Cumulonimbus clouds are rain clouds that form when warm moist air evaporates as it rises. The older, larger, and heavier they become, forces their shape to change into the shape described as cumulonimbus. Nimbus, the second half of this compound word means a dark appearing rain bearing cloud. To clarify, these rain clouds could be thought of an accumulation of dark rain bearing clouds.
Rain clouds are far larger than cumulus clouds and are taller and are vertical as opposed to lower lying horizontal fair weather clouds. In science fiction terminology, you could say cumulus clouds morph into cumulonimbus clouds. Going a step further with cumulonimbus cloud lore, sometimes simple observation make them appear as mansions in the sky. Especially this is so when there's just enough of dark to outline them which tends to emphasize their distinctive shapes. This changes within minutes however as right before our eyes the heavenly vision rearranges itself into other shapes larger, darker and thinner.
What's going on up there? Any number of weather-related phenomenal things are possible: Convective updrafts - large amounts of warm moist air - give the height to these rising cumulative clouds giving them shapes and making them - to imaginative earth born poets and writers - heavenly homes; Small amounts of ice crystals forming from parts of the cloud being pushed higher and into more frigid temperatures. When the weight of the cloud is more than the warm cool air can keep afloat, the bottom part of the cloud leaks. In other words rain drops fall.
Basically, what happens when cumulonimbus clouds forms, it rains. The fierceness or lack thereof depends on the severity or the force of the wind and the temperature gauge whether freezing or extremely hot, there can be rain, hail, snow or a combination of each. To make this easier to understand lets pause here and describe the atmosphere. While this is only the air surrounding us it too is much more complicated than that. Atmosphere is divided into five layers:
The first layer is the troposphere, the lowest level is at ground level where air and humans interact. The second layer is the stratosphere. Jets fly in this level and this where the much talked about ozone level calls home. The third level is the Mesosphere, an extremely hot area that melts rocks and meteor fragments. The fourth level is the thermosphere, the place where the space shuttle resides. The fifth level is the exosphere. This probably is the outer limits and not much is known about it other than it is extremely thin.
Cumulonimbus clouds or vertical growth clouds abound in the lower, middle and highest level of the troposphere and sometimes even invades the second level the stratosphere. Clouds form when moist warmed air - that always rises - or updraft as meteorologists speak of it, condenses. Warmer air beneath pushes it upward and this accumulation is what gives the fair weather clouds, cumulus clouds, its frothiness. An over accumulation of rain drops that is no longer held aloft by the warm air underneath, is forced to spill itself out.
Factors influencing this downpour are the seasons, thus the temperature of the high and low air, the height reached by the vertical cumulonimbus clouds and the resistant hot and cold air reactions. And as such, most of us can do nothing about the weather except talk about it. Like an overwrought child, when it has had too much of this or that explodes. Or in other words, it too can have temper tantrums. That in meteorological terms is called thunder and lightening. Venting is one thing humans and the atmosphere have in common, at various times, we need to cry, it, the weather rains.