Tornado formation, explained | CNN

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Tornadoes are one of Earth’s most violent forces of nature. They have occurred in all 50 states and have been spotted throughout the year. While they are most common in the United States, tornadoes can occur all over the globe.

They have whipped up in all 50 US states – and across the globe – at various times of year, sometimes causing untold devastation. But despite strides in their study, there’s still so much scientists don’t know about them.

“Something you learn from studying tornadoes, especially in the field, is that there is no perfect formula for forming a tornado – every storm is unique,” says Miller.

“Sometimes, it may appear that a storm is in a perfect environment for a tornado to form, yet it never does. Conversely, tornadoes frequently form in marginal environments where it seems like one or more ‘ingredients’ to storm formation is missing or lacking,” says Miller.

Homes damaged by a tornado are seen on March 26, 2021, in Newnan, Georgia.

Scott Cunningham/Getty Images

While there’s no perfect formula as to which storms will produce tornadoes, there’s commonality in the anatomy of the systems that result in tornadoes.

The development of a thunderstorm

As a thunderstorm develops, air rises, helping the cloud grow taller and taller.

As warm air rises, clouds begin to grow taller and taller. This is the developing phase of the thunderstorm. You can see fluffy white cumulus clouds grow taller with your own eyes. This is before you see rain or hear thunder.

The clouds grow taller and taller, even before rain or thunder appears.

Then, the bottom of the clouds darken and the very top could flatten out, creating a protruding anvil shape that indicates very cold air at the top and could be a precursor to hail.

When a thunderstorm becomes a supercell

As warm as rises, cold air is pushed down. This results in wind speed and direction changing with height within the storm – a phenomenon called wind shear.

Wind shear helps the storm begin to rotate and become what’s called a supercell.

Cold air rushing out of the storm cloud, producing a gust front.

During this mature phase of the thunderstorm or supercell; heavy rain, lightning, hail and very strong winds are expected.

Sometimes, a strong gust of wind – called a gust front – blows a few moments before rain arrives. It’s caused by cool air being forced down from the storm cloud. When the cold air hits the ground, it spreads out quickly ahead of the storm and is a sure sign a storm is near.

“Nearly all supercells produce some sort of severe weather (large hail or damaging winds) but only 30 percent or less produce tornadoes,” said the National Oceanic and Atmospheric Administration.

The formation of a tornado

Tornadoes are usually formed from supercells. These supercells develop when there are several ingredients in place to help them thrive: instability, lift and wind shear.

• instability in the atmosphere, which allows air to rise;

• lift, the rising itself;

• and most critically, wind shear: when winds at different heights within the supercell blow in different directions.

Within the supercell, wind shear becomes a crucial component to the storms development. Shear is when winds change directions with height. This creates spin within the thunderstorm cloud.

Another force that’s acting inside the supercell is a downdraft. This is an area of drier air that is pushed down from the storm. As it’s forced down, it wraps around the backside of the storm.

• the updraft – or the rising of warm air – lifts the horizontal air column,

• while the downdraft – an area of drier air pushed down from the storm – twists the column so it’s vertical, then wraps around its backside.

The updrafts and downdrafts work in tandem to pull the horizontal column of air down to form the tornado. This vertical column of air is called a funnel cloud until it touches the ground – at that point it becomes a tornado.

“The most telltale sign that a tornado could be forming, when you are looking at a close-range severe thunderstorm, is the ‘wall cloud,’” says Miller.

The wall cloud is a lowering of the cloud base that will lead to the vortex, or spinning of air.

“You will know you are looking at a wall cloud because it will hang noticeably lower than the rest of the thunderstorm, and you may notice it is rotating if you look closely,” Miller said.

A tornado dies

Tornadoes can disappear as quickly as they appear – often morphing from roaring funnels of fury into nothing in seconds – when a key ingredient is lost, Miller said.

Sometimes as a thunderstorm evolves, its source of warm, moist air is cut off, causing a tornado to die, he said.

Tornado-producing supercells also can merge with other storms, forming into a so-called “squall line,” which generally kills a tornado, Miller said. Squall lines, though, have their own dangers, including gusty winds that can stretch for hundreds of miles – and even spin up quick tornadoes of their own.