because, it seems, it is exactly analogous to teh speed of sound, as I mentioned in changing the header
> If light can be slowed when passing through a transparent > medium; what can't it be given a boost by a moving source?
THUS: clouds redistribute the light, all according to the least-time principle (brachistochrone problem of Liebniz et al, and a particular generalization, with its own prefix), but also in accord with the most basic part of "conduction" in the broadest sense of electromagnetic radiation, where one considers the specific density & specific heat of every object in Universe
without water, the surface would get hotter & colder diurnally, just like Moon extremes (Al Gore's "dark side," Apollonauts dayside).
also, clouds waveguide an enormous amount of energy, viz blazing sunsights & sunclipses (-set and -rise), especially mod the arctic circles. I'd suppose that this is "even more so" with infrared, but I have forgotten whether water vapor's huge absorption spectrum includes much of that.