A: In one way or another, the creation of almost ALL the various effects in these images involved using stencils to lay many iterations of a single cycling texture down over itself in multiple color and/or value variations. The raindrop or snowflake that seems to be a dot or vertical dash moving across the background surrounded by transparency is actually just a dot or dash of noticeably contrasting color on a long ‘path’ filled with equally opaque proceeding and following colors carefully adjusted to ‘match’ the background colors they are crossing, and thus seem invisible. If you look closely, you should be able to see at least portions of some of these ‘apparently transparent’ trails in the rain and snow scenes. The actual process to produce these trails is simple but tedious to do, and describing it in detail here would bore you all to catatonia! So I won’t.
As for why you can’t find the raindrop trails or their palette gradients in the ‘sunny’ day version of the picture, that’s because they aren’t there! Occam’s razor
The waterfall image is one of those scenes designed to do a large array of different things over time, and palette space had to be used with even more strenuous economy than usual. Therefore, though I could have baked the rain lines into any or every version of the scene, and adjusted their palette space to disguise them during ‘clear daylight’ phases, just as all but the visible ‘drop or flake’ portion of the paths is disguised during the rain effect itself, it was just easier to clear both the paths and their unique palette space out whenever the scene wasn’t displaying ‘rainy’ conditions. The screen of raindrops, and the screens of snowflakes, are some of those few ‘overlays’ we baked into the image only when needed, using their rows of palette space for rainbows or stars at other times since none of those things would ever appear simultaneously with rain or snow. It would always be one or the other, so why bother building them all in together?