The July 1995 Heat Wave in the » Midwest: A Climatic Perspective and Critical Weather Factors Kenneth E. Kunkel,* Stanley A. Changnon,* Beth C. Reinke,* and Raymond W. Arritt+ ABSTRACT A brief but intense heat wave developed in the central and eastern United States in mid-July 1995, causing hundreds of fatalities. The most notable feature of this event was the development of very high dewpoint temperature (Td) over the southern Great Lakes region and the Upper Mississippi River Basin. At many locations, hourly values of Td set new records. The combination of high air and dewpoint temperatures resulted in daily average apparent temperatures ex- ceeding 36°C over a large area on some days. A comparison with past heat waves shows that this was the most intense short-duration heat wave in at least the last 48 years at some locations in the southern Great Lakes region and Upper Mississippi River Basin. An analysis of historical data for Chicago, where the majority of fatalities occurred, indicates the intensity of this heat wave was exceeded only by a few periods in the 1910s and 1930s. Impacts in the Chicago urban center were exacerbated by an urban heat island that raised nocturnal temperatures by more than 2°C. An analy- sis of radiosonde data indicates that maximum daytime boundary layer mixing depths were only a few hundred meters in the core region of the heat wave. Simulations using a single-column version of a three-dimensional mesoscale model strongly suggest that this contributed to the very high values of Td since soil moisture in the central United States was near to above average and evapotransporation was likely high, causing a rapid moistening of the shallow boundary layer. 1. Introduction rected toward conditions at Chicago, where 65% of all heat deaths in the nation occurred. An intense heat wave developed over the central United States during mid-July 1995. Although brief, this heat wave was of sufficient intensity to cause 2. Data sources and methods hundreds of fatalities. This paper describes the 1995 heat wave, compares it with past heat waves of simi- Hourly surface observations at all available airport lar duration, and examines the weather conditions locations in the central United States were used to causing the heat wave. Although fatalities were re- assess spatial patterns of dewpoint (7^) and apparent ported in 19 states, 87% occurred in the Midwest (see temperature (7^) for mid-July 1995. Hourly values of Changnon et al. 1996). Consequently, the atmospheric air temperature (TJ, Td, and wind speed were used to analysis focused on this region where the greatest calculate hourly Tap, which were interpolated from impacts were realized, and specific attention was di- Table 5 in Steadman (1979a). A wind speed adjust- ment was obtained from Table 2 in Steadman (1979b). *Midwestern Climate Center, Illinois State Water Survey, Historical assessment of this heat wave and other simi- Champaign, Illinois. lar heat waves was accomplished by using hourly ob- department of Agronomy, Iowa State University, Ames, Iowa. servations in the National Climatic Data Center's Corresponding author address: Dr. Kenneth Kunkel, Midwest- TD-3280 dataset of hourly data. Long-term digital ern Climate Center, Illinois State Water Survey, Department of Natural Resources, 2204 Griffith Dr., Champaign, IL 61820-7495. data are available for fewer stations than were used E-mail: [email protected] in the 1995 analysis for which data were captured in In final form 16 February 1996. real time. The historical analysis was restricted to ©1996 American Meteorological Society those hourly stations with at least 35 years of digital Bulletin of the American Meteorological Society 1507 Unauthenticated | Downloaded 10/10/21 03:12 PM UTC data, and most of the stations used had data available term 1901-1995 record of T max and T mi.n available at beginning in 1948. Daily average values of 7, Td, and suburban Aurora, which is located 65 km from the T were calculated for each day in the period of lakeshore and unaffected by lake breezes. There were record. no comparable suburban measurements of humidity; Because of the high number of fatalities in the thus, the possible effects of the lake breeze on Td were Chicago area in July 1995, a special effort was made not considered. The values of Ta, in this second series to extend the comparative analysis of Chicago's con- were the same as those in the first series. However, ditions into the early part of the century. Because of in the second series, T ap and T a were calculated byJ station moves and the effects of lake breezes, two time substituting° Aurora T max and T mi.n data prior r to 1959 series of daily data were constructed. The extension instead of temperatures at the various Chicago loca- of both series back to 1901 was accomplished by us- tions. Daily T was estimated as the average of the ing a special digital dataset developed at the Illinois Aurora T max and T mm. DailJy averag0 e Tap for Chicag0 o State Water Survey for 1901-1964. It included daily was calculated using the morning and afternoon val- maximum (Tv )7 and minimum v (T . 7 ) temperature1 s and max min ues of T,a with the Aurora values of T min and T max for relative humidity values for early morning (R.xm) and 1901-1958. This second series (hereafter called the late afternoon (Rpm) derived from hourly observations "Aurora/O'Hare" series) of daily data consisted of the for the official station in the Chicago area in the city's compositr e of 17 ) Aurora Tma x and Tmi .n data combined business district (1901-1925), at the University of with Chicago city Td data for 1901-1958 and Chicago (1926-1945), and at Midway Airport (1946- 2) Chicago O'Hare hourly data for 1959-1995; nei- 1964)7 . BJy assumin°g that/?p m was coincident with Tmax 7, ther site is significantly affected by lake breezes. The and R am was coincident with T min , earlJy morning0 and analysis described below was performed with both afternoon values of Ta, and T ap were calculated for the time series to assess the sensitivity of the ranking of 1901 -1947 period. Daily values of Ta, Td, and T were the 1995 heat wave to the method used for construct- then computed as the average of the values at these ing the historical time series. two times. After initial analysis of the data indicated For the 1995 analysis, potential evapotranspiration high rankings of heat waves in June-July 1931, July (PET) was estimated using hourly surface airways 1934, and July 1936, hourly data for these months was observations. Cloud cover observations were used to digitized and these hourly values were used instead estimate solar radiation with the model of Meyers and of the twice-daily values. The first time series (here- Dale (1983) as implemented by Petersen et al. (1995). after called "city/O'Hare" series) of daily average Ta, Net radiation was estimated following Weiss (1983). T(P and Tap values was constructed as a composite from Finally, PET was estimated with the Penman- 1) the twice daily values in the above special dataset Monteith formula (Monteith 1965). This formula re- for 1901-1947, 2) the hourly values available in digi- quires the specification of aerodynamic and surface tal form for Midway Airport for 1948-October 1958, resistances. The aerodynamic resistance (r ) was es- and 3) the hourly values for O'Hare Airport for timated assuming a logarithmic wind profile and neu- November 1958-present. tral stability, as Because of these station moves, the city/O'Hare time series has some inhomogeneities due to the location-dependent effect of lake breezes on station Jn(z/z0)f air temperature conditions. The lake breeze penetrates Ka ' u(z)k2 ' the downtown area and the University of Chicago sta- tions (both close to the lake) on 36% of summer days (Lyons 1972), reaches the Chicago Midway Airport where U(z) is wind speed at height z\ k is von Karman (13 km from the lake) on 14% of the days, and only constant; and z0 is roughness height equal to 0.01 m, rarely penetrates to the Chicago O'Hare station (22 km a typical value (Rowntree 1991) for mown grass sur- from the lake). Therefore, the combination of Chicago faces characteristic of airports where the observations O'Hare's record (beginning in November 1958) with are taken. Surface resistance was set to 50 s nr1, typi- the downtown, university, and Midway sites provides cal for freely transpiring growing crops (Rowntree a historical record with shifts that significantly alter 1991). The above procedure was applied to hourly the temperature continuity. This problem was ad- data, and daily PET was calculated as the sum of the dressed by creating a second time series using a long- hourly estimates. 1508 Vol. 77, No. 7, July 1 996 Unauthenticated | Downloaded 10/10/21 03:12 PM UTC 3.The July 1995 heat wave An upper-level ridge developed over the southwest- ern United States on 7 July and resulted in rising tem- peratures in the Great Plains, with maximum temperatures exceeding 35°C in some areas. By 10 July, maximum temperatures exceeded 40°C in parts of Kansas. Over the next few days, the upper- level ridge and accompanying heat spread northward and eastward across the United States (Fig. 1). The upper-level ridge, which had developed over the southern Rocky Mountains, was centered in eastern Kansas at 1200 UTC on 12 July (Fig.