Term Paper on "Tornadoes What Causes a Tornado?"

Term Paper 7 pages (2242 words) Sources: 1+

[EXCERPT] . . . .

NWS tells people to watch the sky for color changes after a warning is issued. If they feel that it's getting dark (or greenish), this indicates that tornado is about to hit the area. People are advised to go to basements, lowest possible floors, near hallways and far from things that can fly off due to strong winds. Other warnings of an approaching tornado include sighting of wall clouds, large hail or a loud noise that resembles roar of a freight train.

Tornado warning also includes educating people about its movement and the ways it could possibly damage property and lives. Tornadoes are known to move in any direction but the predictable direction of their motion is thought to be from southwest to northeast. A high percentage of damage by tornadoes results from debris that flies off with the fast moving wind. People on the ground can some times spot a tornado themselves but some tornadoes remain hidden behind walls of clouds or rain.

Tornado frequency in certain years

Research and data has shown that in some years, number of tornadoes is considerably higher than the others. The year 1998 had largest number of tornadoes (1440) in recent times. Other big years that had 1400+ tornadoes were 1973 and 1954. 1992 had 1385 occurrences of tornadoes. Lower number of tornadoes were recorded in 1987 (811 tornadoes), 1988 (850), 1985 (877), and 1986 (949). To see the reasons in perspective, scientists have suggested some correlation of tornadoes with global weather phenomena, like global warming, El Nino and La Nina cycles.

During a La Nina season, ocean temperatures decrease in Equatorial and Tropical Pacific, this leaves northwest of the U.S. cold and southwest warm. La Nina occurs every 6 or 7 years affecting weather scene around the globe (earthbulletin). A La Nina year also brings noteworthy changes in the U.S. climate - including frequency of tornadoes, as the weather gets generally harsh. This is mainly due to the reasons that air masses in the north and south go to their extreme temperatures. These temperature differences when clash with each other, tornadoes occur. In areas stretching from the northern Plains to the Great Lakes, air becomes cooler in spring time, whereas in the southeast, air remains significantly warmer. 1998 and 1999 were La Nina years, and showed increased tornado activity. 1998 saw over 1400 tornadoes - a record (impactforecasting). The deadliest tornado of that year was an F5 tornado that left 32 people dead. 1999 saw more than 1300 tornadoes with wind speeds reaching as high as 318 mph.

Creation of supercell thunderstorm in La Nina years also plays a part in creation of tornadoes. Subtropical jets from the southwest blows deep inside the north which doesn't happen in the non-La Nina years. This brings in warm, humid air to a greater distance further into the northern U.S. causing supercell thunderstorm that develops a number of tornadoes in the central region of the U.S. This supercell thunderstorm is not anything like a usual thunderstorm - it has got greater endurance. Both, normal and supercell thunderstorm starts with updrafts - rising action of the air. As the moist air cools down and condenses, it creates rain. And it's this rain which when comes down, causes a downdraft. A normal storm crumples up with the falling hail but a supercell would tilt - one side continues to lift up the air (updraft) and the other side continues to drop down the rain (downdraft). These stronger supercells then create tornadoes. As supercells are associated with a particular weather season that occurs in certain years, tornadoes too, are believed to be generated more in numbers in these years.

Researchers are still trying to come up with ways and means to correctly predict years during which tornadoes are highly likely - and study of La Nina is helping them in the cause. If a firm relation is established between the two, weather department and general public would find itself in a better position to address perils of tornadoes (impactforecasting).

Bibliography http://www.f-5stormshelters.com/faqs.htm basic guide to what causes tornadoes http://www.usatoday.com/weather/wtwist2.htm

The American Museum of Natural History - EarthBulletin - Storms - Where Do Tornadoes Form? http://earthbulletin.amnh.org/C/4/2/index.html

Tornadoes Fact Sheet http://www.nws.noaa.gov/om/torn.htm http://www.nws.noaa.gov/om/brochures/tornado.htm

Weather Watch: Tornadoes http://teacher.scholastic.com/activities/wwatch/tornado/emergency/warning.htm

Severe Weather Pattern Shifts In The United States. Aon Corporation. www.impactforecasting.com READ MORE

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