The severity of heat and cold waves amplified by high relative humidity in humid subtropical basins: a case study in the Gan River Basin, China

High relative humidity (RH) and temperature extremes can occur simultaneously and persist for periods of time, resulting in serious harm to human health, especially in humid regions. Here, the Gan River Basin was used as a case study to investigate the characteristics of heat and cold waves using the apparent temperature (combined air temperature and RH) of the observed and ERA5 reanalysis datasets. Heat waves showed an increasing trend from 1961 to 2018 (particularly from 1997 to 2018), whereas during the same period, cold waves showed a significant decreasing trend. In general, ERA5 reasonably reflected the spatiotemporal characteristics of heat and cold waves, but the ability to simulate cold waves was greater than that of heat waves. The amplifying effect of high RH on heat waves is significantly greater than on cold waves. The increasing rates of heat waves in ERA5 at the three grades other than extreme heat waves were slightly greater than the observations. Cold waves at various grades showed significant downward trends, and the decreasing rates of cold wave in observations were slightly greater than those of ERA5. Using an analysis of the return period (occurrence probability), traditional univariate risk assessment methods based on maximum or minimum temperature may substantially underestimate the risk of extreme events, such as the 2014 heat wave or the 1969 cold wave, because the effects of RH were ignored.