用“雨云”最简略.
Examples：
1.Rain clouds and thunder and lightning were believed to be the dragon’s breath,hence the fire-breathing monster.
雨云和电闪雷鸣据说是龙的呼吸,因此被视为喷着火的怪物.
2.Rain clouds are banking up.Hope they'll blow over.
雨云正在聚积起来.希望它们会散去