"Clouds and Rain" is a rather excellent metaphor for the vertical cyclical process of refining sexual energy.  Think of the (arbitrary) starting point as a pond of water: under the influence of light it ascends as steam, gathers in the sky as clouds, coalesces, and falls as drops of rain - back to the pond.  This process occurs within the vertical context of heaven and earth.

The alchemical process of refinement within a human being is quite similar.  We refine our lower denser-watery energies through the interaction with light, the energy (steam) rises, interacts with our lower-middle-upper aspects, then descends (rains) and coalesces in a more well-blended and pure state.  As in nature, this refinement cycle occurs continuously.  Nourishment includes all parts - our most subtle as well as most dense aspects: roots, trunk, branches, leaves.