Green Comet

Cloud of the Day – Cumulonimbus

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.

rjb

Cloud of the Day – Towering Cumulus

Towering cumulus, also known as cumulus congestus, is what happens to fair weather cumulus if the atmosphere is more unstable. The stability of the atmosphere depends on several factors, such as the temperature gradient and the amount of water vapor in the air. When the temperature drops rapidly with height, the air is more inclined to boil up, and more water vapor means there’s more energy to help it. Where there is nothing to limit vertical development, rising air and moisture can build up into these robust towers. Identified by their substantial vertical size, which is often well over six thousand meters, and their patented “cauliflower” tops, these clouds are the big brothers in the cumulus family. Still no rain though, from towering cumulus. I know what Wikipedia says, but I think the definition of towering cumulus ends when precipitation starts.

rjb