Characteristics of summer heat stress in China during 1979-2014: climatology and long-term trends

Heat stress describes the joint effects of high temperature and humidity. It poses severe impacts on public health and socioeconomic activities. However, its characteristics have not been fully investigated. Here we conduct a comprehensive examination of the climatological pattern and long-term trends of summer heat stress occurrences across China during 1979-2014. We find that China experienced severe heat stress conditions, particularly in densely populated and urbanized areas such as North China, the Yangtze River valley, and South China. Heat stress events in all these subregions are accompanied by decreased relative humidity (RH), and suppressed surface pressure and precipitation. Heat stress in China is mainly caused by high temperature, rather than high RH. Moreover, heat stress in most regions of China has increased noticeably during the past decades. Nationwide apparent temperature (AT, or heat index) and temperature (T) have increased by 0.37 and 0.35 degrees C decade(-1), respectively. The yearly frequencies of cautionary, extreme cautionary, and dangerous heat stress days have increased by 0.92, 0.67, and 0.03 days decade(-1), respectively. It is noticed that AT, T, and absolute humidity increase and RH decreases in most parts of China, suggesting that saturated water vapor pressure increases faster than actual water vapor pressure with rising temperature.