Weather clouds take on all sorts of shapes and sizes, but cloud icons tend to appear in just one form: four circles with a flat bottom. This simple design's most famous use is as Apple's iCloud image, but it also shows up in the icons of many cloud-based apps.
Many applications and services that use remote storage have very similar icons -- four circles connected by a flat bottom
Microsoft employee Scott Hanselman has collected a bunch of these similar-looking clouds and explains that part of what gives the icon its appeal is that the circles are sized in close approximation to the golden ratio, 1:1.6. Hanselman says that this cloud is available in several commercial icon packs, including one that carries the Creative Commons license. From there, designers can customize the cloud with their own colors, sizing, and additions, but the basic four-circles-and-a-flat-bottom image stays the same.