This is a java mod, not new textures. There are more clouds being generated, all based on the existing cloud textures. Additional clouds can be a different type and height to original. My install is a modded BAT 4.2.
Stock at weather/clouds setting 'good':

Modded at weather/clouds setting 'good':

Stock at weather/clouds setting 'hazy':

Modded at weather/clouds setting 'hazy':

Note FRAPS FPS counter in last two images.
Stock:

Mod:

Stock:

Mod:

I need to test further and tidy up. All above images have three cloud re-generations. Different effects can be had with more/fewer re-generations and with varying the height of each. There is a FPS hit for each additional re-generation.
It would be nice to direct each re-generation to its own set of textures. Also to be able to access the random generation method, so that the clouds can be spread further around.
The additional clouds won't cause visibility checks or link to any other class that reference the default cloud object, although it should be possible to have the additional clouds incorporated into those methods.
"What were the skies like when you were young?"

Regards,
M