The study analyzed rainwater samples in Chengdu to understand where the water came from. They found that rain in the dry season has different isotopes than in the wet season, showing different moisture sources. Rain in Chengdu mainly comes from marine air masses, with isotopes getting enriched along the way. The isotopes also show that raindrops evaporate before hitting the ground. The 17O-excess in rainwater can help understand evaporation conditions at the water source. Overall, the study provides insights into how rain forms in Chengdu.