Introduction

Thunderstorms are deep convective clouds that have a large vertical extent all the way from the boundary layer to the tropopause. Thunderstorms often bring a variety of severe weather such as heavy rain, hail, lightning, damaging winds, and, occasionally, tornados. Thunderstorms are sometimes thought to be synonymous with cumulonimbus clouds, though not all cumulonimbus clouds are thunderstorms.

Thunderstorms

The primary energy that drives thunderstorms is the conversion of moist air into clouds and precipitation, which releases significant amounts of latent heat in the condensation process. Over land, thunderstorms occur most frequently in the late afternoon and early evening. Storms are most likely to occur soon after the warmest surface temperatures, which helps to give warm air parcels the initial buoyancy they need to begin to rise. Thunderstorms can last anywhere from less than an hour to more than 12 hours. Because of their rapid growth, relatively small (mesoscale) size, and sensitivity to environmental conditions, it is difficult to predict the exact time and location of a storm. Typically a forecast will say there is a chance of thunderstorms over an area—meaning that although conditions are conducive for a storm, the forecasters don’t know exactly where or when it will occur.

Thunderstorm Life Cycle

The three main stages of a thunderstorms are as follows:

1. Developing stage
2. Mature stage
3. Dissipating stage

In the developing stage, rising air called “updrafts” are dominant in this stage and towering cumulus clouds form. This type of cloud is also known as a cumulus congestus. Next is the mature stage where the towering cumulus becomes 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 as they fall, creating a downward current of air in 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 has thunder and lightning, it is called a thunderstorm. Lightning is caused by a charge separation in clouds. For this charge separation to occur, a cloud must have ice crystals inside of it, which is why thunderstorms are always mixed-phase or ice-phase clouds.

Airmass Thunderstorm

The thunderstorm described above is called an airmass thunderstorm. In general, the ingredients necessary for a thunderstorm to occur are warm moist air, some type of instability, and a trigger that can initiate the storm system. Airmass thunderstorms typically occur in environments with similar properties and little wind shear. Airmass thunderstorms are the most benign types of thunderstorms and typically short-lived, lasting less than a few hours.

The reason airmass thunderstorms are so short-lived and benign is because the storm shuts off its ability to maintain itself. This is due to two primary reasons:

1. Below 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 to drive further convection, the storm loses its strength and dissipates.
2. Rain falls within the updraft of the storm, reducing its strength, again causing dissipation.

Another diagram of an airmass thunderstorm or single cell thunderstorm is shown below. The initial small cumulus cloud grows to a cumulus congestus, which has mostly updrafts. It then grows further into a mature cumulonimbus cloud with well defined updrafts and downdrafts. The final stage is the dissipating stage, which has mostly downdrafts.

The trigger for a thunderstorm can be a warm humid air mass heated from the bottom by daytime solar heating or forced lifting by terrain (orographic thunderstorms). Collision of airmasses can be another trigger, as you might find along a warm front, cold front, or dryline.

Thunderstorm Features

The main features of a mature thunderstorm includes updrafts, downdrafts, overshooting tops, and an anvil. An overshooting top occurs at the top of the thunderstorm. Overshooting tops are dome-shaped as a result of strong updrafts in intense thunderstorms reaching the tropopause and pushing 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.

Severe Thunderstorm

While basic airmass thunderstorms typically consist of one “cell”, severe storms often contain multiple cells, consisting of more than one rising thermal connected with the same storm system. In a multicell storm, each cell is typically at a different life cycle stage and they continue on one from another.

In order for a thunderstorm to become severe, one important additional ingredient is necessary. In addition to warm moist air, some sort of 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 easy; wind shear helps to 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 much longer than airmass thunderstorms. Severe thunderstorms come in many different forms. A few will be discussed below.

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 linear propagating severe thunderstorm. Squall lines consist of a long, narrow line of merged thunderstorms, often triggered by a cold front, dry line, or gust front. Squall lines can be many hundreds of kilometers long, but their width is usually much shorter, between 15 and 100 km. They can sustain themselves from several hours to several days.

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 surface of Earth, it spreads laterally, literally the same as 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 Complex

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

Supercell

A supercell is a special type of severe storm that has rotation. 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 very clearly defined structure, which allows them to sometimes form tornados. As shown in the below image, supercells have additional features like 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.

Here is a diagramed image of an actual supercell thunderstorm that created a tornado. Remember that supercells are rotating storm systems that can produce tornados, but they don’t always. In fact, very few supercells produce tornados, but the ones that do get the most attention!

Here is one last image showing the structure of a supercell from the top view. The image shows the location of the gust front, the location of various precipitation types, and the location of downdrafts within the storm system.

Thunderstorm Formation

Let’s review. The ingredients needed for thunderstorm formation include high humidity, conditional instability, and a trigger that initiates rising air. A symmetric short-lived storm is called an airmass thunderstorm. When we add wind shear to an airmass thunderstorm, a severe thunderstorm can result. We discussed three types of severe thunderstorms including squall lines, MCCs, and supercells. Supercells are a type of severe storm with rotation resulting from directional wind shear in the environment.

High humidity in the atmospheric boundary layer is required for thunderstorms to occur. When water vapor condenses, latent heat is released. Latent heat is the primary energy source for thunderstorms. The higher the humidity, the more latent heat is released and the stronger the thunderstorm becomes.

Instability is necessary for convection to occur. The best case scenario is conditional instability, which often occurs when cold air lies above warm moist air capped by a temperature inversion. Cold air in the upper troposphere gives greater buoyancy to the updraft of warmer air from below and, therefore, a greater chance for strong thunderstorms to occur.

Wind shear is the change in wind speed and or wind direction with height. In an environment with wind but without wind shear, thunderstorms would last between 15 minutes and 1 hour as the thunderstorm and boundary-layer air would remain together. In this scenario, an airmass thunderstorm dissipates after it deprives itself of heat and moisture in the boundary-layer air. Strong wind shear pushes thunderstorms away from the depleted boundary-layer air and into areas with warm, humid boundary-layer air. Thus, strong wind shear helps thunderstorms sustain themselves for longer durations and to grow stronger.

A trigger refers to any process that forces an air parcel to rise through a cap of stable air and result in thunderstorm development. Triggers can be caused by frontal lifting, orographic effects, or surface heating.

Atmospheric Instability and Thunderstorms

Because the potential for thunderstorms to develop depends on atmospheric stability and layering, atmospheric soundings (e.g., Skew-T log-P) are used by meteorologists to help forecast storms. The soundings receive their data from the rawinsonde balloon launches, aircraft observations, dropsondes, satellites, or other meteorological data sources.

In atmospheric soundings, there are several labels indicating atmospheric stability. The labels are lifted condensation level (LCL), level of free convection (LFC), equilibrium level (EL), convective available potential energy (CAPE), and convective inhibition (CIN).

Lifted Condensation Level (LCL) was discussed in previous chapters. It is the altitude where the temperature cools to the dew point temperature, resulting in saturation and condensation. The LCL is the location where the cloud base of a thunderstorm develops.

Level of Free Convection (LFC) is the height at which the environmental temperature rate decreases faster than the air parcel’s moist adiabatic lapse rate. This leads to atmospheric instability. In an atmospheric sounding, the LFC can be found where the moist adiabatic lapse rate of the rising parcel returns to the buoyant or warmer side of the environmental temperature.

Equilibrium Level (EL) is the height in the atmosphere where the temperature of the rising air parcel is the same as the temperature of its surroundings. The EL caps the atmospheric instability. This is where the moist adiabatic lapse rate returns to the negatively buoyant colder side of the environmental temperature. This is also where the anvil top of the thunderstorm is typically located.

Convective Inhibition (CIN) is the amount of energy that prevents a rising air parcel from reaching the level of free convection. On a thermodynamic diagram (Skew-T Log-P), it is the negative area between the environmental lapse rate and the air parcel lapse rate. CIN must be overcome for CAPE to be realized.

Convective Available Potential Energy (CAPE) is the amount of energy an air parcel would have if lifted vertically through the atmosphere over a particular distance. It is an indicator of atmospheric instability. On a thermodynamic diagram (Skew-T Log-P), CAPE can be found by the positive area between the environmental lapse rate and the air parcel lapse rate. It is an integrated measure of the total amount of buoyancy available to a rising air parcel.

CAPE can be used to estimate the maximum updraft velocity in thunderstorms.

While the above equation gives a good estimate, the updraft velocity equation typically gives an unrealistically high value as it ignores many important processes. For example, dry air entrainment, liquid-water loading, and frictional drag are ignored. Observational studies find that the typical updraft velocity is roughly half of the maximum updraft velocity value.

Thunderstorms are important features of Earth’s atmospheric system. It is safe to say that there is always a thunderstorm occurring somewhere on Earth at all times. In many places, thunderstorms provide needed rain and are an important piece of the hydrological cycle. However, thunderstorms can also be hazardous and impacts will be discussed in the following chapter.