Three necessary ingredients for thunderstorm formation

For this lab we will be investigating tornadoes; how, when, and where they form!

At the end of the lab you should hand in the completed "SevereActivity" as well as the answers to the questions listed throughout this assignment.

Part I Thunderstorm & Tornado Formation

A tornado is a fast rotating column of air that touches the ground at the bottom and a storm cloud at the top. The first step to forming a tornado is:

1. Create a thunderstorm

Thunderstorms are when warm air rises and condenses, forming a cloud. The three necessary ingredients for forming a thunderstorm are

Lift - This is something that makes the air rise and could be surface heating, a cold front or dry line, or even a mountain.

Instability - When the atmosphere is unstable, air that is lifted continues to rise. Just like a hot air balloon.

Moisture - Moisture is the fuel for the storm and is needed to form the clouds and precipitation.

This video describes the ingredients for thunderstorms:

https://www.youtube.com/watch?v=6UyPr6kt7Jg (Links to an external site.)

2. Make the storm long lasting

In order for a storm to last longer we often need a continued source of lift. This source of lift is often a

Boundary - The transition zone between two types of air. A cold front is the boundary between cold air and warm air, while a dry line is a boundary between moist air and dry air.

3. Create rotation in the storm

To get rotation in the storm you need

Shear - Also known as wind shear this is a change in wind with height in the atmosphere. The shear can either be in the speed of the wind or the direction of the wind, but usually both are present when a tornado forms.

The wind shear creates horizontal rotation in the storm

4. Tilt rotation into the vertical

Next the storm updraft (rising air) tilts the horizontal rotation into a vertical direction. We now have what we call a mesocyclone. A mesocyclone is when there is a big slow rotation in a storm, and is indicative of supercell thunderstorms. Mesocyclones are needed to form tornadoes, but not all thunderstorms with mesocyclones produce tornadoes.

5. Stretch and squish the rotation

Now comes what I like to call the "figure skater effect." The slow mesocylone becomes smaller underneath the storm which makes it speed up, just like a figure skater pulling in their arms. This stretching or squishing usually is caused by the up- and downdrafts of the storm.

This video describes more about the needed conditions and some of what we still don't know about tornadoes:

https://www.youtube.com/watch?v=BMLZpjRYK9Q

So in summary LIMBS (lift, instability, moisture, boundary, and shear) is needed to form supercells, mesocyclones, and tornadoes, but it doesn't guarantee that a tornado will form.

Questions 1: Describe in your own words how tornadoes form.

Part II: Tornado Climatology

Tornadoes are primarily a U.S. phenomena. In this section we will explore where and when tornadoes occur. Watch this short video that describes why we get so many tornadoes in the U.S.:

https://www.youtube.com/watch?v=0yiZveJAEp4

This link shows a map of tornado track in the U.S.:

http://uxblog.idvsolutions.com/2012/05/tornado-tracks.html

Question 2: Where are there very few tornadoes? From what you have learned, why do you think this is?

This link shows a similar map, but broken down by the time of year:

http://uxblog.idvsolutions.com/2012/06/seasonal-tornado-habitats-1950-2011.html

This link shows the occurrence of tornado by month and day of year:

http://www.ustornadoes.com/2012/03/14/total-u-s-tornadoes-by-month-and-by-day/

Question 3: Describe how the locations of tornadoes changes throughout the year.

Question 4: Why do you think we see these changes?

Part III: Tornado Forecasting

This page gives a brief description of tornado forecasting, including the products that severe weather forecasts put out to inform the public about the risks and likelihood of severe weather and tornadoes:

http://www.noaa.gov/features/protecting_0808/tornadoes.html

You will now be using weather data to forecast tornadoes. Complete the following activity and scan or photograph it to hand in, along with the other questions in this lab:

SevereActivity.docx

After you have completed the activity, look at the maps below.

This map shows the severe weather forecast from the storm prediction center on April 6, 2003:

day1otlk_20030406_1200_prt.gif

This map shows the actual reports of severe weather on April 6, 2003:

030406_rpts.gif

Questions 5: How well were you able to predict severe weather using the data in the activity? How close were you to identifying the areas that actually had severe weather?

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.

The storm relative helicity index indicates whether or not the air is rotating. For a tornado to develop, air must be turning or spinning as it rises and the relative helicity index should be greater than 250.

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.