This sentence comes from the East Building of Xianyang City written by Xu Hun, a poet in the Tang Dynasty.
The whole poem is:
When I was in the high city, Wan Li was sad, and the willow was like Tingzhou.
when the clouds begin to sink, the rain is coming.
Birds fall in the green weeds, Qin Yuan Xi, cicadas sing in the yellow leaves and Han Gong Qiu.
Mo Wen was a pedestrian in those days, so the old country came to Wei River in the east.
"The clouds begin to sink in the morning, and the rain is coming." Describing a natural phenomenon, the heavy rain is coming, and the wind fills the castle. The poet looked at the scenery from the high-rise building and suddenly saw a cloud appear. At sunset, dusk arrived, and soon the sky changed. Suddenly, a gust of wind blew over the high-rise building, and a Shan Yu was coming. Yun Qi is sinking, full of rain and wind, and the movement is clear; "The wind is the head of the rain", which has profound implications. On the basis of "the course of facts", it is orderly and shows "the situation is pressing" to people naturally, so that people who are there must make their own choices: wait and see, head-on or retreat? "The rain is coming, and the events cast their shadows before them" is such an artistic conception, and it is a famous sentence that has been sung through the ages.
This sentence is a metaphor for the signs and atmosphere on the eve of a major change in the situation. Later generations often use it as a precursor to the urgency of political struggle or the eve of the outbreak of unexpected events, and it is also used to describe the tense atmosphere before the outbreak of conflict or war.