Clara finds the Doctor's TARDIS upon a cloud. (TV: The Snowmen)
A cloud was a collection of moisture in the atmosphere. They were the source of rain.
The Ethereals were turned into living clouds by the Baggolts. The First Doctor, finding it odd that the clouds could exist in a thin atmosphere and move despite there being no wind, tried to take a sample of one, but it captured him instead. (PROSE: The Cloud Exiles)
One day whilst in his village's market, Moop noticed that the clouds were making strange shapes until a face appeared in them. It claimed to be the creator of Moop's people and asked for a tribute. Moop gave the "god" some of his Grunt. (PROSE: The Tomorrow Windows)
After losing his companions and parents-in-law, Rory Williams and Amy Pond, the Eleventh Doctor parked his TARDIS in or before 1892, atop a cloud, high above London. A staircase provided the Doctor a means of inconspicuous entry access to and egress from the surface, while assuring him solitude. (TV: The Snowmen)
In the early 21st century, flying Cybermen led by Missy took to the skies and created clouds of cyber-pollen above Earth's graveyards. These clouds then rained upon the graves, recruiting the dead into a new Cyberman army. When control of the Cyberman army went to the hands of the recently-converted Danny Pink, he ordered all Cybermen to return to the clouds and self-destruct, destroying themselves and the clouds. (TV: Death in Heaven)
After "the company" had constructed a second sun, Kromon's clouds dispersed. (AUDIO: The Creed of the Kromon)
