AusGeoff,
The sunlight is hitting the same clouds perpendicular as well, as seen from below in the photo that was posted in this thread. The sunlight spreads far a wide obviously. That's why this whole topic of the sunlight illuminating clouds from below is not evidence of a sun disappearing below the horizon.
If you flash a bulb onto a ball in a white room then the whole ball will be illuminated, but not to the same degree of intensity as the side from which the source of light is coming from.
Conversely, a room painted black will have entirely different consequences.
The atmosphere, like a white room will reflect light but these distinctions are not important because the picture we are discussing have almost all sides of the clouds visible.
Maybe it wasn't clear but the illuminated side is the side with the most brightness, the side with the intense light.
That side can never be the bottom on a flat earth.
The original point that I made is that this particular side of the cloud cannot be both the bottom in one part of the world and then the top in some other part of the world if the Earth is flat.
But a round earth? No problem.