Cool.
But ehm...
It would be nice to have some more information.
PC specs?
Drivers?
Who else is suffering from it?
And don't you dare asking how to get your pc specs, there are hunderds of threads on this forum where poor Karyuu explained that time after time

And, it's also possible that this was a GM mistake.
If you set the fog density really thick and you change the RGB values to those of cyan, you would pretty much get that effect.
edit:
Okay, with the GM options "/rain here start" and "/fog here 1500 0 255 255 0" I managed to get the exact image you posted up there.