10.1: Thunderstorms formation and types

Thunderstorm Life Cycle

The three main stages of a thunderstorm are illustrated in Figure \(\PageIndex{1}\), and are as follows:

1. Developing (cumulus) stage
2. Mature stage
3. Dissipating stage

In the developing stage, rising air currents, known as “updrafts,” are dominant, and cumulus clouds with some vertical growth are formed. This type of cloud is also known as a cumulus congestus. Next is the mature stage, where the growth continues, resulting in a towering cumulus, known as a cumulonimbus cloud, featuring both updrafts and “downdrafts” or sinking air. Downdrafts develop as a result of water droplets falling to the ground as precipitation (e.g., snow, rain, hail). The falling drops drag air downward, creating a descending air current within the cumulonimbus cloud. Finally, in the dissipating stage, downdrafts are dominant and cut off the updrafts needed for a cumulonimbus cloud to form and sustain itself. As a result, the cloud dissipates.

If a cumulonimbus cloud is accompanied by thunder and lightning, it is referred to as a thunderstorm. A charge separation in clouds causes lightning. For this charge separation to occur, a cloud must have ice crystals inside of it, which is why thunderstorms are always mixed-phase or contain both ice and liquid droplets.

Figure \(\PageIndex{1}\): The three stages of thunderstorm formation include the developing, mature, and dissipating stages. (Public Domain; NOAA via Wikimedia Commons). Alternative Description of Image.

Airmass Thunderstorm

The thunderstorm described in Figure \(\PageIndex{1}\) is called an airmass thunderstorm. In general, the ingredients necessary for a thunderstorm to occur are warm, moist air, some instability, and a trigger that can initiate the storm system. Airmass thunderstorms typically occur in environments with similar properties and little wind shear. They typically occur in summer, are benign, and last for only a few hours. They are also known as single-cell thunderstorms, where one updraft and one downdraft together form a convective cell. The reason airmass thunderstorms are so short-lived and benign is that they lose the ability to maintain themselves. This is due to two primary reasons:

1. Below the cloud base, the falling rain evaporates, cooling the air in the boundary layer. The storm deprives itself of the heat and moisture in the boundary-layer air. Without the warm, buoyant air that drives further convection, the storm loses strength and dissipates.
2. Rainfall within the storm updraft reduces the storm's strength and further dissipates it.

Another diagram of an air mass thunderstorm or single cell thunderstorm is illustrated in Figure \(\PageIndex{2}\). The initial small cumulus cloud develops into a cumulus congestus, characterized by predominantly updrafts. It then develops further into a mature cumulonimbus cloud, characterized by well-defined updrafts and downdrafts. An overshooting top often occurs at the top of the thunderstorm during the mature stage. Overshooting tops are dome-shaped due to strong updrafts in intense thunderstorms that reach the tropopause and then push upward into the stable region. However, ultimately, because the tropopause is so stable, the air gets pushed back downward and spread laterally. This lateral spreading forms the cloud anvil at the top of the thunderstorm along the tropopause boundary. The thunderstorm is most intense during its mature stage, characterized by heavy rainfall, lightning, and thunder. The final stage is the dissipating stage, characterized by predominantly downdrafts. During the dissipating stage, the towering cumulonimbus begins to evaporate from the base upward, as downdrafts cool the surface and deprive the storm of its heat source. Often, only the anvil top remains after dissipation.

Figure \(\PageIndex{2}\): The typical life cycle of a thunderstorm consists of primarily updrafts during the developing stage, both updrafts and downdrafts during the mature stage, and primarily downdrafts during the dissipating stage. (CC BY-SA 4.0; Antonio Ciccolella via Wikimedia Commons). Alternative description of the image

The trigger for a thunderstorm can be a warm, humid air mass heated from below by daytime solar heating or forced to rise by terrain (orographic thunderstorms). The collision of air masses along a warm front, a cold front, or a dryline can also be a trigger.

Severe Thunderstorm

While basic airmass thunderstorms typically consist of one “cell”, severe storms often contain multiple cells, each with a separate rising thermal associated with the same storm system. In a multicell storm, each cell typically occupies a different life-cycle stage and continues to develop through successive stages.

For a thunderstorm to become severe, one additional important ingredient is necessary. In addition to warm, moist air, some instability, and a trigger, severe thunderstorms need wind shear. Wind shear is defined as a change in wind speed or direction with altitude. The reason wind shear is important for severe storms is that it helps tilt a storm, such that the updraft and downdraft are displaced from one another. Without the competition between upward-moving air and downward-moving air, severe thunderstorms can strengthen and last much longer than air-mass thunderstorms. Severe thunderstorms take many forms.

Severe Thunderstorm Types

The three main types of thunderstorms include squall lines, mesoscale convective complexes (MCCs), and supercells.

Squall Line

A squall line is a line of linearly propagating severe thunderstorms. Squall lines consist of a long, narrow line of merged thunderstorms, often triggered by a cold front, dry line, or gust front as depicted in Figure \(\PageIndex{3}\). Squall lines can be many hundreds of kilometers long, but their width is usually much shorter, ranging from 15 to 100 km. They can sustain themselves for several hours to several days.

Figure \(\PageIndex{3}\): A squall line is a line of thunderstorms, often forming along or ahead of a cold front. (Public Domain; Severe Weather 101 via NOAA/NSSL). Alternative description of the image.

A squall line continues propagating with the help of a gust front. When rain falls into the drier environment below the cloud base, it evaporates, cooling the air. This cool air continues to move downward, sometimes with strong force and reinforcement from a continuous downdraft. When the cool downdraft hits the Earth's surface, it spreads laterally, much like a density current. Because it is relatively cool, it acts like a mini cold front, pushing warmer air up over it as it moves along. This upward push can initiate convection and continue fueling the larger storm system.

Mesoscale Convective Systems

A Mesoscale Convective System (MCS) is a collection of thunderstorms that act together as a system, as depicted in Figure \(\PageIndex{4}\). An MCS can span an entire state and persist for more than 12 hours. On radar, this system may appear as a solid line, a broken line, or a cluster of cells.

Figure \(\PageIndex{4}\): Radar reflectivity from a Mesoscale Convective System over Pennsylvania with a trailing squall line. (Public Domain; David Roth via Wikimedia Commons). Alternative description of the image.

A mesoscale convective complex (MCC) is a collection of thunderstorms of the severe type. The storm system features a cloud shield (anvil) with a diameter of at least 350 km, typically elliptical or circular, and lasts 6-12 hours. MCCs are huge storms that occur multiple times per year, especially in the central United States. They are truly impressive convective masses.

Supercell

A supercell is a type of severe storm characterized by rotation as illustrated in Figure \(\PageIndex{5}\). Supercells form when the environment has directional wind shear, causing updrafts within a cloud to gain vorticity as they rise. While all severe storms have a structure that separates the updraft from the downdraft, supercell storms have a clearly defined structure that enables them to produce tornadoes. The precipitation is located below the shelf cloud and away from the tornado. Supercells have additional features, such as a wall cloud and a flanking line. The wall cloud exists beneath the cloud base and near the border between the downdraft of cold air and the lower-level inflow of warm, humid air. The flanking line is the region of towering cumulus that indicates extensive updrafts ahead of intense thunderstorms. Just like other cumulonimbus clouds, the supercell has an anvil near the top. But the updraft in such a rotating mega storm can be strong enough to overshoot the anvil and deposit moisture into the stratosphere.

Figure \(\PageIndex{5}\): A Supercell consists of a rotating cumulonimbus core with precipitation, such as light rain, heavy rain, and hail, pushed downwind by wind shear. The top of the cloud is an anvil-shaped feature, with some overshooting top clouds. (CC BY-SA 3.0; Vanessa Ezekowitz via Wikimedia Commons). Alternative description of the image.

Figure \(\PageIndex{6}\) is an annotated version of an actual supercell thunderstorm that created a tornado. Remember that supercells are rotating storm systems that can produce tornadoes, but they don’t always. In fact, very few supercells produce tornadoes, but the ones that do receive the most attention, since they are the most destructive.

Figure \(\PageIndex{6}\): An annotated image of a supercell cumulonimbus cloud depicting the wall cloud and tornado near the base and an anvil near the top. Precipitation occurs away from the rotating core. (Public Domain; NOAA via Wikimedia Commons). Alternative description of the image.