Heat wave indices struggle to accurately measure severity

Heat wave indices struggle to accurately measure severity
by Simon Mansfield
Sydney, Australia (SPX) Aug 08, 2024

Despite the increasing frequency and severity of heat waves due to climate change, there is no universal method to measure heat-wave severity. Existing indices vary in their thresholds for defining dangerous heat-stress conditions. Researchers reported on August 7 in the journal Nexus that five out of six current heat-wave indices failed to capture the severity and spatial distribution of recent lethal heat waves in India, Spain, and the USA. The lethal heat-stress index was the exception, accurately identifying dangerous heat-stress conditions, particularly in low-humidity areas.

"We found that some existing indices may not be appropriate to all geographical regions and climate conditions," says senior author and geospatial expert Qihao Weng (@Qihao_Weng) of the Hong Kong Polytechnic University. "It's important that scientific communities, public health circles, and policy makers come together and reconsider the existing indices."

Countries worldwide use different methods to measure and define heat waves, but it's unclear which is most effective or how their performance is influenced by local climatic conditions. Some metrics rely solely on maximum air temperature, while others incorporate factors such as radiation, wind, and humidity.

"Even in relatively low-temperature conditions, if the humidity is high, it can still be dangerous for people with health problems," says Weng. "In humid conditions, even 28 C wet-bulb conditions can lead to severe strain and heat stroke."

To evaluate the effectiveness of existing heat-wave indices, researchers applied six indices to climate data from heat waves in Spain and the United States in 2022, and in India in 2023. They discovered that the lethal heat-stress index outperformed the others, effectively distinguishing between areas impacted by extreme heat stress and identifying the days on which these conditions occurred.

The lethal heat-stress index is a temperature- and humidity-based metric designed to identify conditions likely to cause human death. Unlike other indices, it applies a correction factor to relative humidity, making it more accurate in predicting dangerous heat conditions in low-humidity regions.

"The lethal heat stress index applies a correction factor to relative humidity, which means that it is better at predicting dangerous heat conditions in regions which have very low humid conditions compared to the other indices," says first author Pir Mohammad, an earth scientist at Hong Kong Polytechnic University.

The researchers note that heat waves affect people differently based on factors such as age, pre-existing health conditions, and socioeconomic status, which influences access to adequate cooling and the ability to avoid working outdoors in hot conditions.

While the lethal heat-stress index is currently the best option, the researchers believe it can be further improved with additional research. They also highlight that most heat-related mortalities occur indoors, and future studies should address indoor conditions during heat waves, considering factors like building age and materials.

Ultimately, the researchers aim to help develop a universal definition of dangerous heat conditions.

"We need to deploy a global framework that considers temperature, humidity, and other factors like socioeconomic status and age so that we can mitigate dangerous heat-wave conditions," says Mohammad. "The UK government recently redefined their limiting criteria for heat waves, and it might be a good time for other countries to also think about how we can define heat waves more effectively."

Research Report:Comparing Existing Heat Wave Indices in Identifying Dangerous Heat Wave Outdoor Conditions