Unfortunately this misleading information caused three people to lose their lives as they sought shelter under an overpass during the Moore, OK F5 tornado on May 3rd, 1999. So why not an overpass? Well, the main reason is with most over passes, you are in an open area where debris from the storm can harm you as wind speeds increase, not only from increasing elevation but from the air flow being squeezed between the narrow opening of the overpass. Without going into the subject of fluid dynamics, the best way to explain what happens as winds hit an overpass is to relate the winds to water flowing out of a garden hose. If you put your thumb over the flow of water, what happens? The velocity of the water increases due to the same amount of volume being squeezed through a smaller opening. So the winds in a relatively weak tornado can increase dramatically causing more bodily harm to those who seek shelter under an overpass. Below is a graphic created by the National Weather Service on why Overpasses do not provide adequate shelter from tornadoes.
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