Comparative study of multiple heat indices in revisiting summer heat across China based on meteorological observations

Multiple indices have been created to measure hot conditions that may cause discomfort, stress even death to humans. However, distinctions among these indices and their performance in measuring heat remain undisclosed. We conduct a comparative study of multiple heat indices and revisit the spatiotemporal changes in summer heat across China. The maximum temperature-based index, more sensitive to average and maximum temperatures, suggests a larger increasing trend (0.42 degrees C/10a) in heat intensity than those average temperature-based ones which are more sensitive to minimum temperature. The absolute threshold-based heat-day indices are not so applicable as the relative ones in measuring the increasing heat days over the Tibetan Plateau. During 1960-2018, significant (p < 0.05) increasing trends in heat intensity (0.11-0.42 degrees C/10a) and heat day (0.63-2.67 days/10a) are revealed with a jump-like increase after the mid-1990s at the country level. Stronger heat intensity occurs over the southeast and north China with larger increasing trends over the Tibetan Plateau and northwest China. Northern China with larger increasing heat intensity and days should take effective measures of adaptation to reduce suffering from the summer heat. Given differences among multiple indices and the performance over different regions, a rational selection of heat index considering the research subject of interest and regional climatology is highly recommended.