wall of clouds; bank of clouds
Describes a horizontal stretch of clouds, often appearing as a wall or bank across the sky.
夕方、西の空に横雲が広がっていた。
In the evening, a bank of clouds spread across the western sky.
Compound of 横 (yoko, 'horizontal') and 雲 (kumo, 'cloud'). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning.