If towering cumulus is the big brother of the cumulus family, then cumulonimbus is the bad tempered big brother who’s had a really bad day. After the water vapor has been lifted to such a great height that vertical development begins to run out of steam, the tiny water droplets forming the brilliant white cloud start to coalesce into larger drops. These become heavy enough to fall, dragging some air down with them, though they might be lifted again, possibly cycling up and down many times, growing into large drops. If the temperature is low enough, layers of ice grow into hail stones.
The friction of the counter-moving air builds up an electric charge. At the same time, weakening uplift allows strong winds up there to blow the top of the cloud sideways. Sometimes the cloud continues to grow, resulting in several horizontal fans forming.Eventually, heavy precipitation, rain or hail, pours out of the cloud’s black bottom, bringing cold downdrafts with it, accompanied by spectacular lightning and thunder. The precipitation is why there’s a “nimbus” in the name.
Visions of frowning gods throwing lightning bolts.