- Altocumulus cloud sheet with a ribbed pattern of long billows
![Altocumulus cloud sheet with a ribbed pattern of long billows](http://www.skyartstock.com/peas/W943604Z-SKYART.jpg)
- W943604Z
- A sheet of Altocumulus clouds in the background is blocked by a passing lee wave train which forms a ribbed pattern of long, low billows. Where each crest lifts air locally, cloud thickens into a dark band, while it remains cloud-free between crests. There are signs that the wave crests have cloud at several levels within them.
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