How a thunderstorm can produce a tornado
Lightning strikes as tornado survivors search for items at their devastated home on May 23
By Jennifer Gray, CNN Meteorologist
Tornadoes are one of Earth’s most violent natural forces.
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.
“There is no perfect formula for forming a tornado,” said CNN meteorologist Brandon Miller, a storm chaser who’s has been following and studying these storms since 2005.
“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,” he said.
Still, there are some common factors anyone can look for to understand the life cycle of a developing storm that could produce a tornado. They are:
A thunderstorm develops
In the developing phase of the thunderstorm, warm air rises and fluffy white cumulus clouds begin to grow.
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.
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.
In this maturing phase, 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.
A tornado forms
Tornadoes usually form from supercells when the right ingredients are in place to help them thrive:
• 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.
The wind shear creates a horizontally rotating column of air within the thunderstorm cloud.
Then, two key forces inside the supercell can act on the rotating air column:
• 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 resulting vertical column of air is called a funnel cloud until it touches the ground — when 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,'” Miller said.
The wall cloud is a lowering of the darkened cloud base that leads to the air rotation. While the presence of a wall cloud doesn’t always mean a tornado will form, it certainly ups the odds.
“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.
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CNN’s Jackson Dill and CNN meteorologist Haley Brink contributed to this report.
