Aspects of Tornadoes

Aspects of Tornadoes
How tornadoes form? [image]Many people have seen the movie ?twister? (starring Helen Hunt & Bill Paxton), within the movie there is clear footage that shows the carnage and brutality of the tornado. Tornadoes (from the Spanish words ?tronada? meaning thunderstorm and ?tornar? meaning turn) are nature?s fiercest storms. Tornadoes are funnel-shaped, wedged-shaped and rope-shaped and seem to appear from the bottoms of the huge thunderstorm clouds. Despite their small size when compared to other windstorms like hurricanes, tornadoes produce very strong winds. Tornadoes from due to the engagement of two different air streams which cause development of the hazard. If a tornado is too form often a low-pressure system, with cold and warm fronts, must be present. [image]First warm, moist air (e.g. from Gulf of Mexico) starts off a cold war and meets fast moving stream of cold air (e.g. from Rocky Mountains). A small layer of warmer air gets above the warm and humid air at the surface.
This is like an atmospheric sandwich, with warm and humid air at the surface, the capping layer of warmer air in the middle and cold and dry air above. As this continue the meeting of the air above causes the land mass to warm up during the day, this causes the warm moist air to become unstable and rises through the cold air to form a small, low-pressure system ? mesocyclone. As this continues it causes cumulonimbus cloud development and thus thunderstorms develop. A thunderstorm happens when there is moisture in the atmosphere, a lifting force causing air to begin rising, and unstable air that will continue to rise once it starts and becomes supercharged. The different wind speeds and directions between the warm and cold air cause the wind to rise vertically through the atmosphere. All thunderstorms are characterised by updrafts, rising air currents which supply the warm, humid air that helps develop the thunderstorm, sometimes the column of rising air becomes a vortex ? a funnel cloud or a tornado. Air rises from the ground in the tornado?s vortex and causes a low-pressure area is created near the ground. Air rushes to fill this area, causing additional damage to areas not directly hit by the tornado. [image]Once the air and wind conditions are stable enough to create a tornado the twist effect is put into effect. This occurs when the converging air is deflected and forced into a circular path. When this air is put together with the wind sheers the result is air spinning upwards through the storm. Once this is put into effect the corkscrew motion develops further as the warm air continues to rise within the centre and the cold air descends at the edges. As the storm continues the rotation, which at first was very widespread becomes tighter and faster. This continuos movement creates distinctive precipitation formation as typically hail is produced from tornado forming clouds. As this occurs the tornado becomes ever more powerful and starts to reach its full potential of becoming a hazard, it develops near the edge of the thunderstorm where there is a downward movement of cold air. What are the characteristics of a tornado? All tornadoes have a funnel-shape cloud that is their distinctive characteristic. Most of the time the cloud that creates the tornado is white, grey or dark grey in colour just like any ordinary cloud. As a tornado develops, it extends directly and forces its way to the ground where their winds pick up soil and debris. It is this material that gives tornadoes their colour, in areas where soils are dark, tornadoes appear dark grey or black, in areas where soils are brown, tornadoes will have a reddish colour. [image]Tornadoes vary in size as well. They are on average between 130 - 170 metres wide and can travel between 35 ? 90 kilometres per hour along the ground. These tornadoes usually last for several minutes and can cut a path of destruction a couple of kilometres long. The largest tornadoes that are nearly 2 kilometres across are the most destructive but they are very rare. Their winds can exceed 400 kilometres per hour and can travel at 110 kilometres per hour. Because most tornadoes form in thunderstorms, thunder, lightning, rain or hail are all very common. In fact, the introduction of heavy rain or hail sometimes announces a tornado?s arrival. [image]Tornadoes prone areas also describe a distinctive characteristic of the hazard. As shown from the movie twister, tornadoes are a constant threat in the usa, with 80% of the world?s occurrences of tornadoes occurring within the nation. The hazard is most prone within the Great Plains region, which has been nicknamed ?the Tornado Alley?. This is due to the suitable meeting of different air streams, cold air from the Rocky Mountains and warm moist air from the Gulf of Mexico. But tornadoes also reported in Australia and the UK. Although weak compared to tornadoes in usa the tornadoes in UK and Australia still prove a hazard to the local population. Tornadoes also occur within other countries such as India, Bangladesh, Uruguay, China and Japan. Due the lack of contrasting air masses tornadoes rarely occur within equatorial region countries. The tornado active season in usa is between mid-March and August, and in UK it is most likely to occur between September and January. But it must be taken into account that tornadoes can occur at any time of the day or year if all the factors needed for it to be formed are active. How are tornadoes classified? The Fujita-Pearson Tornado Intensity Scale (or ?F-Scale?) is used by meteorologists to measure tornado wind speeds. The scale was named after the two men who developed it: Dr. Theodore Fujita, and Allan Pearson, head of the Forecast Centre in Kansas City. The scale measures the path length, width and wind speed and using the table and analysis is made on its destructive force and the tornado is classified. F-scale winds character OF damage relative frequency F0 (weak) 40-72 mph light damage: Some damage to chimneys, TV antennas, roof shingles displaced. Small branches broken on trees. 29% F1 (weak) 73-112 mph moderate damage: Roof decking removed, carports overturned, some trees uprooted, automobiles overturned. Unanchored homes sliding. 40% F2 (strong) 113-157 mph considerable damage: Roofs blown off homes leaving strong walls standing. Sheds and other outbuildings demolished, unanchored mobile homes overturned, block structure walls collapsed, roofs peeled back. Small wood missiles observed. 24% F3 (strong) 158-206 mph severe damage: Exterior walls and roofs blown off homes. Metal buildings collapsed or are severely damaged. Forests are flattened. Most block structures collapsed. 6% F4 (violent) 207-260 mph devastating damage: Few walls, if any, standing in well-built homes. Pile of debris on foundation, large steel and concrete missiles thrown far distances. 2% F5 (violent) 261-318 mph (rare) incredible damage: Homes on slabs levelled with debris removed. Schools, motels and other marginally engineered buildings have considerable damage with exterior walls and roofs gone. Top stories demolished. less than 1%

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