Monday, February 23, 2009

The Video That Started It All......................

Today starts Severe Weather Awareness Week here in Texas so I thought we would review the best practices for keeping you and your family safe during severe weather. Today we will discuss where to seek shelter from a tornado if caught outside or in your car. For years the meteorological community has been telling people if you are caught outside or in your car during a tornado the best place to seek shelter in and a ditch or ravine and to cover your head. This is still the best choice for protecting yourself during a tornado if caught outside without sturdy shelter nearby but during April of 1991, a large tornado outbreak hit Kansas and from this event, video of a news team first trying to outrun a tornado and then seeking shelter under an overpass changed the public’s outlook on where to seek shelter.

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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