The most violent storm that can occur at a given point,
	a tornado is a rapidly rotating column of air extending
	from the base of a swelling cumulus or cumulonimbus
	cloud down to the ground. A funnel cloud is an incipient
	tornado that has not reached the ground. Most tornadoes
	have a funnel cloud phase before and after the tornado
	stage. Some funnels never become tornadoes. A tornado's
	narrow, rotating spiral of air is usually larger at cloud
	base than at the ground. The column is made visible by
	the condensation of water vapor, or by the presence of
	dust or debris raised from the ground. Winds on the 
	surface of the column are usually spiraling upward and
	can reach 250 mph (400 kph) in very small areas.
	Almost all Northern Hemisphere tornadoes spin 
	counterclockwise.

	The powerful updrafts inside a tornado can suspend
	tremendous loads of dirt and debris. When the tornado
	weakens or dissipates, the debris cloud may suddenly
	collapse toward the ground, spreading horizontally away
	from the dissipating tornado, much like a microburst.

	The largest and most dangerous tornadoes develop from
	parent mesocyclones (supercell thunderstorms). Both
	require the same conditions:moist,warm low-level air,dry
	air aloft,strong instability, a jet stream, and southerly
	winds at lower levels, westerly winds at upper levels.
	Powerful updrafts inside the supercell thunderstorm
	draw in slowly rotating air circulating around it,
	concentrating the spinning motion. As the updraft
	strengthens, the spinning increases, until the uprdraft
	becomes a narrow, rotating column. Large outbreaks
	of violent tornadoes usually occur east of upper-level
	travelling disturbances. Less intense tornadoes can occur
	near squall lines and gust fronts, beneath rapidly swelling
	cumulus and cumulonimbus, and within hurricanes.

	Non-supercell tornadoes, or land spouts, form when
	updrafts in a rapidly developing cumulus congestus or
	cumulonimbus cloud draw in slowly rotating low-level air.
	The preexisting slow rotation of the low level air can be
	caused by airflow around mountains or ridges or the 
	convergence of sea breezes or gust fronts. The rotation
	of many non-supercell tornadoes begins near the ground
	and grows upward.

	Most tornadoes move from southwest to northeast. But
	they may come from other directions, form a loop, or be
	stationary. Typical speeds are 35 mph (55kph), but some
	move at up to 70 mph (115 kph). Any tornado can cause
	significant damage. A typical tornado lifetime is less than
	15 minutes, but some have lasted up to 7 hours.

	Tornado intensities are rated with the Fujita-Pearson
	Scale. More than half of all tornadoes are rated as weak;
	their winds are typically below 110 mph (180 kph), their
	path lengths are shorter than 3 miles (5 km), and their
	path widths are narrower than 55 yards (50 m). Only 
	2 percent of tornadoes are rated as violent, but the winds
	of the storms can reach 250 mph (400 kph); their path
	lengths can exceed 300 miles (500 km), and their widths
	3 miles (5 km). Strong tornadoes attain values between
	these two categories.

	Over the past century, tornadoes in North America have
	killed between 15,000 and 20,000 people, and injured
	many more. During the 1980s, human loss in the U.S. and
	Canada averaged about 60 per year. Property loss from 
	tornadoes average at least $1 billion a year.

	
	The Great Plains of North America provide the most 
	favorable environment on Earth for the formation of 
	tornadoes. Seventy-five percent of the world's reported 
	tornadoes. (about 1,000 annually) occur in the U.S., and
	another 5 percent touch down in Canada. Most tornadoes
	occur east of the Rocky Mountains in spring. States along
	the gulf of Mexico start the tornado season in the spring.
	During April and May, the highest numbers of tornadoes
	occur in Texas, Oklahomam, Kansas, Nebraska, and the 
	Dakotas. In May through August, the Midwest, northern
	Great Plains, and Great Lakes have the most tornadoes.
	By late fall, the highest frequency is along the Gulf.
	Some hurricanes produce weak but numerous tornadoes
	in heavy rainsqualls at landfall. In most regions, tornadoes
	are most common during the warmest part of the day
	when surface air has heated to become most unstable.
	On the Plains and in the Midwest, most tornadoes occur
	during the late afternoon to early evening. In the Gulf states,
	they can occur during day or night.