Smoke looks too thin when it is on the small square plane. Here is upside-down view and Flow object (small square) settings:
And this is side view and Domain object settings:
Why is it so, and how can I make the smoke look better?
I found that the following parameters influence on smoke's density above other objects:
And of course the light must be good.
Here is what I've got with Density 30, Light Clamping 10, HighRes turned on, and Sun light energy 10: