Two important visible features of a cumulonimbus cloud are
	the icy top and the anvil. When a cumulus has a fuzzy and
	fibrous top instead of a hard outline-indicating that ice
	is present-a cumulonimbus has formed. A cloud also
	qualifies as a cumulonimbus when part of the top is flat like
	an anvil or has an overhang above clear sky. Sometimes the
	anvil blows a long way downwind at cirrus levels; detached
	anvils often give rise to cirrus clouds in subtropical and
	tropical regions and during warmer months elsewhere.

	A cloud with an anvil is always a cumulonimbus. The anvil 
	shows that the cumulus has stopped growing at a high
	level, usually above 30,000' (9,000 m), because of strong
	stability-warming or drying-above the anvil's top.
	Anvils may blow off for a long distance and become
	detached or much larger than the parent cumulus cloud.
	Sometimes the anvil may partially detach or be the only 
	part of the cumulonimbus that remains. A full cumulonimbus
	and anvil structure may also be formed in a cold winter
	environment. Some cirrus clouds formed by anvils are
	thick enough to be gray on the side away from the Sun.
	

	A pileus is a small horizontal accessory cloud-at a high
	level and quick changing in form-that often occurs
	above a cumulus or cumulonimbus; it may appear as a hood
	that is draped across the top of the cloud tower beneath it.
	A pileus is formed when a relatively shallow layer of moist
	air is lifted by a strong updraft and cooled below it's dew
	point. Most updrafts that are strong enough to produce a 
	pileus cloud will also result in a cumulonimbus. A pileus
	may become draped over a cumulus  cloud as the latter
	grows to the cumulus congestus or cumulonimbus stage. 

	Called CB in the aviation and Musicological communities
	and abbreviated as such on weather maps, cumulonimbus
	are very important clouds to monitor whenever there is a 
	good supply of low-level moisture and strong upward
	forcing of the air. The earlier in the day that a
	cumulonimbus forms, or the more organized and vigorous 
	that it appears, the more likely it is to become significant
	in terms of storms and weather.

	When cumulonimbus becomes arranged in rows or
	complexes, the likelihood of severe weather increases.
	Severe weather that can accompany cumulonimbus
	includes tornadoes, waterspouts, and funnel clouds; brief
	to prolonged heavy rain, hail, sleet, snow, and flash floods;
	strong winds and turbulence at any level of the
	atmosphere, including gust fronts, squall lines, and
	microbursts; and lightning from cloud to ground, within
	clouds, and between clouds.

	These clouds are much more common during warm
	months. In the eastern U.S., cumulonimbus with low bases
	form most often in the spring and summer. In humid areas,
	adequate moisture and instability aloft-the requirements
	for the formation of cumulonimbus -often occur near
	cold fronts and large-scale traveling systems. Over the
	larger mountain ranges of the dry regions in the western
	U.S. and Canada, summer cumulonimbus clouds occur
	with strong afternoon heating over or along mountain
	slopes, more vigorously when there is moisture aloft.	

	Along the west coast, dry and sinking air aloft inhibits
	most cumulonimbus growth, even though surface air
	may be humid. Cumulonimbus with any anvil may
	also form during cold weather in areas near water, for 
	example, in the lee of the Great Lakes (that is, downwind
	from the lake); such clouds often occur in lines and
	produce heavy snow.