I was in one once. A towering Cumulus. A big dark one. Weather said
just rain, no thunderstorms. It started raining. Then I lost some
altitude. Looked up and my airplane was coated with ice! Clear, but
ragged ice, about 1/2" thick on all forward facing surfaces.
Fortunately I had warm VMC under me, so I descended and shed the ice. I
don't fly into dark Cumulus clouds anymore. Only reason I did that time
is I was pretty ignorant of weather. I was just happy to have a
clearance and be able to fly in actual. I was in and out of IMC. Here
comes a big dark one. In I went. Coulda been worse, coulda been hail....