Atmospheric conditions and formation mechanisms of tornadoes

TORNADOES

What causes a tornado?

Tornado' comes in English from a Spanish word 'tornada' that means 'thunderstorm'. As defined by Nation Weather Service, a tornado is a 'violently rotating column of air in contact with the ground and pendant from a thunderstorm'. Tornadoes result when there is stuffiest amount of moisture in the atmosphere at lower and mid levels, and when this condition gets combined with warm unstable air that rises above due to a lifting force. Initially, there should be some reason to cause air to flow upwards and to contribute in the formation of a tornado. This happens due to the fact that warm air is considerably lighter in weight than cold air (Earthbulletin). The buoyant air is produced as atmosphere gets heated near the ground surface. The heated air, which is warm and light, begins to rise upward and if during this phenomenon, a thrust of cool air is met, it further instigates the upward motion of air and could possibly trigger a thunderstorm. As the air of the area becomes unstable, it continues to rise once the rising motion is setoff - called the updraft. Given that all conditions are suitable, warm air rise high above cold air - that's what takes place inside a thunderstorm and it's usually a thunderstorm that gives rise to tornados. During a thunderstorm, the air (after rising) gets cooled and the humidity and moisture starts to condense, forming clouds. The speed with which air flows upward in an updraft can be more than 100 mph; due to such strong updraft force, the resultant clouds (containing hail - raindrops and ice balls) can attain a height of several thousand feet above the ground level.

Tornadoes are formed because of this rapid (high-speed) rise of air. When cold or dry air comes in contact with warm, humid air inside a thunderstorm; the strong wind currents create an invisible spinning effect that is horizontal in direction. The spinning radius is greater at heights and narrower at the end near to the ground - this motion ultimately creates a huge funnel like tornado of rotating air that moves at great speeds. Area of rotation can be anywhere between 2 to six 6 miles. Updraft continues to pull the moist air upwards and tornado rises above. As the pressure at the center of tornado funnel is lower than the surroundings, the winds start to flow-in with enormous speeds from all around this giant rotating column of air. This is the reason why a tornado damages areas hundreds of meters way from its actual vortex. The winds bring dust, dirt and debris into the tornado and the funnel gets darker in color. At the edges of updraft, tornadoes are strongest and they shed hail from around from the downdraft (created by the air that comes down from thunderstorm carrying with itself rain and balls of snow). That's how heavy rain or a hail shower often occurs just before a tornado hits.

The terrain of U.S. makes it very susceptible to formation of deadly tornadoes. Tornadoes are most common in the areas that occupy large flat and dry regions. Tornado Alley is the name given to the region of U.S. that is more vulnerable than other to tornadoes attacks - Northern Texas, Oklahoma, Kansas, and Nebraska (Central United States). In the U.S., a low-level-jet (3000 to 5000 feet from the ground) blowing humid and warm air from the south when faced by advancing column of cool air, assist creation of fierce thunderstorms that often give rise to tornadoes. Some thunderstorms get more violent because of the dry wind blowing at 10000 feet above the ground level from the southwest. One of the most dangerous forms of thunderstorms called supercells are aided by jet-streams at 25000 feet above ground level, these jet-streams help the warm air to rise and contribute into formation of tornadoes.

In the early spring, these climatic conditions are responsible of creating tornadoes in the south of U.S. As the spring passes, tornadoes become more likely in the northern parts of U.S. - the Plains and the Midwest. In the months of April and May, tornadoes strike both the south and the Plains. In this season, tornadoes condition can last up to several days due to large system of storms (Usatoday). Presence of 'dryline' in the Central Plains which separates eastern warm, moist air from the western hot, dry air is responsible for developing thunderstorms. As the dryline shifts to the east during afternoons, the result comes out as thunderstorms which give the plays a major part in formation of tornadoes. Mesocyclone (rotating air column) is produced when the air going upwards from the Gulf of Mexico comes across winds that tends to turn it (rising air), and causes change in its direction and speed. As humid air from the Gulf of Mexico flows north, crossing the Midwest and Great Plains, it is met by the westerly jet stream and southwesterly stream - all winds create enormous force as they come from varied attitudes and speeds. These forces air from the Mexico Gulf to rotate and conditions for a tornado becomes ripe (earthbulletin). In the regions of High Plains in the south and along Rocky Mountains, tornadoes are produced when air in the lower terrain rise towards higher terrain (f-5stormshelters).

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