Rainbows in clouds

When the sun is behind you and you look at some rainstorms, you see a rainbow. Since the droplets in a cloud are (I assume) spherical, why don't we see rainbows in clouds? My hypothesis is that they are there, but they are superimposed on a bright cloud where secondary scattering dominates, so the rainbow is just washed out by contrast. Recently I was on a flight around noon and could see below me the shadow of my plane and when the clouds became thin enough that primary scattering probably dominates, there was a perfect circular rainbow with the dark terrain below it. So why no rainbows from thin clouds when the viewer is not on a plane? My guess is the sky behind it is bright. But I am unsure. Another possibility is that what I saw was not a rainbow. But the ring's angular radius looked about right...