During the winter of 2018/2019, Southeast China had a lot of heavy rain. Scientists studied the water vapor in the air to see where the rain was coming from. They found that the rain came from five main places: South China, East China Sea, South China Sea, Bay of Bengal, and Northwest China/Mongolia. The water vapor changed quickly during the rain, showing that the rain came from different places at different times. This switching between moisture sources is important for causing long periods of heavy rain in the winter.