While "alto" is not a cloud type itself, it's a prefix used to describe clouds in the middle layer of the atmosphere, between 6,500 and 20,000 feet (2,000 and 6,000 meters).

Here are some examples of alto clouds:

* Altostratus: A sheet-like cloud that often covers the entire sky.

* Altocumulus: White or gray patches of clouds arranged in rounded masses or sheets.

* Altostratus lenticularis: Lens-shaped clouds that form over mountains.

The highest cloud type is Cirrus, followed by Alto clouds, and then Stratus clouds.