Clouds in Earth's sky
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. One type of cloud was the cumulonimbus.
A cloud could also refer informally to a concentration of matter drawn together by gravity. For example, nebulae were sometimes known as interstellar dust clouds. (TOS: "The Immunity Syndrome")
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.
Fighter jets sometimes flew under the clouds. (TOS: "Tomorrow is Yesterday")
Watching clouds was a pastime enjoyed by some. In 2267, Spock and Leila Kalomi watched the clouds on Omicron Ceti III. He commented that one looked like a dragon. It was something he'd never taken the time to do before. (TOS: "This Side of Paradise")
Clouds on the holodeck
Clouds also appeared on other planets, for example, the planet housing the Vulcan monastery at P'Jem and Angel I. (ENT: "The Andorian Incident"; TNG: "Angel One")
Janeway watching the clouds
The holographic program having Natasha Yar speak to her friends following her death included several clouds (TNG: "Skin of Evil") as well as the holographic recreation of Café des Artistes in Paris on Earth. (TNG: "We'll Always Have Paris")
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")
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