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:
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:
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.:
This link shows a map of tornado track in the U.S.:
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:
This link shows the occurrence of tornado by month and day of year:
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:
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:
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:
This map shows the actual reports of severe weather on April 6, 2003:
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?