An average thunderhead weighs about half a million kilograms
The towering storm cloud called cumulonimbus holds roughly 500,000 kilograms of water on average, and its top can climb to 12 kilometres, sometimes 20 or more. It is the largest cloud there is, the engine behind lightning, hail and tornadoes, and even a small one dwarfs every cloud around it.
The name blends Latin cumulus, heap, with nimbus, rain cloud. It begins as water vapour condensing low in the troposphere, carried upward by strong buoyant currents. Higher up the vapour freezes into ice crystals, snow and soft pellets called graupel, and collisions among these particles produce hail and the charge separation behind lightning. Cumulonimbus grows out of swollen cumulus congestus clouds and can mature into part of a supercell.
The base may span several kilometres or only a few tens of metres, forming anywhere from about 200 to 4,000 metres up. A mature cloud flattens into an anvil where it hits the stable layer near the tropopause, and the anvil can stretch many kilometres ahead of the storm. Air with enough momentum punches above that level as an overshooting top, a sign of severe weather. Forecasters name many features: bubbly mammatus pouches underneath, a cap-like pileus on top, and the tuba, a hanging column that can become a funnel or tornado, sometimes dropping to about 6 metres above the ground. Wildfires and volcanoes can build their own version, the flammagenitus, fed by heat from below.
Individual storm cells are short-lived. Most die in about 20 minutes, once falling rain drags down more air than the updraft lifts and the energy drains away. On hot, humid days, though, the gusts and moisture pouring out of one cell can spark another a few kilometres off within tens of minutes, or hundreds of kilometres away hours later, keeping storms going for hours or even days.
Beyond lightning, cumulonimbus can bring torrential rain, flash floods, straight-line winds and large hail. In winter it can produce thundersnow with intense snowfall and blizzard conditions. A downburst over dry ground can raise a wall of dust known as a haboob. The clouds are most common in the tropics and in moist mid-latitude summers.
Source: Cumulonimbus cloud
