Lenticular clouds

Smooth lens- or almond-shaped clouds that stand still over hills while the wind blows through them.

Latinlens-shapedLevelMiddle, sometimes highBase2,000–7,000 m

What it looks like

Smooth, lens- or almond-shaped clouds, sometimes stacked like a pile of plates, hanging over or downwind of a ridge. Air rising over the hill cools and condenses at the crest of a wave; on the way down it warms and the cloud evaporates, so the cloud stays put while the air passes through.

What it means

Strong wind aloft and a stable layer. Glider pilots love them; walkers should expect a rough, cold day on the tops. They can last for hours and account for a fair share of historical flying saucer photographs.

Not to be confused with

Ordinary altocumulus, which drifts with the wind, and cap clouds, which sit on a summit rather than hovering downwind of it.

Field note. Look on the lee side of any long ridge on a windy but otherwise clear day, especially around sunrise.

Seen one? Add it to your log or check with the identifier.