Using quality-controlled observational daily temperatures in China, three independent types of heat waves, including daytime HWs, nighttime HWs and compound HWs are defined. A daytime (nighttime) HW consists of at least three consecutive hot days (nights) without any accompanying hot nights (days); while a compound HW contains at least three consecutive days with simultaneous extreme daily maxima and minima. Different types of HWs showed distinctive preferences in occurrence locations and timing. Compound HWs were frequently observed in the Yangtze River Valley. About half of compound HWs in northern parts of China occurred during early summer. Most of daytime HWs concentrated in northern and western parts of China, and were inclined to appear as dry events with low relative humidity. The nighttime HWs preferred to occur in South China and Southwest China. Such kind of HWs mainly covered peak-late summer, and were accompanied by sharply heightened moisture level.
Compound HWs occurred more frequently, with significant lengthening duration and increasing intensity in North China, the Yangtze River Valley, South China and Southwest China. Spatial extents covered by compound HWs expanded markedly during the past few decades, especially after 1995. Trend maps for daytime HWs showed quite contrasting patterns. Daytime HWs in central-eastern China experienced significant decreases in occurrence frequency, duration and intensity. The cooling of daily maxima and increasing transitions from daytime events to compound events jointly led to narrowing spatial coverage of independent daytime HWs. Notably, such increasing transitions were most striking in highly populated big cities. All the indicators of nighttime HWs showed significant positive trends across China. The spatial extent covered by nighttime HWs has been expanding since 1961, with a linear trend of 61.29×104 km2 decade-1. Regional relative humidity changes triggered an intriguing phenomenon that dry HWs became wetter and wet HWs became drier, regardless of HW definitions. However, the trends of relative humidity were substantially amplified during HWs (2-3 times larger than summer-mean humidity trend), especially during compound HWs.
The above comparisons highlight that increasing HWs have behaved more typical of nighttime-accentuated events, in particular form of nighttime events and compound events respectively. In particular, the compound HW is promising to become the dominant type in the future. Above thorough comparisons among diverse heat wave types would provide more accurate and better-targeted information for future adaption and mitigation in tackling heat-related consequences.