You need of course a spectral atmophere for this. Metaclouds are not needed, but can be used just as the regular spectral cloud layer, but they ae less easy to animate, you will need to animate the material in the advanced material editor, and even move the spheres about for realistic results. The layer doeen't need to be close to the ground, it's really up to you, if you want to fly through fog near the ground, or clouds higher in the sky.