Decoding Popular Heat Index Metrics
Heat metrics are a useful way of understanding how we feel when it's hot outside. They help us assess whether we are comfortable with the heat and if we are experiencing any problems with heat stress in different regions and environments. This article provides a quick look at several of the most common heat index metrics used around the globe. We will examine their parameters, how they're used, and their practical benefits.
1. Heat Index (HI), created by Lans P. Rothfusz in 1990, combines air temperature and relative humidity to estimate how hot it feels, known as apparent temperature. It shows how the temperature feels compared to the actual temperature, especially in high-humidity conditions where perspiration does not evaporate effectively and cool the skin. It helps to determine the risk of heat-related illnesses and guides public health advisories during hot weather.
2. Wet Bulb Globe Temperature (WBGT) is a comprehensive index that takes into account air temperature, humidity, air movement (natural wind or artificial sources such as ventilation) and, what is really important, radiant heat which might be direct sunlight or sources such as furnaces or any heating systems. This metric is usually measured by a specific device called a WBGT meter, or if it's not available, it can be calculated using weather data. It's widely applicable in professional environments, the military, and sports, and it evaluates heat stress more accurately, considering exposure to radiant heat, and influences safety protocols in outdoor activities and industrial workplaces.
3. The Climate Vulnerability Index (CVI) is a tool designed to show how climate change might impact different regions. It helps policymakers decide where to focus resources and efforts to protect vulnerable areas. The CVI considers several factors. First, it looks at exposure, which means how likely an area is to face climate change effects like more frequent heat waves, stronger storms, or rising sea levels. Next, it evaluates sensitivity, which shows how easily a place could be damaged by these changes. Finally, it assesses adaptive capacity, which is about how well an area can cope with or recover from climate impacts, considering things like financial resources, technology, and social systems. By combining all this information, the CVI gives each place a score. A high score means the area is more vulnerable to climate change, while a low score indicates it is better equipped to handle the impacts.
4. The Discomfort Index (also known as the temperature-humidity index) is a way to measure how uncomfortable the weather feels to people. It uses two types of temperatures: dry-bulb and wet-bulb. Dry-bulb temperature is what you usually think of as the air temperature. It's what a regular thermometer measures. Wet-bulb temperature is a bit different. It considers both temperature and humidity. To measure it, you wrap a wet cloth around the thermometer bulb and measure the temperature as the water evaporates. The wetter the air, the less the water evaporates, and the closer the wet-bulb temperature is to the dry-bulb temperature. The Discomfort Index combines these temperatures to show how hot and sticky the weather feels. When it's really hot and humid, it feels much worse. The index can vary depending on where it’s calculated, especially between urban and rural areas, which can lead to different levels of discomfort.
5. Apparent temperature (AT) takes into account temperature, humidity, and wind speed. It estimates what the temperature feels like to a clothed adult. The Australian Bureau of Meteorology uses it to issue heatwave warnings and inform the public about what to do during extreme heat events.
6. Humidex considers both heat and humidity to describe how people feel when it's hot. Used by Canadian and European meteorologists to indicate how hot the weather feels to the average person, ranging from mild discomfort to dangerous conditions. This helps public health efforts and influences people's behavior during heatwaves.
7. Physiologically Equivalent Temperature (PET) stands for "perceived equivalent temperature", which is defined as the air temperature at which the body's heat balance is maintained, considering typical indoor conditions. (without wind and solar radiation). It helps to manage heat stress risk in various outdoor activities and urban planning, providing insights into heat-related health risks and ways to mitigate them.
This is just a brief overview of globally applicable indexes. As you can see, some of them are quite similar in meaning, differing primarily in their calculation methods. However, they all help assess and manage heat stress across various geographical and environmental contexts. From guiding public health interventions to influencing urban planning and workplace safety protocols, these indexes significantly contribute to enhancing resilience against heat-related challenges. Understanding and utilizing these metrics empowers policymakers, health professionals, and the public to better prepare for and mitigate the impacts of extreme heat events, fostering climate resilience and public health safety.
Want to stay safe in the heat? Download the Body Temperature App today and use our widget and monitor the heat index based on your location to safeguard yourself against heat-related conditions that could pose health risks.
Sources:
Marina I. Stathopoulou, Costas Cartalis, Iphigenia Keramitsoglou, Matheos Santamouris, "Thermal remote sensing of Thom’s discomfort index (DI): comparison with in-situ measurements," Proc. SPIE 5983, Remote Sensing for Environmental Monitoring, GIS Applications, and Geology V, 59830K (29 October 2005); [https://doi.org/10.1117/12.627541](https://doi.org/10.1117/12.627541)
Saud Ghani, Ahmed Osama Mahgoub, Foteini Bakochristou, Esmail A. ElBialy, "Assessment of thermal comfort indices in an open air-conditioned stadium in hot and arid environment",Journal of Building Engineering, Volume 40, 2021, 102378, ISSN 2352-7102, https://doi.org/10.1016/j.jobe.2021.102378.
Höppe P. The physiological equivalent temperature - a universal index for the biometeorological assessment of the thermal environment. _Int J Biometeorol_. 1999;43(2):71-75. doi:10.1007/s004840050118
