 By Johnnie Thompson, City Manager
This week we will discuss what a tornado is and what precautions we should all take if a tornado warning is issued.
What is a tornado? The word tornado comes from the Spanish language and means to twist or turn. According to the Glossary of Meteorology (AMS 2000), a tornado is "a violently rotating column of air, pendant from a cumuli-form cloud or underneath a cumuli-form cloud, and often (but not always) visible as a funnel cloud." In simpler terms a tornado is a narrow, violently rotating column of air that extends from the base of a thunderstorm to the ground. Due to the invisible nature of wind, it is difficult to see a tornado unless the rotating column of air picks up water droplets, dust and debris. Tornadoes are the most violent of all atmospheric storms and no other weather phenomenon can match its force and destructive powers. How do tornadoes form? This is not a simple question to answer. In general a tornado requires the presence of varying layers with contrasting temperatures, moisture levels, density and wind flows. Several theories have been developed on how tornadoes form but none have won general acceptance in the meteorology field. The two most frequently discussed theories envisions tornado development as either the effect of thermally induced rotary circulations, or development by converging rotary winds. Scientists seem to agree that neither independently create a tornado and agree that it is likely a combination of both thermal and mechanical forces. Tornadoes develop into a wide array of shapes, sizes and severity from weak (the most common) to the extremely violent and large monster tornadoes, like those that recently plagued Oklahoma. Weak tornadoes comprise nearly 69% of all tornadoes reported and cause less than 5% of tornado related deaths. They usually have a life span of 1 to 10 minutes and have winds at less than 110 miles per hour. Tornadoes with winds between 110-205 miles per hour are generally classified as Strong tornadoes and comprise 29% of reported tornadoes; they cause nearly 30% of tornado related deaths and may last 20 minutes or longer. Violent tornadoes only comprise 2% of reported tornadoes but they cause 70% of tornado deaths and have a life of 1 hour or more. Tornadoes normally move from the southwest to northeast or west to east but may appear from any direction. In Texas, tornadoes spawned from hurricanes may move from the southeast to northwest. Forecasting Tornado forecasts are only issued in the U.S. by the National Weather Service (NWS). Local offices of the NWS (ours is located in New Braunfels and services the Austin/San Antonio area) issue warnings, while the Storm Prediction Center issues watches and general serve weather outlooks. The U.S. has some of the most advanced scientific forecasting tools in the world. With the development of Doppler radar network in the U.S., enhanced satellite imagery, and very sophisticated computer modeling programs the National Weather Service has been able to improve from little to no advance warnings in the 70's to about 5 minutes in the 80's to an average of 15-20 minutes today. The future in weather forecasting is the development of a Phased Array Radar; this will allow forecasters to analyze storms much faster and eventually lead to even better forecasting and issuance of warnings. Forecasting Terminology Severe Weather Outlook-Something that we've seen the past few years on TV is the Severe Weather Outlook Map issued by the Storm Prediction Center (SPC) in Norman, Oklahoma. Each day the SPC issues an outlook detailing the areas in the US which have a chance of severe thunderstorms and/or tornadoes. The area of risk varies from slight to severe. This outlook forecast is the first level of warning to the public. Tornado Watches-Issued by the SPC to alert the public to potential development of a tornado in your area. the conditions are very favorable for the development of a tornado but no tornado has been reported. Tornado Warning-Issued by a local NWS office when an actual tornado has been spotted or indicated by radar. Safety-To prepare for these severe and violent storms some of the precautions you can take are: - Seek inside shelter immediately go to the interior part of the lowest floor, such as a bathroom or closet. Stay away from windows and if possible get under something sturdy like a table or bench. Take a portable radio or TV with you to your shelter.
- Mobile Homes and Travel Trailers...are particularly vulnerable to overturning and destruction during strong winds and tornadoes. Tie downs will generally not protect a mobile home a tornado. If possible leave the mobile home and go to a community shelter. If none is available a ditch or culvert may offer better protection. Prepare a plan of action before the storm hits.
- In Schools...Whenever possible, follow advance plans to a basement, an interior hallway, or the lowest floor.
- Avoid the ends of hallways that open to the outside.
- Avoid classrooms with windows or outside walls.
- Stay out of auditoriums, gymnasiums or other structures with wide, free-span roofs.
- Rest rooms in the middle of building can offer some shelter from flying debris.
Do not board or stay on school buses in a tornado warning School buildings offer more protection and safety.
- Open Country...Seek shelter if shelter is available nearby and time permits. If there is no time, lie flat in the nearest depression, such as a ditch or culvert, and protect your head with your arms.
Weather Radio-One of the most important items you can purchase to help you be prepared for serve weather is a weather radio. Please click below for a fact sheet on weather radios and their merits. Weather Radio Info Sheet
BOTTOM LINE... Be Safe... plan ahead, watch local weather conditions, stay informed and know where the nearest shelter is.
Research Publications and Organizations: National Weather Service/Storm Predication Center Texas Department of Emergency Management Tornado History Project | 
