The 1995 heat wave in Chicago, USA

A remarkable heat wave occurred throughout the American Midwest in July 1995. It affected a large part of central and eastern USA (1), with screen temperatures up to 42° C, notably in Nebraska and Kansas (Note 3.C). Maximum daily average dewpoints in Wisconsin, nearer the Great Lakes, exceeded 26° C, because of local evaporation. An unusally warm mid-troposphere subsided to warm even more and trap the heat and humidity in the PBL. The ‘apparent temperature’ (Section 6.5) exceeded 48° C in places.

The heat wave greatly affected Chicago, where urban heating in the city amounted to 1.6 K in the afternoon, and up to 2.5 K at night. The high humidity and warm air aloft kept the nocturnal minima unusually high, around 26°C. The apparent temperature in Chicago rose to 48°C and stayed above 31°C for 48 hours (2). The result was an estimated 830 deaths, 525 of which occurred in the Chicago area (3), however most fatalities were amongst elderly people and those with frail health. We do not wish to understate the severity of the environmental discomfort, yet the attribution of the cause of death was sometimes ambiguous.

Heat waves are by far the greatest weather-related killer in the USA. Causes of death include inadequate warning, power failures, inadequate ambulance and hospital facilities, urban heating, an aging population, an inability to ventilate houses properly by opening windows, for fear of criminal intrusion, and poverty which prevented buying, repairing or using air-conditioners.

Reviewing apparent temperatures during heat waves in Chicago since 1960 (2), it appears that their maximum values increased by more than 3 K, whereas daily minimum values did not change, which is unlike global trends of dry-bulb temperatures. This inconsistency is partly explained by a changing of the standard hygrothermometers in 1986. In addition, trends outside Chicago were less evident, indicating the effect of increased urban heating there, and also summer mean apparent temperatures showed no increase. As a result, it is hard to prove any rise in the likelihood of heat waves, due to global warming, for instance. Because the 1995 Chicago heat wave was so extra-ordinary, a repeat of the Chicago is not to be expected soon.

A system to predict the occurrence of oppressive airmasses (4) worked well during the 1995 Chicago heat wave, except that the system overpredicted mortality towards the end. This is possibly because most of the vulnerable people had already succumbed to the earlier episodes, or maybe people were taking evasive action in view of the success of previous predictions.

(1) Kunkel, K.E., S.A. Changnon, B.C. Reinke and R.W. Arritt, 1996: The July 1995 heat wave in the Midwest. Bull. Amer. Meteor. Soc. , 77, 1507-18.

(2) Karl, T.R. and R.W. Knight 1997: The Chicago heat wave: how likely is a recurrence? Bull. Amer. Metor. Soc.,78, 1107-19.

(3) Changnon, S.A., K.E. Kunkel and B.C. Reinke, 1996: Impacts and responses to the 1995 heat wave. Bull. Amer. Meteor. Soc., 77, 1497-1506.

 (4) Kalkstein, L.S., G.Tan and J.A. Skindlov, 1987: An evaluation of three clustering procedures for use in synoptic climatological classification. J. Climate Appl. Meteor., 26, 717-30.