Thunderstorms and atmospheric instability
Thunderstorms are complex weather phenomena that can have both beneficial and harmful effects, typically forming under warm and humid conditions. They contribute essential rainfall to drought-affected areas but can also lead to severe outcomes, such as flooding, hail damage, and even wind-related disasters like tornadoes. Each year, thunderstorms cause significant economic damage and result in numerous fatalities, primarily from lightning strikes and flash flooding.
The formation of thunderstorms relies on three primary factors: heat, instability, and moisture. As the sun heats the Earth's surface, pockets of warm air rise, contributing to the development of cumulonimbus clouds. The process involves several stages, starting from the cumulus stage to the mature phase, which can reach impressive heights and generate strong winds and precipitation. Thunderstorms are classified into various types, including single-cell, multicell, multicell lines, and supercells, each with distinct structures and potential for severity.
Current research indicates that climate change may influence the frequency and intensity of thunderstorms. While rising temperatures could lead to increased atmospheric instability in some regions, the overall effects on storm patterns—such as the potential for drier conditions or more intense events—remain complex and challenging to predict. Understanding these dynamics is vital for assessing future weather-related risks and impacts.
Subject Terms
Thunderstorms and atmospheric instability
Definition
Thunderstorms can be both beneficial and detrimental. Normally formed under warm summertime conditions, they can bring needed rainfall to dry, parched soils. They can also, however, deliver rain with such force that it can cause severe soil erosion and property damage by flooding. Additionally, hail from these storms can wipe out crops and kill people and livestock. Each year, thunderstorms cause over a billion dollars’ worth of damage to property, livestock, and crops in the United States. Tornadoes and other high wind events can also be spawned by these storms. However, lightning associated with thunderstorms is the major severe-weather-related cause of death in the United States. More deaths occur from lightning each year than from severe winter storms, hurricanes, and tornadoes combined. Thunderstorms are also associated with flash flooding, another major cause of U.S. weather-related deaths. Over one hundred U.S. deaths occur each year as a result of flash flooding due to thunderstorms, more than the average annual death count associated with tornadoes.
Worldwide, over two thousand thunderstorms are active at any given time. The basic preconditions for a thunderstorm are heat, instability, and moisture. Insolation from the Sun (heat) warms the land but does so inconsistently within the target zone of the storm. Some land surfaces may warm faster than others, creating pockets of warm and cool air. Where this differential heating warms the atmosphere, the air begins to rise, expand, and cool adiabatically. This rising, unstable air releases heat, which in turn generates greater atmospheric instability, more adiabatic cooling, and additional condensation. As this cycle continues, the storm builds.
Thunderstorms develop through a series of stages. The first stage is the cumulus or towering cumulus stage. Cumulus clouds are “cotton ball” type clouds, usually associated with fair weather. However, these clouds develop as a result of atmospheric instability and differential heating. During the cumulus stage, updrafts lift the air to levels at which condensation occurs, sometimes accompanied by precipitation. As precipitation begins to fall from the upper portions of the cloud, the mature stage emerges. This stage is characterized by both an updraft and a downdraft component within the storm. From its beginnings to the mature stage, the storm may reach as high as 6,096 meters.
As the storm develops, more precipitation falls. This precipitation cools the lower levels of the storm and cuts off its updraft component. This dissipating phase is the final stage in storm development. Thunderstorms can spawn other storms around themselves as their gust fronts—cold air flowing out of the main tower of the storm—mechanically lift warm air in the vicinity of the main storm. Thunderstorms at their full development can reach upward to over 12,200 meters. Winds from these storms can range from mild to severe. Downbursts and straight-line winds emerging from the gust front can reach over 97 kilometers per hour. Tornadoes from these storms can produce winds from 64 kilometers per hour to in excess of 312 kilometers per hour.
Thunderstorms can be classified into four general types: single cell storms; multicell, or cluster, storms; multicell lines; and supercell storms. These classifications relate to the characteristic structure of the storm, its duration, and its severity. Single cell storms are rare but can produce 20- to 30-minute storms. Multicell storms contain more than one storm in a cluster. Each storm is in different stages of development. Line storms are multicell clusters associated with squall lines and are notorious for producing heavy rain, large hail, and sometimes tornadoes. The supercell is a single cell multiple kilometers across. The energy of these storms can rotate the storm, producing a mesocyclone. Although rare, supercells pose the greatest threat of damage. High downburst winds in excess of 128 kilometers per hour are possible, along with baseball-sized hail and violent tornadoes. Studies from the 2020s predict that as anthropogenic factors continue to cause climate warming, hailstorm severity will also increase, resulting in even larger hail.
Significance for Climate Change
Additional heat added to the lower atmosphere through general warming of the climate might contribute to an increase in atmospheric instability; however, moisture must be present to generate a thunderstorm. Some studies suggest that precipitation in the tropics, a region known for thunderstorms and atmospheric instability, has increased as a result of warming temperatures. However, the increase in precipitation tends to be over oceanic areas rather than continental regions. In contrast, some climate models suggest that although fewer storms may occur over continental areas, their intensity may be increased by global warming—albeit without accompanying precipitation.
With some areas of the world expected to become drier as the climate changes, so-called dry storms could develop, producing more wind and more lightning. If warming is accompanied by drought, the potential for wildfires fueled by lightning could increase. Additionally, some models suggest that wind shear, the lateral movement of the wind, would give way to more violent updraft conditions. It is generally understood that the combination of wind shear and updraft has the potential to produce tornadic conditions in these kinds of storms. Some climatologists suggest that given the complexity of the physical geography and atmospheric environment, predicting the potential for increased tornadoes in these kinds of storms is not possible.
Bibliography
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Battan, Louis J. The Nature of Violent Storms. Garden City, N.Y.: Doubleday, 1961.
Raupach, Timothy H., et al. "The Effects of Climate Change on Hailstorms." Nature Reviews Earth & Environment 2.3 (2021): 213-226. Accessed Jan. 15, 2023.