Dementors, at least a hundred of them, gliding in a black mass around the lake towards them.

得蒙特，至少有一百多个，黑黑的一群在湖面上滑翔。

The blue sky was still visible, except directly overhead, where this black mass of cloud was sweeping swiftly northward.

蓝天依旧，只有头顶正上方那团乌云飞快地飘向北方。