Cumulonimbus are isolated to highly organized clouds
	made up of water droplets in their lower portions and ice
	particles in their upper portions; they have dark bases.
	Precipitation always falls from cumulonimbus clouds, and
	severe weather most often occurs with them; if there is 
	lightning or thunder; the cloud is considered to be a
	cumulonimbus. In arid regions, the rain or snow may 
	evaporate before it reaches the ground; this phenomenon,
	called virga, is often accompanied by gusty winds.

	A cumulonimbus is formed when rising currents of warm
	air cool to saturation; water vapor in the currents condenses
	into droplets, forming a small cumulus. Continued rising
	warm air currents create a swelling cumulus and, finally, a 
	cumulonimbus. For this stage to be reached, atmosphere 
	must be very unstable in a deep layer. Cumulonimbus
	clouds are deep enough that cloud bases often are not
	visible at a distance when the rest of the cloud can be
	seen. Bases of cumulonimbus range from about 2,000' to
	4,000' (600-1,200 m) in humid regions to as high as
	8,000' to 15,000' (2,400-4,500 m) in dry regions.