Aerial photo shows a rainbow at the Hukou Waterfall scenic spot of the Yellow River at the border area between north China's Shanxi Province and northwest China's Shaanxi Province on Dec. 11, 2018. (Xinhua/Liu Hongda)

Aerial photo shows a rainbow at the Hukou Waterfall scenic spot of the Yellow River at the border area between north China's Shanxi Province and northwest China's Shaanxi Province on Dec. 11, 2018. (Xinhua/Liu Hongda)

Aerial photo shows a rainbow at the Hukou Waterfall scenic spot of the Yellow River at the border area between north China's Shanxi Province and northwest China's Shaanxi Province on Dec. 11, 2018. (Xinhua/Liu Hongda) 

Aerial photo shows a rainbow at the Hukou Waterfall scenic spot of the Yellow River at the border area between north China's Shanxi Province and northwest China's Shaanxi Province on Dec. 11, 2018. (Xinhua/Liu Hongda)

Aerial photo shows a rainbow at the Hukou Waterfall scenic spot of the Yellow River at the border area between north China's Shanxi Province and northwest China's Shaanxi Province on Dec. 11, 2018. (Xinhua/Liu Hongda)

Aerial photo shows a rainbow at the Hukou Waterfall scenic spot of the Yellow River at the border area between north China's Shanxi Province and northwest China's Shaanxi Province on Dec. 11, 2018. (Xinhua/Liu Hongda)

Aerial photo shows a rainbow at the Hukou Waterfall scenic spot of the Yellow River at the border area between north China's Shanxi Province and northwest China's Shaanxi Province on Dec. 11, 2018. (Xinhua/Liu Hongda)

Aerial photo shows a rainbow at the Hukou Waterfall scenic spot of the Yellow River at the border area between north China's Shanxi Province and northwest China's Shaanxi Province on Dec. 11, 2018. (Xinhua/Liu Hongda)