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SevereActivity.docxAnalyzing Severe Weather Name_______________________
Introduction
Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes. The thunderstorms in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east.
Tornadoes are violent windstorms associated with severe thunderstorms. Meteorologists carefully monitor atmospheric data to predict where thunderstorms might develop. They also attempt to predict whether these storms might spawn powerful tornadoes.
The necessary ingredients for severe weather can be remembered using the pneumonic LIMBS:
L: Lift (something to make the air rise)
I: Instability (once the air starts rising, it keeps going)
M: Moisture (the fuel for the storm)
B: Boundaries (provide a location for storms to continue to form)
S: Shear (change in the winds that creates rotation in the storm
Meteorologist use thermodynamic indices to help them diagnose whether LIMBS is present.
Thermodynamic indices are sets of numbers that indicate the state of the atmosphere at a given time and place. Three important thermodynamic indices are the dew-point temperature, the lifted index, and the storm relative helicity index.
The dew-point temperature indicates the amount of moisture in the atmosphere. The higher the dew point, the more moisture in the air. The dew-point temperature of an area usually needs to be at least 50°F for a tornado to develop.
The lifted index indicates how fast or slow air will rise or sink. Air must be rising for a thunderstorm - and therefore, a tornado - to develop. A lifted index needs to be -4 or less for a tornado to develop.
In this investigation, you will use thermodynamic indices and weather maps to predict where a tornado might strike.
Part A: Analyzing Dew-Point Index
1. Look at the map in Figure 1. This map shows dew-point temperatures in the United States on April 6, 2003.
2. Which part of LIMBS is the dew-point temperature?
3. Lightly shade in regions that have dew-point temperatures greater than 50°F conducive to the formation of tornadoes.
4. List the states that have parts shaded. Figure 4 shows a map of the U.S. with states labeled.
Part B: Analyzing Lifted Index
1. Look at the map in Figure 2. This map shows the lifted index for the United States on April 6, 2003. Data Table 1 includes a scale for interpreting the lifted index.
2. Which part of LIMBS is the lifted index?
3. Lightly shade in regions whose lifted index is -4 or less, and conducive to formation of tornadoes (aka: -4, -5, or -6).
3. List the states that include shading.
Part C: Analyzing Storm-Relative Helicity
1. Look at the Storm-Relative Helicity Index map in Figure 3. This map shows the storm-relative helicity index for the United States on April 6, 2003. Data Table 2 includes a scale for interpreting this index.
2. Which part of LIMBS is the storm-relative helicity?
3. Lightly shade in regions whose storm-relative helicity is greater than or equal to 250 and is conducive to the formation of tornadoes.
3. List the states that include shading.
Final Analysis
1. Are there any states where all three thermodynamic indices favor the formation of a tornado? If so, name them.
2. List all the states that have only two of the indices for formation of a tornado (and include which two criteria they have).
3. List all states that have only one index for tornado formation (and which criteria).
4. Imagine that a cold front moved across the SE U.S. on April 6, 2003. Which part of LIMBS is a cold front?
5. Would this front increase or decrease the chances of tornado formation? Explain.
6. On Figure 4, sketch with concentric circles where you think there is a (a) high, (b) medium, and (c) low chance of severe weather and tornadoes on this day.
