A tornado in the US has claimed 9 lives as it ripped through Greensburg, Kansas on Friday (4th May 2007). Around 90% of the small rural town of Greensburg has been destroyed. Greensburg, is in Tornado Alley, the central part of the US which is frequently hit by tornadoes. Stunning video footage of the tornado (courtesey of Sky News http://news.sky.com) can be seen here. The BBC News article can be found here.
9 people were killed just under two weeks ago when a strong tornado struck a small Texan community on the US border before passing into Mexico.
Find out more about these deadly natural phenomenum in this Tornadoes special from CNN with quizzes, photos and tornado facts and figures.
Stunning images of tornadoes can be found in the NOAA Tornadoes Photo library.
Tornado Formation
So how do Tornadoes form? This excellent animation from CNN shows the processes involved in the formation of a Tornado. The movement of air within a tornado is visualized by scientists in this animation.
The most severe "supercell" thunderstorms which often produce tornadoes from in unstable atmospheric conditions where cold dry polar air meets warm moist tropical air (ref). Tornado Alley in Central America is where some of the most devastating tornadoes occur in the US. The area includes parts of Texas, Oklahoma, Kansas and Nebraska. The area, broadly speaking is between the Rocky and Appalachian Mountains; the flat nature of the Great Plains of Tornado alley results in cold dry polar funnelled south from Canada meeting warm moist tropical air which is travelling north from the Gulf of Mexico (ref).
Tornadoes mostly form in the northern hemisphere between March to June when the conditions for the development of severe thunderstorms are optimum (ref). "A tornado is a violently rotating column of air in contact with and extending between a cloud (often a thunderstorm cloud) and the surface of the earth" (ref). The exact process of tornado formation is debated but the deadliest tornadoes results from supercell tornadoes (long-lived rotating thunderstorms which last several hours) and begin with air rising from the ground into the bottom of the thunder cloud. Changing wind speeds and wind directions result in the formation of a spinning vortex. This spinning updraft of warm air stabalises and feeds the thunderstorm (hence the long-lived characteristics of the supercell). Next a strong downdraft of cooler dense air forms with increasing speed bringing the rotating vortex downwards. As it reaches the ground, some of the downdraft spreads, as it does so a visible condensation funnel forms, when the downdraft reaches the ground, the funnel cloud becomes a tornado. As the updraft of warm moist air continues it feeds the tornado and it grows until it reaches the mature stage at which point it lasts from between a few mins to over an hour. Gradually the tornado weakens as it's air supply is reduced by the cold downdraft which wraps around the funnel (ref).
Find out what it is like inside a tornado through this amazing footage on the National Geographic site - a probe fitted with 7 video cameras captures footage from inside the tornado! Find out more about how this is done through an interview with the storm chaser.
See also this excellent flash from National Geographic looking at characteristics, formation and effects of Tornadoes.
Finally there are more stunning Tornado visualizations here.
The Effects of Tornados: A Case Study
Oklahoma Tornado - May 3rd 1999
A particularly severe Tornado (an F5 on the fajita scale) hit Oklahoma on May 3 1999 with winds of 318mph. It was the first stage of a severe weather system which latest 3 days. 36 people were killed in the tornado and it became the fourth most costliest single tornado in US history with $1.1 billion damages. A total of 10,500 buildings and 47 businesses were destroyed (ref).
The following links provide more details on the case study:
- Wikipedia - Oklahoma Tornado Outbreak
- NOAA - the Great Plains Tornado Outbreak - May 3rd 1999
- USA Today - May 3-6th 1999 tornadoes - a look back at the deadly outbreak
- BBC - US tornado break records
- BBC - Tornado terror in pictures
- BBC - Dozens killed by tornadoes
- BBC - In pictures - the eye of the storm
- BBC - Search for tornado victims
Footage of the May 1999 Oklahoma F5 tornado can be seen in this YouTube video.
Tornado Preparedness
Here is an excellent pdf file on Tornado Preparedness including information on Torndao formation as well as safety advice and disaster plans - helps to give a sense of how people living in tornado prone areas plan for the possible even of a tornado.
Other links:
- Tornado Preparedness and Response
- Safety tips from FEMA
- NOAA Tornado Portal
- Surviving a Tornado (USA Today - excellent)
Tornado Footage
This YouTube video contains footage of a collection of violent tornadoes and their aftermath.
