The kanji character below is the Japanese word ame (rain) for those who want to practice their calligraphy.
There are various ways that rain can be depicted in sumi-e paintings. These three practice sketches illustrate some of them.
The first sketch shows a series of short senbyō-hō lines using only the tip of the brush. This is good for depicting light rain. For heavier rain, use more lines.
The second sketch is made using a hake (wide brush) and a kappitsu-hō (dry) stroke. Medium rain can be depicted this way.
The third sketch demonstrates heavy rain using a hake brush. Load about ⅓ of the length of the bristles with chūboku (medium) sumi. The strokes begin as junpitsu-hō (soggy) strokes and become kappitsu-hō strokes as the brush moves down the paper.
My demonstration painting should look familiar. I used Hiroshige's ukiyo-e from 12.1 as a model. The rain strokes look a lot smoother in my original. The scanner doesn't do it justice.