Clouds through the windows of Janeway's ready room in 2371
A cloud was a visible mass of droplets or frozen crystals floating in the atmosphere above the surface of a planet or other astronomical body. The most common form of clouds on M-class planets were made of water vapor. Low-level clouds very near or at a planet's surface were called fog.
A cloud could also refer informally to a concentration of matter drawn together by gravity. For example, nebulae were sometimes known as interstellar clouds. {{incite
A common consequence of water vapor clouds on planets is the meteorological phenomenon known as rain. Storms, lightning, and thunder are also associated with clouds.
Clouds also appeared on other planets, for example Angel I. (TNG: "Angel One")
In 2371 after the USS Voyager landed on a planet in the Delta Quadrant, Captain Kathryn Janeway watched the clouds through the windows of her ready room. (VOY: "The 37's")