Opinion / Column: Facts about the tornado | | dailyprogress.com

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People stand on the porch of a house destroyed by tornadoes that ravaged the area in Dresden, Tennessee, on Sunday.


Gerald Herbert


Alisa Hass and Kelsey Ellis The Conversation

On the night of December 10-11, 2021, an outbreak of powerful tornadoes ravaged parts of Arkansas, Mississippi, Missouri, Kentucky, Tennessee and Illinois, killing dozens of people and leaving wrecks for hundreds of kilometers. Hazard climatologists Alisa Hass and Kelsey Ellis explain the conditions that generated this event – including what could be the first “four-state tornado” in the United States – and why the Southeast is vulnerable to these disasters all year round, especially at night.

What factors came together?

On December 10, a powerful storm system approached the central United States from the west. As the system brought heavy snow and slippery conditions to the colder west and northern Midwest, the south enjoyed near-record heat, thanks to warm, humid air coming in from the northern Gulf of Mexico. .

The storm system brought cold, dense air into the area, which interacted with the warm air, creating unstable atmospheric conditions. When hot and cold air masses collide, warm, less dense air rises to cooler levels in the atmosphere. As this warm air cools, the moisture it contains condenses into clouds and can form storms.

When this instability combines with strong wind shear – winds changing direction and speed at different heights in the atmosphere – it can create an ideal pattern for strong rotating storms to occur.


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