When he looked into a frozen puddle, he didn't see a cloud anymore. He saw four tiny paws, two coal-black eyes, and a wagging tail. He had become a puppy, but he was still as round and white as the sky he came from.
The image you're referring to, , is a popular internet photo of a small, fluffy puppy (often identified as a "Pomeranian") that looks like a round, white snowball with tiny black eyes and a nose.
One afternoon, during a particularly enthusiastic sneeze from a passing wind, Nimbus lost his grip on the atmosphere and tumbled downward. He didn't rain, and he didn't evaporate. Instead, he tumbled through the freezing mountain air until he landed with a soft poof in a patch of fresh snow.