Yaks are seen on the grassland in Qilian County, northwest China's Qinghai Province, Dec. 7, 2020. (Xinhua/Zhang Hongxiang)

Aerial photo taken on Dec. 7, 2020 shows scenery of snow-covered mountains in Qilian County, northwest China's Qinghai Province. (Xinhua/Zhang Hongxiang)

Aerial photo taken on Dec. 7, 2020 shows yaks on the grassland in Qilian County, northwest China's Qinghai Province. (Xinhua/Zhang Hongxiang)

Aerial photo taken on Dec. 10, 2020 shows the scenery of Heihe gorge in Qilian County, northwest China's Qinghai Province. (Xinhua/Zhang Hongxiang)

Aerial photo taken on Dec. 10, 2020 shows winter scenery of Qilian County, northwest China's Qinghai Province. (Xinhua/Zhang Hongxiang)