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Publication Date
27 November 2019

Hot, Humid Extremes Linked More to Moisture than Dry Heat

A study of ‘wet bulb’ events that combine extreme temperature and humidity points to the need for more research on heat waves amplified by humidity.
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The spatial distribution of China’s averaged daily maximum wet-bulb temperature (TW), in degrees Celsius, during May and September over the period 1960-2015.

In the face of extreme hot-weather events and their mounting death toll worldwide, it is important to know more about the confluence of contributing factors, which include anomalous high temperatures, local circulation patterns, high humidity, stagnant winds, remote sea-surface temperatures, and air pollution. Despite such complexity, extreme-heat events are largely analyzed by a single variable: extreme “dry-bulb” temperatures―that is, those that consider only a mercury reading. A recent study based on 55 years of data in China offers a way to analyze such extreme events in terms of both temperature and humidity. Such a “wet-bulb” temperature (TW) analysis, the authors say, provides an effective integrated measure for characterizing extreme heat events. The paper points to the significant role of moisture in extreme-heat events and to the need for more research into the role of moisture in heat stress.


Under very humid and hot conditions, the human body undergoes extreme stress that may lead to increased rates of discomfort, injury, and death. Understanding the origin of such conditions has immense societal value. This study, the first of its kind in China, explores the relative role of temperature and humidity in extreme wet-bulb events and spurs further research into how these factors may change the frequency and intensity of such life-threatening events in the future.


In a world increasingly beset by extreme-temperature events that result in social disruption and death, it is important to understand the factors that create and prolong such events beyond just the single variable of extreme dry-bulb temperature. In a recent paper, scientists at Nanjing University and the U.S. Department of Energy’s Pacific Northwest National Laboratory (PNNL) analyzed observations of temperature and humidity. They classified extreme wet-bulb temperature (TW) events into those dominated by temperature and those dominated by humidity.

The researchers explored differences in large-scale environments in northern and southern China. They found that extreme TW events dominated by temperature lasted longer than events dominated by humidity, but that moisture contributes significantly overall to extreme TW events. Their analysis of observations from 1,710 stations across China between 1960 and 2015 shows that specific humidity (q) contributes more than temperature (T) to extreme TWs, especially in northern and northwestern China. Based on the relative contributions of q and T, the researchers classified regional extreme TW events as q-dominated and T-dominated, respectively. They found important differences in large‐scale environments for q‐dominated and T‐dominated extremes in wet-bulb temperatures. Consistent with the contrasting environments, for instance, T‐dominated extreme events tend to last longer than q‐dominated extremes. (Convection during q-dominated events redistributes the heat and moisture and reduces wet-bulb temperatures near the surface.)

Using cluster analysis of global reanalysis data, the paper shows that extreme TWs are generally accompanied by increased surface air temperature and humidity, as well as anomalous high pressure and notable water vapor flux convergence. Given the significant role of moisture in extreme wet-bulb heat events, the researchers say more research is needed to understand the impacts of combined temperature and humidity on heat stress now and in the future.

Point of Contact
L. Ruby Leung
Pacific Northwest National Laboratory (PNNL)
Funding Program Area(s)