An air-mass thunderstorm, also called an "ordinary", "single cell", or "garden variety"

thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are some ...

, is a thunderstorm that is generally weak and usually not severe. These storms form in environments where at least some amount of

Convective Available Potential Energy In meteorology, convective available potential energy (commonly abbreviated as CAPE), is the integrated amount of work that the upward (positive) buoyancy force would perform on a given mass of air (called an air parcel) if it rose vertically thro ...

(CAPE) is present, but very low levels of

wind shear Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizont ...

and helicity. The lifting source, which is a crucial factor in thunderstorm development, is usually the result of uneven heating of the surface, though they can be induced by

weather front A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Disturbed and unstable weather due to these differences often arises along the boundary. For in ...

s and other low-level boundaries associated with wind

convergence Convergence may refer to: Arts and media Literature *''Convergence'' (book series), edited by Ruth Nanda Anshen *Convergence (comics), "Convergence" (comics), two separate story lines published by DC Comics: **A four-part crossover storyline that ...

. The energy needed for these storms to form comes in the form of

insolation Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ...

, or solar radiation. Air-mass thunderstorms do not move quickly, last no longer than an hour, and have the threats of

lightning Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions, both in the atmosphere or with one on the ground, temporarily neutralize themselves, causing the instantaneous release of an average ...

, as well as showery light, moderate, or heavy rainfall. Heavy rainfall can interfere with

microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...

transmissions within the atmosphere. Lightning characteristics are related to characteristics of the parent thunderstorm, and could induce

wildfire A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identi ...

s near thunderstorms with minimal rainfall. On unusual occasions there could be a weak

downburst In meteorology, a downburst is a strong downward and outward gushing wind system that emanates from a point source above and blows radially, that is, in straight lines in all directions from the area of impact at surface level. Capable of pro ...

and small

hail Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...

. They are common in temperate zones during a summer afternoon. Like all thunderstorms, the mean-layered wind field the storms form within determine motion. When the deep-layered wind flow is light,

outflow boundary An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air ( outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually ...

progression will determine storm movement. Since thunderstorms can be a hazard to aviation, pilots are advised to fly above any haze layers within regions of better visibility and to avoid flying under the anvil of these thunderstorms, which can be regions where hail falls from the parent thunderstorm. Vertical

is also a hazard near the base of thunderstorms which have generated outflow boundaries.

Life cycle

The trigger for the lift of the initial cumulus cloud can be

heating the ground producing

thermals A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...

, areas where two winds converge forcing air upwards, or where winds blow over terrain of increasing elevation. The moisture rapidly cools into liquid drops of water due to the cooler temperatures at high altitude, which appears as ''

cumulus Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin ''cumulo-'', meaning ''heap'' or ''pile''. Cumulus clouds are low-level clouds, gener ...

'' clouds. As the water vapor condenses into liquid,

latent heat Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition. Latent heat can be underst ...

is released which warms the air, causing it to become less dense than the surrounding dry air. The air tends to rise in an ''updraft'' through the process of

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the c ...

(hence the term

convective precipitation In meteorology, the different types of precipitation often include the character, formation, or phase of the precipitation which is falling to ground level. There are three distinct ways that precipitation can occur. Convective precipitation is g ...

). This creates a low-pressure zone beneath the forming thunderstorm, otherwise known as a

cumulonimbus cloud Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. ...

. In a typical thunderstorm, approximately 5×10⁸ kg of water vapor is lifted into the

Earth's atmosphere The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...

. As they form in areas of minimal vertical

, the thunderstorm's rainfall creates a moist and relatively cool outflow boundary which undercuts the storm's low level inflow, and quickly causes dissipation.

Waterspout A waterspout is an intense columnar vortex (usually appearing as a funnel-shaped cloud) that occurs over a body of water. Some are connected to a cumulus congestus cloud, some to a cumuliform cloud and some to a cumulonimbus cloud. In the ...

s, small hail, and strong wind gusts can occur in association with these thunderstorms.

Common locations of appearance

Also known as single cell thunderstorms, these are the typical summer thunderstorms in many temperate locales. They also occur in the cool unstable air which often follows the passage of a

cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern ...

from the sea during winter. Within a cluster of thunderstorms, the term "cell" refers to each separate principal updraft. Thunderstorm cells occasionally form in isolation, as the occurrence of one thunderstorm can develop an outflow boundary which sets up new thunderstorm development. Such storms are rarely severe and are a result of local atmospheric instability; hence the term "air mass thunderstorm". When such storms have a brief period of severe weather associated with them, it is known as a pulse severe storm. Pulse severe storms are poorly organized due to the minimal vertical wind shear in the storm's environment and occur randomly in time and space, making them difficult to forecast. Between formation and dissipation, single cell thunderstorms normally last 20–30 minutes.

Motion

The two major ways thunderstorms move are via advection of the wind and propagation along outflow boundaries towards sources of greater heat and moisture. Many thunderstorms move with the mean wind speed through the Earth's

troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. Fro ...

, or the lowest of the

. Younger thunderstorms are steered by winds closer to the Earth's surface than more mature thunderstorms as they tend not to be as tall. If the gust front, or leading edge of the outflow boundary, moves ahead of the thunderstorm, the thunderstorm's motion will move in tandem with the gust front. This is more of a factor with thunderstorms with heavy precipitation (HP), such as air-mass thunderstorms. When thunderstorms merge, which is most likely when numerous thunderstorms exist in proximity to each other, the motion of the stronger thunderstorm normally dictates future motion of the merged cell. The stronger the mean wind, the less likely other processes will be involved in storm motion. On

weather radar Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly pulse- ...

, storms are tracked by using a prominent feature and tracking it from scan to scan.

Convective precipitation

Convective rain, or showery precipitation, occurs from cumulonimbus clouds. It falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds such as thunderstorms have limited horizontal extent. Most precipitation in the

tropics The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also refer ...

appears to be convective.

Graupel Graupel (; ), also called soft hail, hominy snow, or snow pellets, is precipitation that forms when supercooled water droplets in air are collected and freeze on falling snowflakes, forming balls of crisp, opaque rime. Graupel is distinct fro ...

and

are good indicators of convective precipitation and thunderstorms. In mid-latitudes, convective precipitation is intermittent and often associated with baroclinic boundaries such as

squall line A squall line, or more accurately a quasi-linear convective system (QLCS), is a line of thunderstorms, often forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front (which often are accom ...

s, and

warm front A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold f ...

s. High rainfall rates are associated with thunderstorms with larger raindrops. Heavy rainfall leads to fading of microwave transmissions starting above the frequency of 10 gigahertz (GHz), but is more severe above frequencies of 15 GHz.

Lightning

Relationships between lightning frequency and the height of precipitation within thunderstorms have been found. Thunderstorms which show radar returns above in height are associated with storms which have more than ten lightning flashes per minute. There is also a correlation between the total lightning rate and the size of the thunderstorm, its updraft velocity, and amount of graupel over land. The same relationships fail over tropical oceans, however. Lightning from low precipitation (LP) thunderstorms is one of the leading causes of

Aviation concerns

In areas where these thunderstorms form in isolation and horizontal visibility is good, pilots can evade these storms rather easily. In more moist atmospheres which become hazy, pilots navigate above the haze layer in order to get a better vantage point of these storms. Flying under the anvil of thunderstorms is not advised, as

is more likely to fall in such areas outside the thunderstorm's main rain shaft. When an outflow boundary forms due to a shallow layer of rain-cooled air spreading out near ground level from the parent thunderstorm, both speed and directional wind shear can result at the leading edge of the three-dimensional boundary. The stronger the outflow boundary is, the stronger the resultant vertical wind shear will become. T. T. Fujita (1985). "The Downburst, microburst and macroburst". SMRP Research Paper 210, 122 pp.

References

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External links

Glossary - NOAA's National Weather Service
Atmospheric electricity Weather hazards to aircraft Mesoscale meteorology Storm Weather hazards Severe weather and convection