pass the Mississippi River and Rockford area, seem to weaken as they approach Lake Michigan. But, many of the severest storms seem to brew rapidly, giving little time for warning. Frequently, the local media broadcasts first warnings for areas that have already experienced storms for at least 20 minutes. The storms are already 2 or 3 suburbs east of the suburbs receiving the warnings.

Longer periods of continuous precipitation occur mostly in autumn, winter, and spring. From November through April -- on average -- more than half of the months are days with full overcast. About one-half the precipitation in winter, and about 10 percent of the yearly total precipitation, falls as snow. Snowfall from month to month and year to year is greatly variable. There is a 50 percent likelihood that the first and last l-inch snowfall of a season will occur by December 5 and March 20, respectively. The first hint of any snow usually occurs around Thanksgiving and brings a mess to rush hour as many drivers have forgotten the effects of slick roadways. Significant snowfalls (6 inches in 24 hours) have occurred as early as mid October. Snow that falls in February and March often melts rapidly -- within a few days in a strengthening sun. Snow in December and January frequently covers the ground for weeks, which is excellent for skiers, but not so good for runners and joggers. Fog is most common in March. Much of this fog is advection fog, which occurs when warmer air passes the cold, snow-covered ground. Snow storm tracks seem to be difficult to predict. Winter Storm watches are frequent, but the storms often turn to the north or pass to the south affecting Wisconsin or Central Illinois, respectively. Many surprises occur when storms that were expected to hit Central Illinois turn toward Chicago for a direct hit.

The City of Chicago is known as the "Windy City"; but, the average wind speed is not greater than in many other parts of the U.S. The "Windy City" nickname came from a reference to political hot air, not the weather. Channeling of winds between tall buildings often causes locally stronger gusts in the central business area, helping the city of Chicago keep the nickname. Occasionally in winter, huge chunks of ice have fallen off high rises and caused injuries and even fatalities. March and April are the windiest months. July is the calmest month, excluding thunderstorm gusts.