When FlightGear switched from PLIB to OSG the 3D clouds were temporarily lost. Currently efforts are going on to re-implement the clouds. As of October 2008 the clouds are back and committed to FlightGear CVS, and released with FlightGear 1.9.0 that December.
Cloud shapes and layers
A 3D cloud layer is defined in the <layers> section. It contains of a number of randomly located <boxes>, containing a set of <clouds>.
Each cloud is generate with random size, based on the following tags:
<min-cloud-width-m> <max-cloud-width-m> <min-cloud-height-m> <max-cloud-height-m>
The cloud consists of <num-sprites> individual sprites. Each sprite is generated with random size, based on the following tags:
<min-sprite-width-m> <max-sprite-width-m> <min-sprite-height-m> <max-sprite-height-m>
The <texture> defines the texture file to be used for the sprite. The texture file may contain more than one texture in a grid, by setting the <num-textures-x> and <num-textures-y> tags.
Cloud are darker towards the bottom. bottom-shade indicates the shading factor of the bottom of the cloud compared with the top.