Made an account to confirm I, too, am having this issue. It appears to begin above the cloud layer.
CPU: AMD 4300 Quad Core 3.8GHZ
GPU: AMD Radeon R7 260, Catalyst 13.12 (14.x won't recognize I have two monitors, so I'm stuck with this driver and can't update)
Knew I should have gone Intel/nVidia, but money was tight at the time I had to build this PC :(
EDIT: Disabling triple buffering did nothing, also tried disabling overclocking (which I don't use, so that probably didn't do anything)
I realized I have this issue on the /r/hoggit server when they have the weather set to overcast, BUT everything is fine in singleplayer when there are few clouds. Odd.