r.VolumetricRenderTarget.MinimumDistanceKmToEnableReprojection

This is the distance in kilometer at which the `cloud surface` must be before we enable reprojection of the previous frame data. One could start with a value of 4km. This helps hide reprojection issues due to imperfect approximation of cloud depth as a single front surface, especially visible when flying through the cloud layer. It is not perfect but will help in lots of cases. The problem when using this method: clouds will look noisier when closer to that distance.