Tornadoes are tricksters, swirling displays of incredible force that can follow straight corridors or deviate in erratic paths. Scientists still are not 100 percent on the hows and whys of tornado formation — which is one of the reasons storm chasers and research meteorologists come out en masse during tornado season to study them in depth.
Not all thunderstorms create tornadoes and not all tornadoes are created by thunderstorms. But the most common method of violent tornadogenesis stems from a supercell thunderstorm — one that contains a vortex called a mesocyclone at its core — which indicates there's a decent chance a tornado is in the works. So, the first few factors on our list are all components needed to build a supercell and, in turn, a tornado — and the last couple are ones that could contribute to the tornado formation itself.
5. Wind Shear and Updraft:
One characteristic that's generally necessary for a strong supercell — and subsequent tornado — is wind shear, a difference in the wind speed at varying altitudes or directions. Air currents are often more powerful at higher altitudes, for example, where jet streams can send them tumbling into a horizontal loop.
As the thunderstorm begins to form, the accompanying updraft can also encourage horizontal spin, but it's this next event that's key — for a thunderstorm to generate a tornado-spawning mesocyclone, the supercell's powerful updraft must succeed in raising, sustaining and tightening the central vortex at a near vertical alignment.
If this step is reached, powerful forces within the whirling mesocyclone can achieve a balance between the inward and outward flow of air and create what is known as the dynamic pipe effect. The low-pressure center of the vortex (the pipe) sucks additional air up into the storm and lengthens the rotating tube that could potentially become a funnel cloud.
4. Air Temperature:
A second important component needed for supercell formation and tornadogenesis is warm air, especially in relation to parcels of air higher up in the atmosphere. Many, although not all, supercell thunderstorms associated with tornadoes form at the boundary between cold, dry air and warm, moist air.
At the border between these two air masses, the rising warm air helps ignite rotation by fueling the towering updraft that pulls additional warm air up in its wake. The powerful updrafts associated with thunderstorms are aided by uneven heating of the Earth's surface — one of the reasons they're common in the afternoon hours.
Once warm air reaches the top of the storm, it spills over the backside as a rear flank downdraft and is recycled back into the storm. If the air is too cold, the storm weakens and tornadoes don't form.
3. Moisture and Instability:
Another ingredient necessary for tornadogenesis is moisture. All the warm air discussed in the last paragraph was moisture-laden saturated air that likely rode in from the tropics. As the air begins to rise, its dew point lowers and the moisture cools and condenses to form the storm cloud.
Water vapor releases incredible amounts of heat as it condenses into raindrops, and it's this heat that fuels the storm, revving up a convective cycle of moving air. Thunderstorms are super powerful, and all their energy and furious wind speeds create a positive feedback loop that propagates until the supercell reaches mammoth proportions.
It's also important to note that instability — the difference between the temperature and moisture levels of air from the bottom to the top of the parcel — works to intensify a storm.
2. Gravity Waves:
Once the stage is set for a mesocyclone supercell, another atmospheric occurrence that could up the chances of a twister is a set of incoming gravity waves. Gravity waves are like ocean waves; disturbances in the fluid atmosphere, say powerful gusts of wind or soaring mountain ranges, can cause the air to ripple and roll. Enter the GrITs theory: Gravity wave Interactions with Tornadoes.
According to Tim Coleman and Kevin Knupp — the former a research scientist and the latter a professor in the department of Atmospheric Science at the University of Alabama in Huntsville — gravity waves can have an effect on storm development.
By studying information recorded from previous supercells and tornadoes, the two meteorologists found instances where atmospheric waves interacted with mesocyclones, compressing and shrinking them. By decreasing the size of a rotating object, you cause it to gain speed.
Coleman and Knupp, whose work was published in the March 2008 issue of the Monthly Weather Review, concluded that in cases where gravity waves interact with mesocyclones, they can potentially give a boost in terms of rotational speed — which might just jumpstart the formation of a tornado.
1. Raindrop Size:
New research recently published in Geophysical Research Letters suggests that a storm's spread of raindrop sizes could be an indicator of the likelihood a tornado will form. (Raindrops vary in size depending on the conditions under which they're formed.)
Nathan Snook, a graduate research assistant, and Ming Xue, a professor in the School of Meteorology and Director of the Center for Analysis and Prediction of Storms — both at the University of Oklahoma — conducted a 3-D study of the microphysical conditions inside a supercell storm.
They studied a number of atmospheric variables and found that large raindrops tend to cluster together more than smaller ones, and are also less prone to evaporation. Since evaporation cools the air (as opposed to the heating that occurs with condensation) this means the air beneath a storm cloud will be warmer than if smaller drops were present — and we know that warm moist air is just the thing to fuel a massive supercell that spawns tornadoes.
But while this could help explain why a tornado doesn't debut with every supercell, tornadogenesis is still an elusive process. Many factors only just now being considered could also prove to be playing a key role in tornado formation.
Are Tornado Patterns Changing?
The question of whether tornado patterns are changing is somewhat difficult to answer, in part because very little is known about tornado occurrences before the 1880s. Overall, records were pretty weak even after that point until a more rigorous tallying system commenced in the early 1950s — before that time, not a lot of effort was made to report the vast majority of tornado touchdowns.
The Modern Era of Tornado Tracking
There are a couple of reasons for this lack of data. For one, the nation wasn't as densely populated as it is today — in order to know the number of tornadoes that struck each year, somebody had to be there to see them. Just as importantly, someone had to decide to follow up and report them. Smaller tornadoes — especially ones that didn't cause a lot of damage — had the tendency to slip through the cracks.
Other factors to keep in mind include the gradual improvement in weather tracking technology. Plus, in the 1950s, the number of reported tornadoes started to skyrocket as national weather agencies began to take an increased interest in tornado prediction.
On a similar note, once meteorologists started taking a more intense look at tornadoes, enthusiastic networks of storm spotters and storm chasers began springing up around the country, which added greatly to the amount of tornadoes being reported. Thanks to all of those factors, enhancements in reporting have continued to progress nicely over the intervening years.
Charting the Trends
Because early data is somewhat sketchy, analysts have adopted some clever methods for trying to chart tornado trends over the past 100-plus years. One strategy for trying to determine long-term patterns has been to analyze tornado data focusing only on those tornadoes at the powerful end of the spectrum — often the F2 to F5 strength storms (although those are now classified on the Enhanced Fujita scale). The theory is that stronger storms — while less frequent than weaker ones — had a better chance of being reported in the days before weather radar and other helpful meteorological innovations.
If that assumption is the case, then the data appears to show little indication that there's any overall trend when it comes to violent tornadoes, according to the National Severe Storms Laboratory of the National Oceanic and Atmospheric Administration (NOAA). Some years they're very frequent, other years they're not.
What a New Climate Could Do
All these fluctuations make it hard to tell whether tornadoes in general are becoming more frequent or more intense, but many are worried that might begin to change as global temperatures rise. Some researchers suspect climate change will alter weather patterns, potentially leading to increased numbers or heightened intensities of hurricanes, thunderstorms, tornadoes, droughts and floods, although whether this is the case remains to be seen.
One study published in the journal Environmental Research Letters found that the tornado that struck downtown Atlanta in March 2008 could have gained the momentum to form or been intensified by interactions between less-frequently studied inputs. Although more research is needed on a number of factors to form definitive conclusions, the study proposed the idea that mild rain during drought conditions could've played a role in the occurrence of this rare "downtown tornadic event." Increased urbanization and the heat island effect are also under scrutiny as possible contributors.
The Calm Before the Storm
You've probably experienced it at some point: the Emergency Alert System disrupts your favorite TV show and warns of an impending storm. The severity of it is frightening, but you can't help to step outside to take a look at what you're in for. The wind whips up, the sky grows dark, and the trees sway and bend at improbable angles. Then something unsettling happens — everything grows quiet, the wind dies down and an eerie calm hovers. Then the storm hits, and things go back to the violent chaos that had just vanished.
So, is there really calm before a storm, and if so, is there an explanation for it?
What is a storm?
Storms generally include heavy rains, wind, thunder and lightning. In a nutshell, a storm forms when an extreme difference in air pressure occurs in the atmosphere. This pressure is driven by the movement of cold and warm air. Rain joins the party when cloud droplets become so large that the clouds can no longer support them. This causes them to fall to the ground and ruin your picnic. Lightning says hello when liquid and ice particles above the freezing level collide. This causes them to build up large electrical fields in the clouds. Once the electric fields become large enough, sparks of lightning occur (not so different from the static electricity that gives you a slight shock on your carpeted floor). Thunder accompanies lightning because it's actually caused by the bolt of electricity produced in a lightning strike.
Storms don't last forever because at some point, either the warm or cold air dissipates and equilibrium is reestablished. This means the winds die down and calm prevails once again. How long does this last? It depends on the storm. It can be as long as a week or as short as a couple of hours.
Is there calm?
This isn't exactly a yes or no question. Sometimes there is a calm before the storm, and sometimes there isn't — it depends on the conditions.
Here's how it happens when conditions are right: The fuel of a storm is the warm, moist air that's drawn in from the surrounding environment. When this air is pulled in, it creates a low-pressure vacuum. As air travels up and through the storm cloud, it takes the warm moist air with it. All of this air rises and eventually spills over the sides of the cloud. The air coming back down is warmer and dryer than it was on the way up and, therefore, more stable. Once enough of this air descends, it helps to stabilize the surrounding air. This is the calm before the storm. Of course, it's a relatively brief experience and has nothing to do with whether a storm will actually materialize.
So while not every storm can meet these conditions, they sometimes do, creating the eerie calm that has resulted in the famous saying.
What's a Supercell?
Supercells are some of the most powerful thunderstorms in nature. Although not all tornadoes are spawned from supercells, many of the most violent and deadly ones do originate from these types of storms. Supercells are also known to produce powerful winds and enormous chunks of hail, along with being responsible for many instances of severe flash flooding.
The Sight of a Supercell
While some supercells are characterized by heavy precipitation, the amount of rainfall generally depends on the conditions under which they formed. Supercells can happen in many geographical regions, but are very common in the central United States because the landscape favors collisions between cold arctic and warm tropical air — an important, though simplified precursor of supercell formation.
Supercells frequently last longer than other storms and are often identifiable on weather radar because they commonly feature a hook-shaped signature on one side. They also tend to have very distinct characteristics such as a section called the anvil, which forms if the cloud hits a certain altitude and the jet stream tugs at its top.
The Start of the Storm
For any storm to start, one basic tenet must be met: the rising of warm, moist air. That being said, there are a couple of ways this can be accomplished. Mountains can push the air skyward in a process known as orographic lifting, as can colliding or converging air fronts.
Basically, as warm humid air gains altitude, it cools and condenses, often forming clouds under the right conditions. When moisture condenses, it releases heat. In thunderstorms, this heat helps fuel further growth, by keeping the inside of the cloud warmer than the air surrounding it. Then the warm, moist air, now cool and dry, pours out the top and down the sides of the storm.
Supercells come in a couple varieties, but what unifies them is a feature called the mesocyclone — an extremely organized, persistently rotating updraft. These storms are often kick-started by variations in wind speed and direction — what's called wind shear. For example, if wind is blowing stronger at a higher altitude, and in the opposite direction from air at a lower altitude, this can cause the air to begin to rotate. Surface heating (the sun warming air close to the ground) can also help get things spinning, which is why storms are frequent on warm afternoons and evenings.
Updrafts eventually alter the wind shear pattern, shift the horizontal rotation to a vertical rotation, and the mesocyclone is born. A mesocyclone is often several miles wide and has a low-pressure core that sucks in further parcels of air like a vacuum cleaner hose. That low-pressure core, coupled with centrifugal force, makes mesocyclones experience what is commonly known as the dynamic pipe effect, a phenomenon that balances inward and outward airflow.
The Eye of the Tornado
The greater the rotation and the larger the contrast between the two original air masses, the more powerful supercells become. Not all mesocyclones produce tornadoes, but the ones that do are a force to be reckoned with.
Once a supercell has started cooking, it often fuels itself to even more magnificent heights in a positive feedback loop that usually works something like this: The mesocyclone pulls in increasing amounts of air, which intensifies its rotation. This in turn causes it to extend downward, which then makes it contract. Contracting causes it to rotate more quickly, which further leads to an increase in airflow.
Mesocyclones can stretch from the top of a supercell all the way to the bottom, sometimes forming what is referred to as a wall cloud. This is a cloud that extends below the generally flat base of a supercell and is where a tornado is most likely to sprout.