Click to Release have created a 3D cloud generator, rendered with CSS using the 3D Transforms property.
The generator supports Google Chrome and Safari and Firefox, with a slight speed boost if using the Nightly build version of Firefox.
You can generate regular white and dark grey storm clouds. The tool even lets you generate “Michael Bay” style explosions seen below!
The team have published a tutorial on how to make the clouds yourself here.
Source: Click to Release