All thunderstorms, from the smallest rumblers to the biggest supercells, go through the same 3-stage lifecycle. Like anything that is actually living, thunderstorms do require a healthy balance and a constant fuel source to maintain them.
Stage 1: Cumulus Stage
If you’ve ever been outside on a nice day, you’ve probably seen fair weather cumulus clouds. All thunderstorms start off as similar cumulus clouds, but do require that the atmosphere be unstable. In a stable atmosphere (a nice bright sunny day), an air parcel that is forced upward will sink back its initial position, preventing updrafts from forming. In an unstable atmosphere, an air parcel that is forced upwards will continue to accelerate upward, creating an updraft. The faster the parcel accelerates upwards, the stronger the updraft is.
In the cumulus stage, a thunderstorm consists solely of updrafts and has very few restrictions for strengthening, as it feeds off of the warm, moist air underneath it. If updrafts are strong enough, these cumulus towers can reach heights of 50,000 feet.
Stage 2: Mature Stage
As you increase in height in the atmosphere, the temperature drops. Cold air cannot hold as much moisture as warm air, so moisture in the air begins to condense into water droplets (or ice crystals). These water droplets are held in the cloud by the updraft. As more and more water condenses, it gets heavier and heavier, and eventually the updraft will not be strong enough to hold it in the cloud. When this happens, the condensation falls to the ground as precipitation, either as rain or hail, depending on what temperatures are in the cloud. As precipitation falls, it is accelerated downward by gravity, forming a downdraft. A thunderstorm at its mature stage contains both an updraft and a downdraft.
At the top of the storm, the updraft rapidly slows down and spreads out, forming an anvil shape. The height of the anvil is the boundary between stable air above the anvil and unstable air beneath the anvil. On days that updrafts are particularly strong, the updraft will overshoot the anvil and into the stable air before falling back down to the anvil and spreading out. Meteorologists refer to this as the “overshooting top,” which is commonly seen in supercells. The area underneath the overshooting top will often spawn severe weather.
Stage 3: Dissipating Stage
In a typical thunderstorm, the downdraft will begin to form on top of the updraft and they will begin to fight (think of it as a “push-of-war”). Unfortunately for the updraft, the downdraft has both gravity and the heavier weight of the precipitation on its side, so the downdraft will always win. Adding insult to injury, the falling precipitation will also pull cold air down from aloft and cuts off the storm’s fuel supply of warm, moist air (remember, cold air means less moisture too). Visually, as the updraft weakens, the anvil and overshooting top will disappear from the top of the storm. The base of the storm will begin to disintegrate and eventually all you are left with is a rain shaft.
This ends the basic lifecycle of a thunderstorm. I will now go into talking about some of the basic phemonena that cause severe thunderstorms to form, so if you are interested, please continue reading.
Wind Shear: The Secret Ingredient for Severe Thunderstorms and Tornadoes
There are two types of wind shear: speed shear and directional shear. I’ll start with speed shear. Speed shear is different wind speeds at different heights, with roughly the same wind direction (stronger winds aloft). In a non-sheared environment, the downdraft and updraft of a thunderstorm fight over the same area, and the downdraft always winds, which chokes off the storm’s fuel supply and causes it to dissipate. In a sheared environment, the updraft and downdraft begin to separate, allowing the storm to continue to tap into its fuel supply. As the updraft and downdrafts separate, they interact with each other less and less, and thus allow for the storm to tap into more and more of its fuel supply. When speed shear is strong enough to separate the updraft and downdraft so they no longer interact with each other, the updraft has no restrictions on its access to its fuel supply since the precipitation falls far enough away from it. The result here is often stronger severe thunderstorms as well as supercells. Speed shear alone, however, is still not enough to spawn tornadoes.
In order to spawn tornadoes, directional wind shear is needed. Directional wind shear is exactly what you’d expect: different wind directions at different heights. Directional shear is maximized when the difference in wind direction is 90 degrees (a right angle). If you take a column of air and push it in different directions at different heights, it will begin to spin. This rotation is called the mesocyclone. If the directional shear is strong enough, the mesocyclone will begin to tighten, much like a spinning figure skater pulling their arms in. As it tightens, it rotates faster and faster and will eventually begin to lower into a funnel cloud. As soon as the funnel cloud touches the ground you have a tornado. Since the predominantly southwest-to-northeast flow from the jet stream dominates the upper atmosphere, directional shear is usually caused by winds in the lower atmosphere. The low-level jet as well as flow from surface low pressure and high pressure systems are two common contributors to directional wind shear. If you couple strong vertical shear with strong directional shear, the result is usually the long-track supercells that produce violent tornadoes.
Severe Straight-Line Winds
As precipitation falls, it pulls cold air down with it, which is accelerated by gravity. The general rule of thumb is that the taller the cloud is, the faster the precipitation will be travelling as it heads toward the ground. When the air being pulled down with the precipitation hits the ground, it spreads out in all directions, forming a gust front. The strongest part of the gust front is where the wind is blowing from the same direction the storm is travelling from. From this information, you can probably figure out that the faster the air is traveling as it hits the ground, the stronger the gust front will be. When these winds reach 58 mph or greater, they are classified as severe wind. In certain scenarios, a burst of air will accelerate very rapidly down from the cloud as spread out very quickly in all directions. This phenomenon is called a microburst, which have been known to cause extensive damage in non-tornadic thunderstorms. I’ll leave you with this photo of straight line winds from a squall line associated with the West Florida Tornado Outbreak on March 31, 2011.