Overview

A historic heat wave in mid-July of 1995 overwhelmed most of the Midwest, and particularly the city of Chicago. The peak of the heat wave occurred between July 13th and July 14th, when a deadly combination of very hot air and high dew points contributed to extreme heat index values in excess of 115°F during the daytime. Little relief also occurred during the nighttime -- especially in Chicago, where the urban heat island effect contributed to 100+°F heat indices persisting late into the night.

Here is a summary of just a few of the notable statistics from this heat wave:

• The heat wave claimed over 500 lives in the city of Chicago alone and hundreds more across the Midwest.

• The highest temperature recorded at any Chicago-area reporting station during the heat wave was 106°F at Midway Airport on July 13th. The only other time when temperatures this high were recorded in the city of Chicago was in the mid 1930s during the Dust Bowl, when temperatures climbed as high as 109°F at Midway Airport.

Maximum daytime temperatures (in degrees Fahrenheit) at Chicago-area reporting stations on July 13 and 14, 1995. Source: NOAA/NWS July 1995 National Disaster Survey Report.

• Incredibly high dew points were observed (near or above 80°F at the peak of the heat wave). These dew points, combined with the very hot temperatures, resulted in oppressive heat indices being observed both during the daytime and the nighttime. On the worst day of the heat wave (July 13th), heat indices in the city of Chicago got as high as 124-125°F during the afternoon and remained above 100°F well after sunset.

Hourly timeline of temperature, dew point, and heat index observations at O'Hare and Midway airports on July 13, 1995

Timelines of temperature, dew point, and apparent temperature/heat index observations from July 10 through July 17, 1995 at (clockwise, from top left): O'Hare Airport, Midway Airport, Rockford Airport, and Meigs Field/Northerly Island.

The 1995 heat wave was a significant weather moment in Chicago’s history. Following the event, the National Weather Service (NWS) and the City of Chicago collaborated on developing a new impact-based set of Extreme Heat Watch and Warning criteria based on this heat wave's impacts on human health. Warning communication and emergency coordination were also enhanced with the goal of better protecting life and property in future significant heat waves. The strong partnership forged between the NWS Chicago office and the City of Chicago in the aftermath of this historic heat wave persists to this day with both entities working closely together to ensure that the city and its inhabitants are prepared not just for heat waves, but for other extreme weather events as well.

Meteorology

Mid-July 1995 was dominated by an unseasonably hot and humid air mass that was slow to move out of the Midwest. An abnormally strong upper-level ridge and a dome of high pressure controlled the overall weather pattern during the peak of the heat wave. Typically, upper-level ridging is associated with warmer temperatures beneath the ridge, and this was very much the case during mid-July 1995, when near-surface air temperatures were 10-20°F above average. The most extreme temperatures in the Chicago area were observed on July 13 and 14, 1995, when the upper-level ridge was centered squarely over the Midwest.

500 mb geopotential heights and wind speeds from 7/12/1995 through 7/16/1995. These maps show the northward and eastward expansion of the broad upper-level ridge/high pressure area in the region that was the primary weather feature driving the heat wave.

A chart showing 500 mb geopotential heights at 7 PM CDT on July 13, 1995. This chart reflects the broad upper-level ridge/high pressure area over the Midwest.	500 mb geopotential height anomaly for the heat wave event. Large positive values (warm colors) indicate the above average strength of the upper-level ridge that was in place over the Midwest during the heat wave.	Near-surface air temperature anomaly in degrees Celsius. The warm colors (higher positive values) across the Midwest show that this event was marked by well above average air temperatures. Most of the Upper Midwest experienced temperatures 15-20°F above average during the heat wave.

Southerly winds at the surface transported very humid air into the region, resulting in persistent upper 70s/lower 80s dew points in the Chicago area -- well above the average for July. The subsequent hot, humid air mass in place during the heat wave resulted in extreme heat index values across the region. These unseasonably high heat indices were the hallmark of the 1995 heat wave and contributed to the high number of casualties during the event.

Regional peak heat indices on 7/12/1995	Regional high temperatures on 7/12/1995	Regional peak dew points on 7/12/1995
Regional peak heat indices on 7/13/1995	Regional high temperatures on 7/13/1995	Regional peak dew points on 7/13/1995
Regional peak heat indices on 7/14/1995	Regional high temperatures on 7/14/1995	Regional peak dew points on 7/14/1995
Regional peak heat indices on 7/15/1995	Regional high temperatures on 7/15/1995	Regional peak dew points on 7/15/1995
Regional peak heat indices on 7/16/1995	Regional high temperatures on 7/16/1995	Regional peak dew points on 7/16/1995

One other factor that likely contributed to the intensity of the heat wave was an abnormally wet May in the Midwest earlier in the year. A 2003 research paper found that late spring rainfall anomalies in the "Corn Belt" region of the Midwest correlate to increased levels of evapotranspiration from crops during the mid-late summer. Enhanced evapotranspiration leads to increased quantities of moisture being emitted into the air, resulting in higher dew points and relative humidity. Thus, the above normal rainfall observed in the region in May 1995 likely played a role in why the dew points in mid-July 1995 were as oppressively high as they were.

More on the meteorological diagnosis for this event can be found here.

Historical Context

A paper published in the Bulletin of the American Meteorological Society found that the 1995 Chicago heat wave was not an unprecedented event in Chicago’s history, at least in terms of maximum high temperatures. The authors noted that heat waves in 1911, 1916, 1934, and 1936 were of comparable (or greater) intensity. How does the 1995 heat wave compare to more recent significant heat waves, particularly July 29-31, 1999 and July 4-7, 2012?

The heat wave that affected the Chicago area in late July 1999, was eerily similar to the 1995 event with uncomfortably high dew point temperatures affecting Chicagoland. Dew points soared into the upper 70s and lower 80s, peaking at 82°F at both Midway and O’Hare airports. The resulting dangerous heat indices of 119°F at Midway and 113°F at O’Hare occurred on both July 29 and 30. The peak high temperatures during the 1999 event occurred on July 30 when Midway and O’Hare hit 103°F and 100°F, respectively.

One major factor that influenced the severity of both the 1995 and 1999 heat waves was the urban heat island effect (click here for more information about urban heat islands). Temperatures at Midway Airport were generally warmer on the order of 1-2 degrees compared to O'Hare Airport during daytime peak heating, possibly due to more pronounced urban heat island effects with the airport largely being surrounded by brick buildings, concrete, and asphalt. Additionally, the rate of overnight cooling in urban areas is much less pronounced than surrounding areas due to the urban heat island effect. Such was the case in 1995 and 1999 with overnight lows in the upper 70s to lower 80s offering no respite for those afflicted by the maximum daytime heat and humidity. Increased heat awareness following the 1995 heat wave helped alleviate significant impacts during the 1999 heat wave.

More recently in 2012, Chicago experienced a heat wave that was warmer than previous heat waves, including the 1995 heat wave. Maximum high temperatures in early July 2012 were generally a few degrees higher than in the July 1995 or July 1999 heat waves. In fact, July 4-6 and 3-6, 2012 were the second warmest 3-day and 4-day periods, respectively, in Chicago's recorded history. Average high temperatures during these periods were 91.3°F and 90.1°F, respectively, trailing only 3-day and 4-day periods from July 1916! However, unlike the 1995 and 1999 heat waves, the dew point temperatures were considerably lower in 2012. Thus, the resulting heat indices during the 2012 heat wave were far less impactful than the 1990s heat waves. That said, this heat wave, combined with the subsequent lack of rainfall and above average July temperatures, contributed to the extensive drought in the Midwest in July 2012 and impacted agriculture throughout the region.

Top five 1-day through 4-day average temperature rankings for the city of Chicago

Number of hours with a heat index greater than 100°F and 110°F during the 1995, 1999, and 2012 heat waves

Average daytime heat indices (in degrees Fahrenheit) during the 1995, 1999, and 2012 heat waves

Advancements

Advancements in Forecasting

Technology has obviously evolved in the years since the 1995 heat wave, but so has the knowledge of heat wave weather patterns -- so much so that the ingredients for a significant heat wave can be recognized several days in advance, even a week or more at times. The increases in technology and computer power have contributed to vast improvements in the numerical weather prediction (NWP) models that meteorologists use for forecasting. Models today are highly sophisticated in how they model the atmosphere, especially compared with those in used in 1995. Additionally, contemporary high-resolution models account for the influences between land-atmosphere and ocean-atmosphere processes, further enhancing their utility in forecasting potential excessive heat episodes. The increased technology at forecasters' disposal has proven tremendously useful to the forecast process in the decades since July 1995.

The analog technique is another highly useful forecasting tool that compares the forecast weather pattern and relevant parameters with past significant events. Analogs allow forecasters to look at the ingredients associated with previous high impact weather events to see how those ingredients and events compare with what is currently being forecasted. Analogs can help a forecaster identify a potential significant weather event or a pattern conducive of significant weather that warrants monitoring several days in advance.

Our ability to detect potential heat waves in advance has steadily improved over the past several years, especially with longer range (7-14 day) forecasts. The Climate Prediction Center issues 6-10 day, 8-14 day, and 30-day temperature and precipitation outlooks. These outlooks are updated frequently and highlight the current thinking regarding the long-range weather pattern. Such information can increase awareness and lead-time regarding potential heat waves, allowing for preparedness measures to be enacted well in advance of their onset.

Advancements in Preparedness

Following the 1995 heat wave, the Chicago NWS office and the City of Chicago worked together to develop the following impact-based extreme heat criteria specific to the City of Chicago and Cook County to enhance the warning communication for future heat waves:

1. Any day the maximum heat index is forecasted to reach at least 110°F.
2. Two consecutive days with maximum heat indices forecasted to reach at least 105°F.
3. Three or more consecutive days with maximum heat indices forecasted to reach at least 100°F.

These criteria recognize that the human body’s ability to properly deal with extreme heat dramatically decreases as the heat index rises. In general, heat indices between 95°F and 105°F can be tolerated with appropriate precautions, but when heat indices exceed 105°F, the ability of the body to keep its internal temperature regulated decreases. Heat index values of 115°F or higher can have significant health impacts on otherwise healthy individuals. Reference the Additional Info tab for more information on the health impacts of extreme heat.

In 2025, as part of its Hazard Simplification Project, the National Weather Service updated the names of its highest-level heat products in an effort to better communicate the danger posed by extreme heat. Excessive Heat Watches were renamed to Extreme Heat Watches, while Excessive Heat Warnings were renamed to Extreme Heat Warnings. In an effort to mitigate potential health and commercial impacts, Extreme Heat Watches are issued between 24 and 72 hours before the onset of extreme heat when confidence is relatively high that the criteria for extreme heat may be met. Upon the issuance of an Extreme Heat Watch, the City of Chicago will begin to mobilize for a potential heat emergency. Should it appear that the extreme heat criteria will likely be met within 24 to 36 hours, an Extreme Heat Warning will be issued. Upon the issuance of an Extreme Heat Warning, the City of Chicago will fully implement its emergency response plan for extreme heat. If notable heat (heat indices greater than or equal to 105°F) is expected within 24 to 36 hours, but the criteria for an Extreme Heat Warning will likely not be met, then a Heat Advisory will be issued instead.

The NWS and public safety partners have many more ways to communicate threats now than in 1995. Ahead of potentially high-impact weather events, the NWS can now message the potential for impactful weather in a variety of ways to a larger audience and can even do that in advance of watches and warnings. The NWS does this via briefings to core partners and forecast graphics (such as Weather Stories) that are easily consumable and can easily be shared in multiple locations, including on social media. These far-reaching means of conveying weather hazard potential can reach a larger audience, and messaging the threats to specific vulnerable groups can now be done too. This is where collaboration with the City of Chicago is especially important. 

Unlike other forms of extreme weather, heat waves do not possess telltale signs of impending hazards. Severe thunderstorms produce dangerous weather phenomena (tornadoes, lightning, hail, etc.) that are easily identified as harmful. Heat waves, however, are often accompanied with sunny skies and relatively benign weather - light winds, no rain, etc. Despite this, heat waves are exceedingly dangerous and should be treated with proper precautions. The collaborative efforts of the NWS, local governments, emergency managers, broadcast media, and you serve to increase public awareness and prompt others to take precautions when extreme heat is forecasted. You are a key part of this effort. In order to successfully mitigate the potential hazards of heat waves and other forms of extreme weather, you can "be a force of nature" by knowing your risk, taking action, and being an example in your community.

Know your risk

• Visit weather.gov for the latest forecast information for your area
• Follow the National Weather Service and your local NWS office on social media to stay up-to-date with the latest weather information
• Read NOAA's National Climate Reports for the latest information on risks where you live
• The NOAA/NWS 1995 heat wave assessment noted that the elderly are especially vulnerable to excessive heat

Take action

• Buy a NOAA Weather Radio so that you always have a reliable method of obtaining warning information
• Put together a disaster supply kit → flashlights, first aid, food, water
• Ensure that elderly family and friends are in good care and have access to indoor facilities with functioning air conditioning during heat waves

Be an example

• Let your friends and family know how you have prepared for adverse weather and help them become weather ready
• Put together a plan to ensure that your family knows exactly what to do when disaster strikes
• Learn more about weather preparedness by visiting the Weather Ready Nation website

Additional Information

Heat Safety

Extreme heat is extremely dangerous! A little preparedness can go a long way to protecting yourself and others. What can you do?

• Drink plenty of water, even if you do not feel thirsty. Limit caffeinated and alcoholic beverages.
• Stay indoors during the hottest part of the day, preferably in a building with air conditioning.
• Limit outdoor activities. If you must work outdoors, take frequent breaks and utilize a buddy system.
• Never leave anyone (including pets) in a parked automobile. Temperatures inside of an automobile can soar to dangerous levels within 10-15 minutes.
• Check on family and friends, especially if they are alone or without air conditioning.
• Seniors, infants, and those who are physically ill are at greater risk for heat-related illness.

NWS Chicago's local heat preparedness and safety web page with infographics like the ones above can be found here. Additional heat safety tips and resources from the NWS can be found here.

For more detailed information on what to do before, during, and after periods of excessive heat, please visit Ready.gov and the Center for Disease Control and Prevention websites.

More Info on Dew Points

The dew point temperature is the temperature at which air will become saturated with water vapor, resulting in the condensation of that water vapor into liquid water droplets. It is one of many different quantifiers for atmospheric moisture content, and during the summertime, it is typically considered to be the best quantifier for how the air truly feels. Dew points below 60°F are considered to be comfortable by most, while higher dew points (particularly as they exceed 70°F) correspond to increasingly humid conditions.

Additional Links

• NOAA/NWS Natural Disaster Survey Report of 1995 Heat Wave
• Illinois State Climatologist Report on 1995 Heat Wave
• July 1999 Heat Wave Event Summary
• July 2012 Heat Wave Event Summary
• Other Local Extreme Heat/Cold Events