Under the background of global warming, severe heatwaves occurred more frequently, which post threats to human health and cause tremendous economic loss. Despite a large number of studies, heatwaves still remain events that have not reached a universal definition. Existing studies generally separate hot days and hot nights artificially. However, they may actually co-occur or occur sequentially. A recent study takes an in-depth look at the structure of heatwaves, i.e., the configuration of hot days and hot nights during heatwaves.
The related article is titled “Changing structures of summertime heatwaves over China during 1961–2017”, which was recently published on SCIENCE CHINA Earth Sciences (2021) Vol. 8 as the cover article. The corresponding author of this article is Zhiyan ZUO, Professor of Fudan University. Based on the latest comprehensive observational data from the National Meteorological Information Center (NMIC) at the China Meteorological Administration, and satellite nighttime brightness index from the Defense Meteorological Satellite Program’s Operational Linescan System, they analyzed how heatwave structures changed over China during the past decades. Their work shows the relationships between hot days and hot nights, and revealed the influence of urbanization and meteorological conditions on heatwave structures.
From the view of epidemiology, hot days and hot nights may have different consequences. Hot days can cause heat stroke and can exacerbate the risk of ground-level ozone pollution, while hot nights can cause sleep deprivation and thermoregulation failure. A concurrent hot day and hot night may be more harmful to human health because human cannot get sufficient recovery from daytime heat at nighttime. Therefore, it is more precise and practical to separately quantify hot days and hot nights when describing heatwave activity.
There is not a one-size-fit-all definition of heatwaves. Factitiously extracting the heatwave periods based on different definitions may come to a different conclusion. Instead of analyzing the characteristics and changes of the frequency, duration, and intensity of heatwaves by a given definition, the present research focused more on the changes of heatwave structures.
This study found that the majority of summertime heatwaves were compound heatwaves that had both hot days and hot nights. During heatwave process, hot days usually occurred one day or a few days before hot nights, which was in accordance with the daily variations in radiation and meteorological conditions, such as the increasing surface humidity and cloud cover, and decreasing solar radiation. During 1961 to 2017, heatwaves became more prevalent at nighttime, and hot days were increasingly replaced by hot nights and concurrent hot days and hot nights (Figure). Urbanization strengthened the coupling between hot days and hot nights.
Knowledge of the past characteristics of heatwaves can help predict future changes. With the global temperature continue rising and urbanization accelerating, it can be predicted that compound heatwaves with more concurrent daytime-nighttime hot extremes are bound to increase in the future. Besides, this study provides instructions for researchers to customize and select appropriate heatwave indices.