An overshooting top (or penetrating top) is a dome-like protrusion shooting out of the top of the

anvil An anvil is a metalworking tool consisting of a large block of metal (usually Forging, forged or Steel casting, cast steel), with a flattened top surface, upon which another object is struck (or "worked"). Anvils are massive because the hi ...

of a

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

and into the lower

stratosphere The stratosphere () is the second-lowest layer of the atmosphere of Earth, located above the troposphere and below the mesosphere. The stratosphere is composed of stratified temperature zones, with the warmer layers of air located higher ...

. When an overshooting top is present for 10 minutes or longer, it is a strong indication that the storm is severe.Chance Hayes, National Weather Service Wichita, Kansas. "Storm Fury on the Plains." Storm Spotter Training. 4H Building, Salina, Kansas. 22 Feb. 2010. Lecture.

Formation

Top of cumulo nimbus viwed from an airplane above DRC

When a thunderstorm forms, clouds build vertically into the atmosphere until the storm's

updraft In meteorology, an updraft (British English: ''up-draught'') is a small-scale air current, current of rising air, often within a cloud. Overview Vertical drafts, known as updrafts or downdrafts, are localized regions of warm or cool air that mov ...

(warm rising air) has reached an equilibrium level (EL); the point where the surrounding air is about the same temperature or even warmer. This point of equilibrium is often marked by the

tropopause The tropopause is the atmospheric boundary that demarcates the lowest two layers of the atmosphere of Earth – the troposphere and stratosphere – which occurs approximately above the equatorial regions, and approximately above the polar regi ...

. Rather than continuing to rise into the stratosphere, the vertical cloud growth abruptly stops, and instead clouds spread horizontally, forming an "anvil" shape on top of the thunderstorm. An overshooting top forms when a thunderstorm's updraft, due to

momentum In Newtonian mechanics, momentum (: momenta or momentums; more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. ...

from rapid ascent and strength of lifting through the free convective layer (FCL), protrudes its equilibrium level, forming a dome-like structure above the anvil. This can occur with any

cumulonimbus cloud Cumulonimbus () 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. Above the lower portions of the cumulonimbus the water ...

when instability is high. Whereas anvils form at the equilibrium level, overshooting tops continue to the maximum parcel level (MPL).

Above-anvil cirrus plume

The strong updrafts delineated by overshooting tops can act as a barrier against the surrounding flow of air. Fast stratospheric winds may rise slightly upon encountering overshooting tops, cooling and producing a turbulent wake of cooler temperatures downstream of the updraft. This interaction also sheds ice and water vapor from the anvil cloud, forming a plume of cirrus emanating from the updraft region, though this is most evident in mid-latitude environments where the tropopause is typically lower and the associated inversion wider. These cirrus plumes may be warmer than the underlying anvil cloud due to the mixing of air from the warmer

. Termed above-anvil cirrus plumes (AACP), the emergence of such features on satellite imagery have been associated with severe weather events. A 2018 study published in ''

Weather and Forecasting ''Weather and Forecasting'' is a scientific journal published by the American Meteorological Society. Articles on forecasting and analysis techniques, forecast verification studies, and case studies useful to forecasters. In addition, submissio ...

'' found that 73 percent of significant severe weather reports in the United States were associated with storms producing AACPs and that AACPs emerged an average of 31 minutes before the issuance of severe weather warnings. Simulations suggest that overshooting tops behave like hydraulic jumps in the presence of strong winds aloft, allowing the transport of over of water vapor into the lower stratosphere per second.

Severe weather

Many thunderstorms exhibit an overshooting top at some point in their life cycle. In weaker thunderstorms, the overshooting top is short-lived, and often takes on a wispy appearance. If the overshooting top is rising and falling in a cyclical fashion with each protrusion persisting only a few minutes, then it could indicate the storm is pulsing and not as strong as a storm with a continuous overshooting top. An overshooting top lasting for more than 10 minutes is a sign of a strong updraft in a thunderstorm, indicating a strong likelihood the storm is producing

severe weather Severe weather is any dangerous meteorological phenomenon with the potential to cause damage, serious social disruption, or loss of human life. These vary depending on the latitude, altitude, topography, and atmospheric conditions. High ...

. If the overshooting top is continuous, it's an indication of enhanced probability that the storm is a

supercell A supercell is a thunderstorm characterized by the presence of a mesocyclone, a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms (su ...

, i.e. a rotating storm. During a strong

tornado A tornado is a violently rotating column of air that is in contact with the surface of Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, although the ...

, the overshooting top may roll or fold over as new activity climbs up the back while the front of the overshooting top collapses into the storm. During a long-track tornado, the entire top of the storm, including the overshooting top, may drop by thousands of feet.

Storm features

A storm powerful enough to produce a lasting overshooting top typically produces the following: * Heavy

rain Rain is a form of precipitation where water drop (liquid), droplets that have condensation, condensed from Water vapor#In Earth's atmosphere, atmospheric water vapor fall under gravity. Rain is a major component of the water cycle and is res ...

; a deluge of

could fall from this storm in a short amount of time. * Strong straight-line

wind Wind is the natural movement of atmosphere of Earth, air or other gases relative to a planetary surface, planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heatin ...

from

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. It originate ...

s; the storm clouds have powerful winds churning inside them. These winds are likely to be felt at the surface and also a threat to aviation aloft. * Sometimes a

may form; most strong tornadoes are associated with

mesocyclone A mesocyclone is a meso-gamma mesoscale (or storm scale) region of rotation ( vortex), typically around in diameter, most often noticed on radar within thunderstorms. In the Northern Hemisphere, it is usually located in the right rear flank ( ...

s located near the interchange of the rotating updraft and the

rear flank downdraft The rear flank downdraft (RFD) is a region of dry air wrapping around the back of a mesocyclone in a supercell thunderstorm. These areas of descending air are thought to be essential in the production of many supercellular tornadoes. Large hail ...

(RFD). *

Hail Hail is a form of solid Precipitation (meteorology), 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 hailsto ...

; if the updraft is strong enough to produce an overshooting top it can also carry large

hail Hail is a form of solid Precipitation (meteorology), 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 hailsto ...

. * A storm that features an overshooting top is a

, and is thus likely to produce

lightning Lightning is a natural phenomenon consisting of electrostatic discharges occurring through the atmosphere between two electrically charged regions. One or both regions are within the atmosphere, with the second region sometimes occurring on ...

References

External links