Decreasing atmospheric visibility associated with weakening winds from 1980 to 2017 over China

This paper presents the climatology and changes in atmospheric visibility (AV) in China during 1980–2017 based on adjusted data from an extended station network. The relationships between AV and the wind speed (WS) from the near surface to the troposphere (850 hPa to 200 hPa) are analysed. The mean visibility at 582 qualified stations in China is 22.03 km, with the highest in summer (25.37 km) and lowest in winter (18.92 km). The annual mean AV has decreased at most stations and all seasons across China since the 1980s. Most regions show a decreasing AV trend mainly before 2005 and an increasing trend in recent years, possibly related to the emission reduction policies in China. The AV has a significant positive correlation with the near-surface WS in all seasons, with the largest correlation coefficient of 0.84 in summer and the smallest in winter. The AV also has a positive correlation with the WS in the lower troposphere in most of China, except in some inland desert regions. Additionally, the long-term trends of WS, surface temperature (T), relative humidity (RH) and surface pressure (Ps) were analysed at each station across China. The results show that both decreasing WS and increasing T could well account for the decreasing trend in AV. The changes in RH and Ps are at least not the main drivers. Overall, the “best” (“worst”) visibility corresponds to the largest (smallest) WS near the ground and in the lower troposphere, but such correspondence tends to overturn towards the upper troposphere.