What does the white rain mean when the black clouds roll over the mountains without covering the mountains, and the white rain jumps into the boat?

It comes from "Drunk Calligraphy at Wanghu Tower on June 27th Part 1" by Su Shi, a famous writer and calligrapher in the Northern Song Dynasty.

Original text

Part 1

Black clouds rolled over the mountains without covering the mountains, and white raindrops jumped into the boat.

The ground wind blows away suddenly, and the water under Wanghu Tower is like the sky.

White rain: refers to the special landscape of summer showers. Because the raindrops are large and fierce, they appear white and transparent against the backdrop of the lakes and mountains