Published: April 1,2017
Deliverance of clouds to the ground, or the process of tornadogenesis, can take anxious hours or sheer seconds, and their wrath can change lives in a matter of moments.
The Ingredients
In the majority of cases, you need to have a strong thunderstorm for tornadoes to form. It takes a specific set of ingredient for those types of thunderstorms to form.Four ingredients known as shear, lift, instability and moisture – SLIM, for short – come together in an area known collectively as Tornado Alley and Dixie Alley better than anywhere else in the world.
Shear – wind shear, as it pertains to severe weather, is the change of wind speed over a distance.
Two different types of wind shear are necessary for tornadogenesis: changes in wind speed and/or wind direction with height.
Two kinds of wind shear that affect thunderstorm formation and development. First, changes in wind speed with height, or vertical wind shear, do two things to help tornadoes develop and grow: spark a rotational column and tilt storms over when they grow so they can be sustained.
Southward plunges, or troughs, in the jet stream bring strong winds aloft at the level where planes routinely fly. Closer to the surface, wind speeds are usually much lighter, especially near the ground, where friction is in play. This difference in wind speed causes air to roll over, and this begins to set the stage for the formation of a tornado.
Later, when a storm is well developed, these changes in wind speed with height allow storms to breathe by pushing the top of the thunderstorm over. This tilting effect allows updrafts and downdrafts to form in different regions of the storm, which permits storms to become stronger. In addition, strong winds at the top of thunderstorms help evacuate air through a storm. When this air is removed from the storm, air must replace it from below. This creates a secondary source of lift.
At the lower levels, clashes of cold and warm air also create low-level jet streams. Sometimes, if these low-level jets get strong enough and have mechanisms to bring them to the ground, they can produce straight-line wind damage.
Secondly, directional wind shear, or changes in wind direction with height, allows the wind to bring in different types of air masses that will support tornado and thunderstorm growth.
(MORE: Where Tornadoes Form Around the World)
Lift – the faster air can rise, the more severe a storm can become.
A few sources of lift include cold fronts, warm fronts, dry lines, sea breezes and mountain ranges. Each of these forces air upward and boosts storms' ability to grow taller and stronger.
The main source of this lift is a change in density. For instance, in a cold front, the cold air is wedged under warm air because cold air is denser. As cold air shoves its way east or south under the warm air ahead of it, that warm air is forced to rise. This usually helps stimulate thunderstorms and can help strong storms reach severe limits.
A stronger cold front will have the densest air behind it, so it will have more associated lift, as compared to a weaker front.
Outflow boundaries, gusts of cooler air that come from deep within a storm, can also collide with other thunderstorms or their remnants and create sources of lift.
Instability – the condition that air is freely available to move upward into the atmosphere. If the atmosphere where clouds are trying to grow is stable, then those clouds will be "capped" or unable to grow. The atmosphere is stable when the temperature warms as elevations rise.
In many winter and even some spring situations, instability is usually the factor that limits storm systems from producing larger severe weather outbreaks, due to a lack of heat.
Another situation where instability can be inhibited is when cloud cover spreads over a region. Clouds limit the amount of heat that can reach the ground and fuel thunderstorms.
Moisture – The more moisture that is available for storms to take root, the more buoyant their environment will be and the more they can grow.
The Gulf of Mexico is very well positioned as a moisture feed. During the springtime months, most low-pressure systems swing from the Rockies across the South. Ahead of these systems, deep southerly winds scoop up moisture from the Gulf and bring it northward in the perfect spot for severe weather, including tornadoes.
Our Rocky Mountains also play a big role in why tornadoes are so prevalent in this country. On a larger scale – say, hundreds of miles – as storm systems move across the Rockies and reach the Great Plains, the low-pressure systems tend to stretch vertically and quickly develop, which provides storms on the tornado scale more dynamic energy with which to work. The higher terrain of the Rockies and High Plains can also send a layer of dry air aloft, sometimes as far east as the Northeast. This dry air can create more instability in the mid- and upper-portions of thunderstorms, helping to feed updrafts.
(MORE: Tornadoes in the Southeast May Be Influenced by Mountainous Terrain)
Thunderstorm Formation
When the ground is heated and the air is moist, thunderstorms can bubble up like popcorn on any given day, but an abundance of those ingredients or an addition of a source of lift, such as a cold front and thunderstorms, will blossom in a variety of ways.If a cold front or dry line is moving through, it will act as a trigger. Storms will be aligned parallel to that boundary as it sweeps across an area.
Storms that form in the warm, soupy air ahead of storm systems and their frontal boundaries are generally more likely to produce tornadoes.
Conceptual structure of a supercell and tornado. (MORE: The Future of Tornado Warnings: More Precise, More Lead Time)
Supercell Development
Although our understanding of tornadogenesis and supercell development is still an ever-evolving study, here is a general idea of how the process works.A decent fraction of tornadoes occur embedded in long lines of severe thunderstorms, or squall lines, but the majority of significant or violent tornadoes come from supercells.
When vertical wind shear is intense near the surface, it creates a rolling tube of air that is parallel to the ground. This is analogous to the cresting of a wave as it gets into shallower water. This rolling tube of air is the origin for what could become a tornado.
Heating near the ground helps lift the horizontal tube of rolling air into the vertical, which could then become a tornado. As the thunderstorm runs into additional instability, this rotating cloud can stretch upward and downward and strengthen.
The rotating tube acts as a suction vacuum bringing up air, and that air falls as rain in areas surrounding and ahead of the strongest part of the storm. This is like the lungs of a thunderstorm that will keep breathing until the storm cannot breathe any longer.
This is what makes a cell a supercell. Supercells do not always contain tornadoes, in fact, the large majority of them fail to produce tornadoes.
Supercells are generally long-lasting thunderstorms that run like self-sustaining engines. These storms can last for hours as long as the environment remains conducive for the storm to breathe and grow.
They are typically isolated, or discrete, from other thunderstorms, which helps them live longer. Other thunderstorms survive on the same set of ingredients, which storms will fight over until one or both storms lose and weaken.
(MORE: Why There's More Than One Type of Tornado)
Funnel Cloud to Destructive Force
As the rotation stretches downward as a funnel cloud, it could reach the ground. When this occurs, the funnel becomes a tornado and can kick up debris.Tornadoes can have winds from 70 to over 200 mph, and their parent supercells can also bring winds over 60 mph and large hail.
Tornadoes are rated on the Enhanced Fujita Scale, a damage scale that ranges from EF0 (weak damage) to EF5 (incredible destruction). Winds are estimated from damage done to structures, cars and even the ground.
A tornado will continue until favorable ingredients lessen.
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