Is there any more info on the weather system? You can create clouds with basic perlin noise which is not terribly computationally expensive, but it really looks like GG spent their time on algorithimic simulation the flocks and herds the bahvoirs etc.
Horizon Clouds tech is being created by the acclaimed FX artist who worked as a Senior FX Technical Director at Blue Sky Studios, where he developed the volumetrics and clouds pipelines for the Rio and Ice Age animated movies.
There was a thread about it in Neogaf talking about it:
http://www.neogaf.com/forum/showthread.php?t=1101497
In fact, here are some of the aspects of this cloud tech:
The clouds lighting uses accurate lighting based on Physically Based Rendering which involves all the aspects of light propagation:
and even one aspect that is not commonly used :
and enhanced by other physical effects and laws:
and finally rendered to match the conditions of atmosphere's elements like the sun's position and height of the clouds which results in different looks and colours:
All those with the different types of clouds in the atmosphere that can vary in height, density, shape and velocity which influence the density and the speed of the precipitations that can impact the gameplay which is heavily tied to weather and day/night cycle.
You can read further details by downloading the PPT file detailing The Real-time Volumetric Cloudscapes here:
http://advances.realtimerendering.com/s2015/index.html
but
Warning: the file is very very heavy and you need a hefty PC to run the videos in those slideshows.