The prefix for clouds at middle altitudes (2,000 to 6,000 meters or 6,500 to 20,000 feet) is alto.

Here are some examples of alto clouds:

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

* Altocumulus: Patches of white or gray clouds that appear as rounded masses.

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

* Altocumulus castellanus: Towering cumulus clouds that resemble castles.