Plain English Meaning

Lens-shaped clouds that sit still in the sky over or downwind of mountains, even when the wind is strong. They mark where powerful waves of air are flowing over the terrain, and the air around them is usually very rough.

Full Definition

Smooth, lens-shaped clouds that form at the crests of mountain waves on the downwind side of a ridge or peak. They appear stationary because the cloud forms continuously as moist air rises and cools at the wave crest, and dissipates as the air descends and warms on the other side, even though strong winds are blowing through them. Their presence is a reliable visual indicator of mountain wave activity and severe turbulence.

Context Anchor

Seen in instrument and discussions, especially when flying near mountains or downwind of mountain ranges.