Water uses it for animation. I don't know if other climates would animate if given other frames, or if it's just a vestigial consequence of sharing the same data structure.
That was my first though too, but the water animation look like this:Obj=ground
# hangtyp * animation
Here the second index is also 0.
They wouldn't animate - it is indeed vestigal. Minimising repetitive patterns would be possible with a small amount of code, quite a large increase in number of stored images and a reasonable way of generating a fixed number to use for each tile.
Yes I tried to add more Climate textures but nothing happened. Makeobj did add them to the PAK file so I guess it would be quite "simple" to have the world generator to pick one randomly.
Unfortunately I have no experience of the code beyond GUI stuff...
The writer calls the first index hangtype
and the second for phase
(if I got the right writer "ground_writer.cc").
Hangtype spans from 0 to 127 while phase spans the range from 0 to a signed int.