The ins and outs of measuring heat
Written by Dr. Luis E. Ortiz and Dr. Daniel J. Vecellio
Summer 2023 has been a scorcher throughout large swaths of the US for several weeks now, and forecasts show that this hot weather will reach Virginia later this week. The consequences of extreme heat can be dire for humans’ physical and mental health, while straining our energy and transportation infrastructure. As information about extreme heat saturates our news and social media, it’s important to understand just what they’re reporting on, and how it affects us. This post will focus on demystifying common measurements of how hot it is and what they actually tell us.
Photograph of a typical weather station temperature sensor with solar radiation shielding. Source: National Weather Service
Air temperature: Also known as dry-bulb temperature or even ambient air temperature, this is the most common measure of how hot it feels, and the one most people are used to seeing. Air temperature is what common thermometers tell us. Historical and current temperatures are often reported from NOAA instruments, which are highly standardized so that their measurements are comparable and not subject to common errors. For example, standardized measurements are often at 1.5 meters above the ground (almost 5 feet) and shielded from the sun’s radiation, which would otherwise heat the surface of the instrument. Air temperatures can stress our bodies, as humans must maintain a core temperature of around 98°F and a skin temperature of around 95°F. If ambient conditions are much warmer, our bodies will have to shed excess heat through sweating and other mechanisms.
Land surface temperature (LST): Also known as skin temperature or even just surface temperature, LST is a measure of how hot a surface is to the touch. This measure will depend on a variety of factors like how reflective to the sun’s radiation the surface is, as well as how sunny it is. LST is often derived from satellite imagery by combining thermal images of the earth’s surface. Because the Earth’s surface absorbs most incoming radiation from the sun, it heats up faster and can be much hotter than the air, especially in the afternoon.
Reports sometimes mistakenly use LST as a proxy for how hot it feels. Think of the difference between a hot paved street and the air on a hot summer day, or how hot a flat top electric stove can get compared to the temperature in a kitchen. During the recent heatwave in Spain, LST reached 140°F in some regions, however, unless laying on the ground, the people of Spain experienced temperatures more like 113°F.
Surface temperature map derived from the European Sentinel satellite mission over Spain. Surface temperatures in Spain reached over 60°C (140°F), although air temperatures peaked at around 45°C (113°F). Source: Copernicus
Wet bulb temperature: The wet bulb temperature is the temperature that a parcel of air would cool to at 100% relative humidity (saturation). Hence, the value of the wet bulb temperature is always less than or equal to the air temperature. The value of the wet bulb temperature can be calculated (given air temperature, humidity, and atmospheric pressure) or measured directly using a wet-bulb thermometer which is simply a thermometer covered by a wet piece of cloth with air flowing over it to allow for evaporation. The evaporation of water from the soaked cloth on the wet bulb thermometer is analogous to the evaporation of sweat from a person’s skin surface, the main mechanism by which humans thermoregulate. This is why wet bulb temperature has routinely been used as the indicator of the environmental limit to human tolerance of extreme heat. A 2010 study theorized that a wet bulb temperature of 35°C marked that upper limit while a 2022 study indicated that in young, healthy, yet unacclimatized individuals, the limit was closer to 31°C in warm, humid conditions, decreasing linearly in hotter and drier conditions.
Wet bulb globe temperature (WBGT): The WBGT was developed in the 1950s by the United States military as a way to measure heat stress in recruits undergoing basic training due to many falling ill due to heat exhaustion and other heat-related complications. The WBGT incorporates the entirety of the ambient environment’s influence on human thermal balance by including direct measurements of radiation (and indirect impacts of wind) into its derivation in addition to air temperature and humidity. Because of the incorporation of radiation into its derivation, it is better equipped to predict outdoor heat stress when compared to other indices such as the Heat Index. The National Weather Service began providing a public WBGT product in 2022, though, it is still not readily interpreted by the layperson. However, the WBGT has gained in popularity over the past few decades in specific fields with those who are trained to understand what its values mean. The WBGT is regularly used in workplaces to monitor the safety of laborers and is part of OSHA guidelines. It is also used across the sports world, such as soccer’s international governing body FIFA, for everything from scheduling summertime practices to determining water breaks in the World Cup.
When communicating climate impacts, it is important to use the proper measurement or index for the phenomenon being described. In using the wrong variable in the wrong context, even unintentionally, conclusions could be construed as climate disinformation just as climate denial talking points are. Using a map of LST instead of air temperature to describe conditions during a heatwave would be an unneeded and incorrect exaggeration of weather conditions felt by people (unless, perhaps, you are falling on pavements in Arizona). Likewise, even though they share many of the same words, wet bulb temperature and WBGT are not equivalent measures and describe different heat stress conditions. Sharing a map of WBGT values which are hovering near 35°C to say that those regions are tipping over the wet-bulb temperature human survivability limit is wrong and could de-sensitize people into not taking heat risks seriously. It is imperative to ensure proper dissemination of the science not only to ensure scientific credibility, but to accurately communicate risk and vulnerability to keep the public safe during these extreme events.
Authors
Dr. Luis E. Ortiz
Dr. Ortiz is a co-PI for the Virginia Climate Center and Assistant Professor in the Department of Atmospheric, Oceanic, and Earth Sciences at George Mason University
Dr. Daniel J. Vecellio
Dr. Vecellio is a Postdoctoral Researcher for the Virginia Climate Center
