6.1 Experimental procedures
Based on the meteorological and environmental characteristics of Dazu Rock Carvings (Chen et al., 2004; Zhang, 2015), the highest and lowest monthly mean air temperatures for 1975-2011 were 39.5℃ (in August) and -3.4℃ (in December), respectively. The mean annual relative humidity was about 83%. The maximum and minimum monthly mean rainfalls were 490.7 mm (in July) and 54.5 mm (in December), respectively. The wet days ranged from 147 to 174 days, accounting for 2/5 to 1/2 of the days in a year. Besides, the proportion of acidic rainfall with pH<5.6 was 77.0 %, where almost of the rainfalls were acidic. In a sense, water is the main cause of sandstone weathering in Niche of Sakyamuni Entering Nirvana. For this reason, the study site might have exhibited three kinds of weathering processes: dry-wet cycle (DW), freeze-thaw cycle and acid rain cycle.