Tornado Tracks: 56 Years Of America´s Most Terrifying Tornadoes Visualized


Scooped by Dr. Stefan Gruenwald onto  Amazing Science
From         www.fastcodesign.com                – Today, 6:31 AM
Tornadoes form under a certain set of weather conditions in which three very different types of air come together in a certain way. Near the ground lies a layer of warm and humid air, along with strong south winds. Colder air and strong west or southwest winds lie in the upper atmosphere. Temperature and moisture differences between the surface and the upper levels create what we call instability. A necessary ingredient for tornado formation. The change in wind speed and direction with height is known as wind shear. This wind shear is linked to the eventual development of rotation from which a tornado may form.
A third layer of hot dry air becomes established between the warm moist air at low levels and the cool dry air aloft. This hot layer acts as a cap and allows the warm air underneath to warm further…making the air even more unstable. Things start to happen when a storm system aloft moves east and begins to lift the various layers. Through this lifting process the cap is removed, thereby setting the stage for explosive thunderstorm development as strong updrafts develop. Complex interactions between the updraft and the surrounding winds may cause the updraft to begin rotating-and a tornado is born.
The Great Plains of the Central United States are uniquely suited to bring all of these ingredients together, and so have become known as “Tornado Alley.” The main factors are the Rocky Mountains to the west, the Gulf of Mexico to the south, and a terrain that slopes downward from west to east.
During the spring and summer months southerly winds prevail across the plains. At the origin of those south winds lie the warm waters of the Gulf of Mexico, which provide plenty of warm, humid air needed to fuel severe thunderstorm development. Hot dry air forms over the higher elevations to the west, and becomes the cap as it spreads eastward over the moist Gulf air. Where the dry air and the Gulf air meet near the ground, a boundary known as a dry line forms to the west of Oklahoma. A storm system moving out of the southern Rockies may push the dry line eastward, with severe thunderstorms and tornadoes forming along the dry line or in the moist air just ahead of it. What is the Fujita Tornado Damage Scale? Dr. T. Theodore Fujita, a pioneer in the study of tornadoes and severe thunderstorm phenomena, developed the Fujita Tornado Damage Scale (F-Scale) to provide estimates of tornado strength based on damage surveys. Since it is extremely difficult to make direct measurements of tornado winds, an estimate of the winds based on damage is the best way to classify them. The new Enhanced Fujita Scale (EF-Scale) addresses some of the limitations identified by meteorologists and engineers since the introduction of the Fujita Scale in 1971. Variability in the quality of construction and different local building codes made classifying tornadoes in a uniform manner difficult. In many cases, these inconsistencies led to overestimates in the strength of tornadoes. The new scale identifies 28 different free standing structures most affected by tornadoes taking into account construction quality and maintenance. The range of tornado intensities remains as before, zero to five, with ‘EF0’ being the weakest, associated with very little damage and ‘EF5’ representing complete destruction, which was the case in Greensburg, Kansas on May 4th, 2007, the first tornado classified as ‘EF5’. The EF scale was adopted on February 1, 2007.
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