The Physical Nature of Heat and Its Measurement

Heat is the defining physical entity of heatwaves. Accordingly, this chapter provides an overview of the physical nature of heat and its measurement. It begins by establishing the relationship between energy and heat, describing heat as a form of kinetic energy. The concepts of the surface radiation and energy balances are introduced, highlighting the various radiative, turbulent, and conductive heat fluxes that contribute to understanding heatwave processes. The chapter then examines the mechanisms of atmospheric heating, drawing a distinction between adiabatic processes and diabatic processes. Significant attention is given to the measurement and monitoring of heat, which is crucial for assessing heat stress levels during heatwaves. Direct measurements using temperature sensors and indirect methods like thermal mapping are discussed for infrastructure applications. For human health, the chapter highlights the complexity of heat stress, which arises from synergistic effects of temperature, humidity, radiation, and other factors. Because the quantification of heat stress is a priority for heatwave related studies focused on exposure and impact relationships, an overview of thermal climate indices used to model heat stress is presented noting that these range from simple empirical indices to complex rational models that solve the human heat balance equation. The strengths and limitations of some of the more commonly used thermal climate indices is discussed with the importance of selecting appropriate indices based on the research problem or application at hand emphasised.