While monitoring the 40-meter band (among others) throughout a typical day/night as propagation changes I have noticed there are often bands of very low noise (black area) and others with much higher noise (some color) in the waterfall display. These are much too consistent, persistent, and wide bandwidth to be intentional signals of any type.
My two-part question is:
What causes those light and dark bands in the waterfall? (More specifically, what causes there to be background noise patterns like that. They are just easier to see on the waterfall obviously.)
If I was looking for a clear frequency transmit on and there was a spot in a dark band and one in a light band would one expect a better signal propagation in one over the other?
Hypothesis: My best guess is that this is something do with signals being absorbed, reflected, transmitted or not transmitted from a distance by or through one of the ionosphere layers. Still, I'm wondering if there is something more specific and how the phenomenon relates to the second part of this question.
Requested example screenshots: I know the WSJT-x "waterfall" is not technically the same as the waterfall display of an SDR but I don't have that hooked up right now. However, the visual effect is similar.