Defining Heatwaves

This Chapter considers the complexities of establishing a universal definition for heatwaves, highlighting the associated challenges and implications. It begins by contrasting two possible framings for defining heatwaves—the structuralist and phenomenological approaches—and outlines the criteria for a “good” heatwave definition. Also addressed is the distinction between heatwaves and warm spells. The chapter then describes the use of absolute and relative criteria for defining atmospheric heatwaves along with the type of variables typically used to define heatwaves. A distinction is made between hazard-based and impact-based heatwave definitions. Following this background material, an overview of heatwave definitions is presented organised under the four broad headings of National Meteorological and Hydrological Services (NMHS), research literature, international working groups and sector based. Unsurprisingly a vast range of definitions are revealed with varying degrees of precision and falsifiability. Setting aside the diversity of heatwave definitions, a general description of heatwave attributes (e.g., duration and intensity) is then presented along with a consideration of their interaction. Building on the idea that not all heatwaves are the same the chapter introduces the idea of heatwave form and presents an example of a heatwave typology for Tokyo, Japan. Further to atmospheric heatwaves, the chapter considers the case of defining marine heatwaves and their categorisation in relation to intensity. The chapter concludes by acknowledging the multifaceted nature of heatwaves and the practical challenges of implementing an interdisciplinary definition that incorporates both physical and societal dimensions.