Heat mortality in Philadelphia, USA

Practical application is made in Philadelphia of what is known about the effects of high temperatures on mortality, by issuing warnings of when ‘oppressive’ conditions are likely (1). This is an advance on methods involving a heat index. The commonly used heat index includes only temperature and relative humidity. Even the most rigorous index, such as the standard effective temperature (Note 6.G) assumes that only temperature, humidity, wind speed and net radiation affect people’s reaction to heat waves, whereas heat-wave duration, night-time minima, and time-of-year are also important. For instance, a heat wave early in summer does more harm than one occurring later when people have become used to warmth. Likewise, the threshold for mortality differs between places, according to the degree of acclimatisation caused by normal temperatures. ‘Oppressive’ conditions are associated with particular airmasses. In places such as Florida and Darwin (Australia), oppressive conditions occur frequently and therefore they are not associated with significantly higher mortality rates. Where these are experienced only occasionally, as along the southeast Australian coast or in the northeast of the USA, the connection between their occurrence and mortality is especially strong.

The system adopted in Philadelphia is based on statistical work by Kalkstein et al (2), allowing categorisation of days by type of prevailing airmass. Days with an oppressive airmass (i.e. maritime tropical) have temperature maxima of 32° C, for instance, dewpoints of 21° C, medium cloud and light southwest winds. For such conditions, the records show a mean daily mortality of about 8.8 deaths above normal in summer in Philadelphia. Days like this occur about 3.5 times a month in summer.

The system worked well during the Chicago heat wave in 1995, 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) Kalkstein, L.S., P.F. Jamason, J.S. Greene, J. Libby and L. Robinson, 1996: The Philadelphia hot weather-health watch/warning system. Bull. Amer. Meteor. Soc., 77, 1519-28.

(2) 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.

(3) 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.

(4) 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.