Horizontal lines showing in volume scatter shader I am creating a landscape scene, and to get clouds I scaled a cube by a lot in x/y directions and set its material as follows:



A quick render resulted in the following pattern:



It is possible to see transparent horizontal lines in the clouds. Above image was rendered with 16 samples. Higher sample counts (like 4096 or more) resulted in smoother, but still visible lines. What could be causing this? Note: a workaround by translating the vector or changing the scale (even slightly) is possible, but I'd like to know the underlaying cause.

Blend file
Volumetrics shader are simulated by "overlaying several layers of thickness". These are called samples. From the Blender reference manual's page: 


  Step Size: Distance between volume shader samples when rendering the volume. Lower values give more accurate and detailed results but also increased render time.


Here's a default UV sphere showing the steps:



Your scene is quite big and deep, so you'll have to decrease the distance (increase the resolution) between these "planes" with the Step size property you can find in the Volume sampling panel in Render tab to hide this effect.

The purpose is to get a finer calculation of the procedural texture you are plugging in.