Background

"Early Tornado Warnings Saved Lives on the Plains" exclaimed the headline for the NOAA Report. On May 3, 1999, sixty-six tornados ripped through Oklahoma killing 48 people. Researchers believe that the death toll could have been much worse. One man avoided being hit with an airborne rental truck by just seconds as he and co-workers took refuge from flying debris under tables in the kitchen of a restaurant. Two children and their parents emerged unhurt from a bedroom closet to find their house demolished around them. Based on the amount of damage and a long historical record, researchers estimate that over 700 lives could have been lost.

Yet, despite widespread destruction, early warnings issued by the Storm Prediction Center and the Weather Service Forecast Office in Norman, Oklahoma saved hundreds of lives. The severe weather that was forecasted earlier in the day had put almost everyone on alert.

To issue severe weather outlooks, such as those used in Oklahoma, forecasters use a variety of tools and techniques. In this case, one of the early indications of unstable atmospheric conditions came from a "sounding". Also known as a Skew-T Diagram, this complicated looking graph provides information gathered from meteorological instruments that are carried by helium-filled balloons high into the atmosphere. The neoprene rubber balloon and its package is called a radiosonde. Over 1000 stations around the world launch radiosondes twice daily. Observations of temperature, dewpoint, and winds at various air pressures are sent back to the launch station by radio. This information is transmitted to the surface, decoded and transformed into a Skew-T Diagram.

The Skew-T Diagram gives a "snapshot" picture of temperature, dewpoint, air pressure, and winds in the atmosphere above a particular point on the Earth's surface to a maximum of about 16 kilometers above sea level. It is a basic tool used in forecasting not only severe storms, but also daily weather.

On the Skew-T Diagram, the "skewed" horizontal axis slants upward to the right hand side of the diagram at a 45o angle, and is the temperature in Celsius degrees; the vertical axis is atmospheric pressure in millibars. Atmospheric pressure decreases with altitude.

Figure 4.1 gives you basic information on temperature and pressure for comparison with the values you see in this activity.

Figure 4.1. Basic temperature and pressure information.

Figure 4.2 is a sample Skew-T Diagram. It is an example of a Skew-T Diagram that weather forecasters use. Although it may look complicated, it is easy to figure out. You will understand it better after you follow the steps in the "Procedure" section of this activity.

Figure 4.2. Sample Skew-T diagram.

The bold solid line on the right is atmospheric temperature measured in oC. It is plotted using the "skewed" lines on the diagram. These diagonal lines are drawn so that a temperature that cools at a normal rate with increasing elevation will have a bold solid line that is almost vertical. You will plot a similar curve on the template provided in Figure 4.3.

The bold dashed line on the left is the dew point temperature, a measure that indicates humidity. Where the dew point line is far from the atmospheric temperature line, the air is dry; where the dew point line is close to the temperature line, the air is moist. You will plot a similar curve on the template provided in Figure 4.3.

Do not use the Skew-T Diagram in Figure 4.2 for your procedure.

Although Figure 4.2 is not the Skew-T Diagram that was used to help forecast the May 3, 1999 tornados, it is one example of a Skew-T Diagram that weather forecasters use. Notice the solid-line temperature curve and the dashed-line dewpoint curve. Since the dewpoint line is close to the temperature line near the surface, the air is moist and precipitation is likely.

On the afternoon of May 3, 1999, forecasters in Oklahoma noticed an abrupt change in atmospheric conditions based on the Skew-T Diagram. Just prior to the devastating tornado invasion, the Skew-T Diagram showed lots of moisture in the lower atmosphere; however, a temperature inversion provided a barrier to developing thunderstorms.

Suddenly, within one hour, the Skew-T Diagram revealed that the temperature inversion had disappeared. The sky opened to rapid supercell convection as moisture laden warm air swiftly ascended into the atmosphere because of its lower density. At the same time, the Skew-T showed a dramatic shift in wind direction, from southeast at ground level to northwest in the upper atmosphere, fueling a fierce spinning motion. Moisture condensed and tornados were born. Forecasters put everyone on alert.

In the past, when meteorologists collected data provided by the radiosondes, they plotted it on charts by hand. Today they use computer software to quickly plot and display this information. Weather forecasters have Skew-T soundings readily available.







































































Procedure

The Forecast Systems Laboratory in Boulder, Colorado provided the data for this activity.

Note: To help you get started, the first five temperature and dewpoint data points from Table 4.1 are already plotted on the bottom of Figure 4.3. Practice plotting these data points, using Steps 1 and 2 that follow, before going on to the rest of the activity.

  1. Using Table 4.1, plot temperature and pressure (in mb) on the Skew-T Diagram in Figure 4.3 that follows this procedure. With black pencil, make a bold, solid curve that stands out.

  2. Using Table 4.1, plot dewpoint temperature and pressure (in mb) on the Skew-T Diagram in Figure 4.3 that follows this procedure. With black pencil, make a bold, dashed curve that stands out.

  3. Notice the solid lines that curve upward from the temperature number labeled on the horizontal axis. These lines are called "dry adiabats" and represent how the temperature of dry air would cool if it were lifted or how it would warm if it were pushed down. Outline the dry adiabats in red.

  4. Notice the dashed lines that curve upward from the temperature number labeled on the horizontal axis. These lines are called "wet adiabats" and represent how the temperature of saturated (wet) air would cool if it were lifted. Outline the wet adiabats in green.

Note: Temperature cannot warm by moving down a wet adiabat because the air would immediately dry to less than saturated conditions.

Table 4.1. Pressure, Temperature, and Dewpoint Data for Skew-T Diagram.


Figure 4.3. Skew-T Diagram Template.


Questions

The information that you plotted comes from one of the Skew-T Diagrams that forecasters used to help predict the outburst of tornados on Monday, May 3, 1999, 18:00 UTC (Coordinated Universal Time). Now that you have plotted temperature and dewpoint, you will interpret your graph, much as a forecaster would use the Skew-T Diagram.

Use the information in Figure 4.4 to help you with your interpretation.

Figure 4.4. Analyzing Your Skew-T Diagram.

  1. What is the air temperature and dewpoint at the surface (lowest level)?

    Is the air dry or wet?

  2. What happens to the amount of air pressure as altitude increases?

  3. What normally happens to temperature in the troposphere (lowest layer of our atmosphere that supports life) as altitude increases?

  4. In this case, what happens to temperature in the troposphere as altitude increases?

  5. At what pressure (mb) is the freezing level (0oC)?

  6. At what pressure (mb) might there be clouds? (Hint: Give a range.)

  7. What happens to the amount of moisture in the atmosphere above 850 mb?

    How can you tell?

  8. The transition between the troposphere and stratosphere is called the tropopause. If temperature increases in the stratosphere as altitude increases, at what pressure (mb) does the tropopause occur? (Hint: Give a range.)

  9. What is the environmental lapse rate between 900 and 800 mb? (Hint: Lapse Rate = Temperature Change/Altitude Change)

  10. Why is there no dew point temperature data above 300 mb?

  11. Between which two air pressure points would the atmosphere be neutral (neither stable or unstable)?

  12. What happens to the temperature between 820 mb and 795 mb?

    What is this weather phenomenon called?

  13. Why does an anvil shape (flat top) form at the top of cumulus clouds? (Hint: temperature)


Conclusion

Review the problem stated at the top of this web page and write a detailed conclusion for this activity on "Forecasting Tornados".

Do you want the complete activity to view/read and print for your use?

Adobe PDF File Available
Adobe Acrobat 5.0 or Greater

SAM II Activity 4 – "Forecasting Tornados: Forecasters Use Temperature, Dewpoint, and Air Pressure"