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Historical Heat Waves

July 1999 Heat Wave in the Midwest

During the last two weeks of July 1999, the Midwest experienced a lengthy series of days with temperatures in excess of 90°F. Before it was over, some 232 deaths were attributed to the heat in the 9-state Midwest region. Below is the full article:

The Nature and Impacts of the July 1999 Heat Wave in the Midwest

Michael A. Palecki and Stanley A. Changnon
Midwestern Climate Center
Illinois State Water Survey
Champaign, IL
August 23, 1999

During the last two weeks of July 1999, the Midwest experienced a lengthy series of days with temperatures higher than 90 degrees F. While only a relatively small number of maximum temperature records were set, the combination of high heat, record dew points, strong solar inputs, and weak winds led to a dangerous situation for people. Before it was over, some 232 deaths were attributed to the heat in the 9-state area served by the Midwestern Climate Center (MCC); there were additional health, infrastructure, and economic impacts that were also quite significant. The nature of the climate anomaly present over the Midwest will be examined first, and then the resulting impacts to society will be discussed.

The Climatology of the Midwest Heat Wave of 1999

Figure 1a
Figure 1a (Click to enlarge)
The month of July had a quite normal temperature history in the Midwest until the start of the heat wave by the 19th in the southern part of the region, and by the 21st in the northern part of the region. The hourly temperature records at St. Louis (Figure 1a) and Chicago (Figure 1b) show that there were actually two components of the heat wave. Figure 1b
Figure 1b (Click to enlarge)
The first stretch of extreme heat ended with the passage of a cold front that temporarily cooled the region on the 26th and 27th in the north, and the 27th and 28th in the south portion of the Midwest. However, after the short break, the heat wave reached its maximum intensity on the 29th and 30th, during which 80% of the Midwest deaths due to the heat wave took place.

While the dew point temperatures were steadily high at St. Louis throughout the period (Figure 2a), a tremendous spike in dew points affected the central portion of the Midwest, including Chicago (Figure 2b). For a sustained period from 6 PM on July 29th to 10 AM on the 30th, the dew point temperatures in Chicago did not fall below 80 degrees F, and a new record was set at 8 AM of 82 degrees F. To characterize the heat wave, the period of July 17-31, 1999, will be described, followed by a detailed examination of the peak day of impacts, July 30th.

Figure 2a
Figure 2a (Click to enlarge)
Figure 2b
Figure 2b (Click to enlarge)

As with most periods of hot summer weather in the Midwest, the region was dominated by surface high pressure and a mid-tropospheric ridge during the last half of July. The 500 mb height anomalies averaged only 40-60 gpm higher than normal over the Midwest during this period (Figure 3a), but the jet stream was clearly diverted to the north edges of the region (Figure 3b).

Figure 3a
Figure 3a (Click to enlarge)
Figure 3b
Figure 3b (Click to enlarge)

Maximum temperature departures from normal were especially strong in the central band of the Midwest, reaching 8 or 9 degrees F in places for the 15-day period July 17 to 31 (Figure 4a). The minimum temperatures were also very high during the evenings, especially in some of the more populated areas such as St. Louis, Chicago, and Indianapolis (Figure 4b).   ^TOP

Figure 4a
Figure 4a (Click to enlarge)
Figure 4b
Figure 4b (Click to enlarge)

The number of days exceeding 90 degrees F in the region ranged from all 15 days in southern Missouri to 9 days in the Chicago area; the number of hot days fell off rapidly to the north of Chicago (Figure 5a). While 90 degrees F heat is not unusual in the southern part of the Midwest in late July, the number of these days was 6 to 10 more than normal in the central portion of the Midwest (Figure 5b).

Figure 5a
Figure 5a (Click to enlarge)
Figure 5b
Figure 5b (Click to enlarge)


Figure 6
Figure 6 (Click to enlarge)
The conditions on July 30th were extreme. Most of the Midwest remained above 72 degrees F overnight, with many stations remaining at 77 degrees F or above, especially in the large cities (green dots in Figure 6).

The midnight-to- midnight average of the 24 hourly temperature readings at the hourly reporting stations in the Midwest indicate an average of more than 90 degrees F in St. Louis, Kansas City, Chicago, and along the Ohio River, and above the mid-80s elsewhere in Illinois, Indiana, Missouri, Kentucky, and the southern portions of Iowa and Ohio (Figure 7a). Therefore, there was no relief or cool down period to recover from the heat accumulated during the previous day. At the same time, the 24-hour average dew point temperatures were above 76 degrees F in the core of the region (Figure 7b).   ^TOP

Figure 7a
Figure 7a (Click to enlarge)
Figure 7b
Figure 7b (Click to enlarge)

To compound matters, wind speeds were quite low over most of the area (Figure 8a), and wind directions were generally from the south and southwest (Figure 8b), not allowing any lake breeze relief to the Chicago area. Finally, skies were clear or only partly cloudy during the daytime in the central and southern Midwest, allowing intense solar radiation to literally turn closed houses and apartments into brick ovens. All of these conditions together resulted in maximal heating of cities and substantial reductions in the capabilities of people to dissipate a heat load through sweating.

Figure 8a
Figure 8a (Click to enlarge)
Figure 8b
Figure 8b (Click to enlarge)


Figure 9
Figure 9 (Click to enlarge)
Most of the people who died on the 29th and 30th lived in large cities with an old infrastructure of non-air-conditioned brick buildings. A comparison of hourly temperatures between stations inside large cities and nearby suburban stations demonstrates the strong urban heat island affects during the heat wave. In St. Louis, the only hourly reporting station that is in the city is at the airport, which is really on the edge of the strongest urban affects. Urban- rural temperature differences averaged about 2 to 4 degrees F over moving 24- hour periods (Figure 9).

Larger temperature differences occur in the late afternoon and early evening, as the city sustains its peak heat level long after the more rural location is cooling down. The comparison was easier to make in the case of Chicago, which has an hourly reporting station in the heart of the city. Midway airport averaged about 5-7 degrees F higher in temperature than nearby Aurora, IL (Figure 9). Conditions in more crowded and built-up neighborhoods were probably even more extreme than at the relatively open airport. The already severe regional heat wave conditions were quite exaggerated in Midwestern cities.

The heat wave was broken by the passage of a strong cold front through the region on July 31 - August 1. Violent storms along the front caused several deaths in Wisconsin and Michigan, and some damage elsewhere. A tremendous toll of deaths and economic losses had occurred in a relatively brief time period. These will be discussed more in the next section.


Heat Wave Impacts

The Human Toll

The major loss of life was in the large cities where the urban heat island amplified temperatures by 3 to 5 degrees F or more. The loss of life attributed to the heat wave by the Associated Press, as of August 7, included 127 in Illinois, 61 in Missouri, 14 in Ohio, 13 in Wisconsin, 8 in Kentucky, 7 in Indiana, and 1 each in Michigan and Minnesota. Most of the deaths in Illinois and Missouri occurred in the Chicago, St. Louis and Kansas City metro areas, with most deaths in the other states occurring in Cincinnati, Milwaukee, and Indianapolis. As in the 1995 heat wave, the majority of those who died were elderly persons, living alone in the inner city regions, and either were without air conditioning or without the funds to pay for continuous operation of their air conditioning units. Heat-related deaths began occurring on July 22, and the initial warm period from July 20-27 was responsible for 46 deaths in the Midwest. The second peak in the heat wave, July 28-31, was far more deadly: 186 deaths occurred then, constituting 80% of the total loss of life of 232.

Although temperature and moisture extremes on July 28-30 matched those during the mid-July heat wave of 1995, many fewer persons died in July 1999. The reduction in fatalities resulted for two main reasons. First, the most extreme period was preceded by 10 hot days during which urban cooling centers and care monitoring procedures for the urban elderly were initiated. The news media provided vast amounts of useful information about behaviors to lessen heat stress and where to seek help. Second, the peak of the heat wave was sharp and brief, lasting 2 days in most of the central and eastern Midwest. In Chicago, for instance, the average maximum temperature of the warmest four consecutive days this year, July 28-31, was 97 degrees F, while maximum temperatures for the period July 12-15, 1995, averaged 101 degrees F. Compared to 1995, the 1999 heat wave had proportionally more victims in Missouri than in Illinois, possibly due to a higher degree of both length and intensity of the heat wave in Kansas City and St. Louis this year. However, the change in regional proportionality of deaths may also have been caused by a more effective response to mitigate the hazardous conditions in Chicago this year.

Electrical Infrastructure Problems

The prolonged period of high temperatures and widespread use of air conditioning brought new daily power use records in all major Midwestern cities including Chicago, Cincinnati, Detroit, Milwaukee, and St. Louis. These records were a result of the combination of high temperatures and the continuing growth in urban demand for electricity. During the peak heat wave period on July 29th and 30th, when 100 degrees F or higher maximum temperatures were common in large Midwestern cities, major power outages occurred in Chicago. Approximately 92,000 customers were without power for at least 4 hours, and many thousands remained without power for 1 to 3 days. The Chicago outages were caused by the overheating of old underground transmission lines. In several cases, the heat generated by the extra large electrical loads could not be dissipated because the ground was also warm. The damaged insulation would then conduct electricity, shorting out the line. In fact, another large outage of electricity occurred in Chicago several days after the heat wave ended due to electric line damage incurred during the heat wave. An even larger outage in Detroit occurred due to the severe storms that ushered in the end of the heat wave on August 1. About 160,000 people lost power for periods of hours to days.

Additional electrical system difficulties occurred due to the new open market for electricity after deregulation. Many electric companies in the region were forced to cut-off large industrial users of electricity, under previous agreements that allow this to occur at times of peak demand. Some electric concerns paid exorbitant rates to acquire extra electricity beyond their production capability. Finally, one major energy supplier, Cinergy in Cincinnati, actually defaulted on contracts to supply electricity elsewhere, losing millions of dollars in the process. The largest financial losses, though, will probably be those faced by Commonwealth Edison in Chicago. Not only must they compensate 100,000 customers for lost refrigerated food and medicine during the heat wave, but they may also be sued to reimburse businesses for up to $100 million in losses by companies that were closed for a day on August 3rd by an electrical outage in downtown Chicago. This power loss affected the Loop area of the downtown, and occurred when power lines previously damaged by the heat wave shorted out.  

Other Impacts

The heat had other major impacts on the Midwest. Major highway damage was noted in several states due to heaving of road surfaces. Crops were stunted in the southern and eastern parts of the Midwest due to the combination of heat and lack of rainfall. Milk production at dairy farms was also hard hit, and thousands of cattle, pigs, and chickens died due to the heat. Large fish kills were reported in Iowa due to a lack of oxygen in the water caused by its heating. Several communities in Illinois and Missouri reported 1-day records for water use. Finally, many other health problems and illnesses were enhanced by high ozone levels, particularly in the larger metropolitan areas. All types of outdoor work, such as landscaping or construction, were reduced in productivity.