曇 means 'cloudy'

A book title mentioned in the 霧 essay collectively refers to clouds, rain, and fog by citing their common "rain" radical. That unites the kanji in this bundle, as well. All four essays explore the realities of damp weather, as when fog grounds planes, frost damages crops, and people seek shelter from the elements. Because of such challenges, these characters make great metaphors. The Japanese associate 霜 with hardship; 曇 with life's ups and downs, as well as gloomy feelings; 霧 with mental fog; and 露 with transience. Because 露 symbolizes vanishing realities, that kanji in particular has inspired poetry.