Development and analysis of a daily heat stress classification for the central United States

Heat stress on humans and livestock is a significant natural hazard in the central United States. NOAA compilations of mortality statistics related to meteorological hazards document the relative importance of heat stress and heat waves in comparison with more commonly recognized hazards, such as hurricanes, tornadoes, and flash flooding. Weather patterns that are associated with individual days of elevated heat stress tend to persist for several days to more than a week during the summer season. Examples of applied climatic aspects for these events have been discussed for heat waves that impacted mid-western US cities in 1966, 1980, 1995, and 1999.

While NWS forecast offices have threshold criteria, such as an apparent temperature exceeding 105 F, for communicating when afternoon temperatures are likely to be a concern, there is currently no available classification of heat stress events that can be applied on a daily basis to assist in communicating the threat level to the general public. Given the magnitude of heat related fatalities in the US, we feel that a classification of daily heat stress events merits development.

Following on a three-year study examining the long-term climatology of livestock-based heat waves and heat stress events in Missouri, Iowa, Nebraska, and Kansas (the MINK region), we have developed and analyzed a five-category heat stress classification system that is designed to be applied on a daily basis. Since livestock producers and haulers are accustomed to the temperature-humidity index (THI), our classification and analyses to-date have used THI as a measure of stressful environmental conditions. Based in part on a 1999 classification of livestock heat waves, we incorporated (into our five category classification system) two measures of the accumulated heat stress over a twenty-four hour period and a measure of whether or not nighttime temperatures cool enough to provide some relief from the stress. The diurnal measures of heat stress utilize the energy accumulation concept associated with the growing degree day calculation, but instead accumulate the amount of heat stress above a selected threshold (such as THI = 79, or a heat index of 90). Nocturnal relief is based on a count of the number of hours when temperatures are below a selected THI or heat index threshold (e.g., THI = 70).

In this paper, we present the results from applying a five category (slight, mild, moderate, severe, and extreme) daily heat stress classification to a number of weather stations from across the MINK region. As was desired, the number of events that are classified into the more stressful categories are much fewer in number than those in the slight and mild classes. For example, in Topeka, Kansas, only two days in the 53-year period analyzed (1948-2000), July 13, 1954 and July 12, 1980 were classified as extreme events. Additional results for Topeka indicate an average of 29.6 heat stress days per year, with a range from 62 days (1980) to 9 days (1950). The frequency and relative magnitude of daily heat stress events declines toward the west and north in the MINK region.