UCSB Science Line
Sponge Spicules Nerve Cells Galaxy Abalone Shell Nickel Succinate X-ray Lens Lupine
UCSB Science Line
How it Works
Ask a Question
Search Topics
Our Scientists
Science Links
Contact Information
Why does the central US have the most tornadoes in the US? What are the causes? Thanks
Question Date: 2013-05-25
Answer 1:

Tornadoes only occur when large thunderclouds form. Many thunderstorm clouds DO NOT produce tornadoes but some do. So, to understand tornadoes you have to understand why thunder clouds cumulonimbus clouds form in the first place.

Basically, it all comes down to 3 main ingredients:

1. There has to be convergence of air at the surface so that air is flowing horizontally from opposing directions. When the air meets it has to go somewhere and so it goes up. Another way to make the air go up is to strongly heat the ground surface due to solar heating. This warms the ground air and because it is less dense the air rises. So, this is called LIFT... there has to be strong lift.

2. The second ingredient is that there needs to be a source of warm MOIST air... that is air that has a high relative humidity. this is important because when warm moist air rises it condenses and makes a cloud... when the water vapor becomes a liquid droplet, heat is given off to the surrounding air which then is less dense and rises even FASTER.... this is why thunderclouds can form so fast and grow to heights up to 20 km!!! In a matter of minutes!

3. The third ingredient is wind shear. This can be understood by recognizing that the wind that blows horizontally does so at a speed that can vary with height. So, say the wind is blowing at 40 mph at 1 km above the surface at some location. Then, at that same x,y spot but at 2 km the speed is say 50 mph... This difference in speed creates “vorticity” or spin.

Now we (meteorologists) use a number called the lift index (LI) to say how all these features come together to trigger possible tornado... this index is called the lift index.

Typically the index is 0 or +1... A severe thunderstorm has LI = -3... The LI over OK last week was -8 !!!!

Tornadoes can develop when the LI is - 5 or -6 or -7 , -8 etc. Strongly negative value of LI.

Now, in that part of the mid west we have warm moist gulf air, we have strong ground heating, and because of the mixing of warm moist tropical air (m T air = marine tropical) and dry cooler continental polar air (cP), you see that we have all the ingredients for intense thunder cell development, and hence the POSSIBILITY of tornadoes.

Click Here to return to the search form.

University of California, Santa Barbara Materials Research Laboratory National Science Foundation
This program is co-sponsored by the National Science Foundation and UCSB School-University Partnerships
Copyright © 2020 The Regents of the University of California,
All Rights Reserved.
UCSB Terms of Use