Environmental Risks and Disaster3

Environmental Risks and Disaster3 - Environmental Risks and...

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Environmental Risks and Disasters Lecture Notes 4 (1/30/2008) The phenomenology of tornados 1. According to a meteorological dictionary a tornado is `a violently rotating column of air, pendant from a cumulonimbus cloud, and nearly always observable as a funnel cloud or tuba'. This definition includes several characteristics: the funnel shape, the rotation, and the association with thunderstorms (the cloud type). Technically, a tornado has to be in contact with the ground and with a cloud in order to be a tornado. 2. Tornados are the most violent of the so-called vortex storms, which also include dust-devils, water spouts, hurricanes and mid-latitude cyclonic storms. Localized whirlwinds such as dust devils are not associated with clouds and are not tornados. Hurricanes are orders of magnitude larger in size, and are energetically driven by different processes than a tornado. 3. The funnel of a tornado is usually observed as being dark. This is simply the effect of the water droplets and dust that are trapped in the vortex and blocking the light. Close to the ground, the tornado stirs up dust and debris creating a cloud. 4. The size of a tornado (meaning the size of the funnel on the ground) can vary quite a bit. The widest tornado observed had a width of approximately 2 miles. More commonly, they are about 300 meters wide, though they can be as narrow as tens of meters. Most tornados are relatively short lived, lasting on average 10- 15 minutes and traveling some 10 km. Some last only for a fraction of a minute, affecting only a localized spot, others can travel for hours and for more than 100 km. Most tornados can be outdriven in a car, if there is a road going in the right direction. 5. Much of what we know about tornados has been deduced by carefully analyzing the damage caused by the tornado. The reason for this is simply that very few instruments can survive the tremendous winds that exist within the tornado funnel. A famous tornado expert was Theodore Fujita, who was the first to characterize many aspects of the tornado phenomenon. He developed the scale that is currently used to describe the damage caused by a tornado, and to estimate indirectly the speed of the wind in the tornado. 6. Tornados generate the highest wind velocities that we observe in any storm. Direct measurements are very difficult to make. In 1999, there was a wind velocity measurement of 318 mph made in Oklahoma using a doppler-radar instrument, which does not have to be in the wind to make a measurement. This measurement was, however, not at ground level, but measured off the side of the funnel at some elevation. It is thus not clear how strong the winds can be on the ground. 7.
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Environmental Risks and Disaster3 - Environmental Risks and...

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