From Muppet Wiki

Cloud is a visible mass of condensed droplets or ice crystals suspended in the atmosphere above the surface of the Earth. He is fluffy and sentimental, and speaks in a throaty whisper.

In Guy Smiley's "Here is Your Life" sketch on Sesame Street, Cloud pays tribute to Oak Tree, who had been watered by Cloud's rain. Oak Tree thanks Cloud and his partner the Sun, and Cloud chokes: "He remembered!"

The character also appeared in the book Grover and the Everything in the Whole Wide World Museum.