Rain and clouds sample , not so photo realistic , see Pilatus Railway demo
https://cesium.com/blog/2017/10/30/its-scary-what-you-can-build-with-cesium/
click the 'rain effect' checkbox.
One way to draw clouds in Cesium is to implement vertex and fragment shader.
see below.
https://cesium.com/blog/2015/05/15/graphics-tech-in-cesium-architecture/
https://github.com/AnalyticalGraphicsInc/cesium/wiki/Fabric
Rendering clouds normaly use 'Perlin noise' algorithm.
see below
http://george-geng.github.io/abovetheclouds.html
Thanks.