A sea of clouds is an overcast layer of stratocumulus clouds, as viewed from above, with a relatively uniform top which shows undulations of very different lengths resembling waves on the sea.

[2] In both cases, the phenomenon looks very similar to the open ocean.

The comparison is even more complete if some mountain peaks rise above the clouds, thus resembling islands.

A sea of clouds forms generally in valleys or over seas in very stable air mass conditions such as in a temperature inversion.

This is a common situation in a high-pressure area with cooling at the surface by radiative cooling at night in summer, or advection of cold air in winter or in a marine layer.