Heat waves in the United States: definitions, patterns and trends

High temperatures and heat waves are related but not synonymous concepts. Heat waves, generally understood to be periods of extremely warm conditions, are of critical interest to a wide range of researchers, health professionals, government agencies, industries, and emergency response experts because of the impacts that extreme heat events have on human health and activities. As such, spatial patterns and trends in heat wave occurrence are a topic of active investigation. However, perhaps because of the diversity of stakeholders engaged in heat wave monitoring and research, there is no single, standard definition of a heat wave event. For example, where climate change scientists might be most concerned with changing distributions of maximum summertime temperatures, health researchers often focus on apparent temperatures, which is more relevant to human health. Experts also differ in how they incorporate duration of warm conditions, relative versus absolute threshold values, and ancillary variables such as humidity and wind speed into their operational definition of a heat wave. While there is value in this diversity of perspectives, the lack of a unified heat wave index can lead to confusion when discussing patterns of heat wave occurrence and associated societal vulnerabilities. Here, we use data from the North American Land Data Assimilation System (NLDAS-2) to examine patterns and trends in 16 previously published heat wave indices for the period 1979-2011 across the continental United States. The Southeast, Great Plains and Midwest regions show the greatest number of heat wave days over this period. The range of average annual heat wave days was smaller for relative threshold definitions than the range for absolute threshold definitions. Positive temporal trends are found across more than 15% of the land area of the Southeast and Great Plains regions for the majority of the heat wave indices. Implications for heat wave studies in the context of climate change will be discussed.