Squall line

A weather radar image of a mesoscale convective vortex (MCV) over Pennsylvania with a leading squall line
Typical evolution of (a) into a bow echo (b, c) and into a comma echo (d). Dashed line indicates axis of greatest potential for downbursts. Arrows indicate wind flow relative to the storm.  Area C is most prone to supporting tornado development.
Cross-section of a squall line showing precipitation, airflow, and surface pressure
How a squall line is depicted by the NWS on weather maps
Shelf cloud on the leading edge of a derecho as photographed in Minnesota

Line of thunderstorms, often forming along or ahead of a cold front.

- Squall line

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Cloud and precipitation structure associated with an area of rainfall which is significantly elongated.

Band of thunderstorms seen on a weather radar display
A February 24, 2007 radar image of a large extratropical cyclonic storm system at its peak over the central United States. Note the band of thunderstorms along its trailing cold front.
Photograph of rainbands in Hurricane Isidore

Rainbands spawned near and ahead of cold fronts can be squall lines which are able to produce tornadoes.


Widespread, long-lived, straight-line wind storm that is associated with a fast-moving group of severe thunderstorms known as a mesoscale convective system.

A shelf cloud along the leading edge of a derecho in Minnesota
Development of derechos
Composite radar image of the June 2012 North American derecho (a progressive derecho) as it moved from Indiana to Virginia
A typical multi-bow serial derecho
A typical progressive derecho
This image shows derecho frequency for the lower 48 United States
Damage to the Väike-Maarja Church in Estonia after the derecho hit on 8 August 2010
Trees felled by downbursts in the Boundary Waters – Canadian derecho of 1999
Barn in Mount Solon, Virginia, destroyed by June 2012 North American derecho

In many cases, convection-induced winds take on a bow echo (backward "C") form of squall line, often forming beneath an area of diverging upper tropospheric winds, and in a region of both rich low-level moisture and warm-air advection.


Thunderstorm characterized by the presence of a mesocyclone: a deep, persistently rotating updraft.

A rotating thunderstorm updraft developing into a tornadic supercell along an outflow boundary from convection to the North.
A supercell. While many ordinary thunderstorms (squall line, single-cell, multi-cell) are similar in appearance, supercells are distinguishable by their large-scale rotation.
Schematic of a supercell's components.
Structure of a supercell. Northwestward view in the Northern Hemisphere
Diagram of supercell from above. RFD: rear flank downdraft, FFD: front flank downdraft, V: V-notch, U: Main Updraft, I: Updraft/Downdraft Interface, H: hook echo
Radar reflectivity map
Schematics of an LP supercell
A low precipitation supercell near Greeley, Colorado
Schematics of an HP supercell
High precipitation supercell.
Satellite view of a supercell
Photo of the 1947 Sydney Hailstorm showing the hail hitting the water at Rose bay

Of the four classifications of thunderstorms (supercell, squall line, multi-cell, and single-cell), supercells are the overall least common and have the potential to be the most severe.

Cumulonimbus cloud

Dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents.

Cumulonimbus calvus cloud in Monterrey, Mexico.
Partial view of a cumulonimbus cloud, possibly an arcus cloud.
Pyrocumulonimbus with pileus
Stages of a cumulonimbus cloud's life.
Transformation from a mature cumulus congestus cloud to a mature cumulonimbus incus
Cumulonimbus calvus
A clearly developed cumulonimbus fibrous-edged top capillatus
A freeze-frame of a Cumulonimbus cloud in the distance exposing a flash of lightning
Arcus cloud (shelf cloud) leading a thunderstorm
A cap (pileus) atop a congestus
Incus with a velum edge
Mammatocumulus with drooping pouches
A funnel cloud (tuba) over the Netherlands
Flanking line in front of a strong thunderstorm
An overshooting top is a dome of clouds atop a cumulonimbus
Cumulonimbus calvus against sunlight with rain falling beneath it as a rain shaft.
Rain evaporating before reaching the ground (virga)

Cumulonimbus can form alone, in clusters, or along squall lines.


Large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above .

An extratropical cyclone near Iceland on September 4, 2003
Comparison between extratropical and tropical cyclones on surface analysis
The initial extratropical low-pressure area forms at the location of the red dot on the image. It is usually perpendicular (at a right angle to) the leaf-like cloud formation seen on satellite during the early stage of cyclogenesis. The location of the axis of the upper level jet stream is in light blue.
Tropical cyclones form when the energy released by the condensation of moisture in rising air causes a positive feedback loop over warm ocean waters.
A fictitious synoptic chart of an extratropical cyclone affecting the UK and Ireland. The blue arrows between isobars indicate the direction of the wind, while the "L" symbol denotes the centre of the "low". Note the occluded, cold and warm frontal boundaries.
A polar low over the Sea of Japan in December 2009
Subtropical Storm Alex in the north Atlantic Ocean in January 2016
2017 Atlantic hurricane season summary map
Hurricane Catarina, a rare South Atlantic tropical cyclone viewed from the International Space Station on March 26, 2004
Cyclone on Mars, imaged by the Hubble Space Telescope
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Strong cold fronts typically feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines.


Sudden, sharp increase in wind speed lasting minutes, as opposed to a wind gust, which lasts for only seconds.

Rainbow after a squall
A shelf cloud such as this one can be a sign that a squall is imminent
A summer squall line in Southern Ontario, producing lightning and distant heavy rains.
A wake low is a mesolow

Usually, this sudden violent wind is associated with briefly heavy precipitation as squall line.

Mesoscale convective system

Complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more.

A shelf cloud such as this one can be a sign that a squall is imminent
Conditions favorable for thunderstorm types and complexes
A mesoscale convective vortex over Pennsylvania with a trailing squall line.
Hurricane Catarina, a rare South Atlantic tropical cyclone viewed from the International Space Station on March 26, 2004
Lake-effect precipitation coming off Lake Erie, as seen by NEXRAD radar, October 12–13, 2006
Typical evolution of thunderstorms (a) into a bow echo (b, c) and into a comma echo (d). Dashed line indicates axis of greatest potential for downbursts. Arrows indicate wind flow relative to the storm. Area C is most prone to supporting tornado development.

A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and generally forms near weather fronts.

Outflow (meteorology)

Air that flows outwards from a storm system.

Radar image animation of an outflow boundary of a storm approaching Tulsa, Oklahoma. The outflow boundary's weak echo moves left-to-right and passes overhead of the Doppler radar station. The outflow produces a gust front that moves ahead of the main thunderstorm.
The outflow boundary indicated by the presence of this shelf cloud preceded a derecho in Minnesota
Structure of a tropical cyclone. The upper level outflow is depicted by cirrus clouds in the upper part of the schematic
A sandstorm (Haboob) approaching Al Asad, Iraq, just before nightfall on April 27, 2005.

Squall lines typically bow out the most, or bend the most convex outward, at the leading edge of low level outflow due to the formation of a mesoscale high-pressure area which forms within the stratiform rain area behind the initial line.

Bow echo

Characteristic radar return from a mesoscale convective system that is shaped like an archer's bow.

Radar image of a bow echo crossing Kansas City at 2:14 AM on 2 May 2008 (NWS Kansas City)
Typical evolution of a thunderstorm radar echo (a) into a bow echo (b, c) and into a comma echo (d). Dashed line indicates axis of greatest potential for downbursts. Arrows indicate wind flow relative to the storm. Note regions of cyclonic rotation (C) and anticyclonic rotation (A); both regions, especially C, are capable of supporting tornado development in some cases.
A bow echo west of the Chicago area

A bow echo is associated with squall lines or lines of convective thunderstorms.

Cold front

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.

The symbol of a cold front: a blue line with triangles pointing in the direction of travel
A cold front over the eastern and central region of the United States of America
An incoming cold front in northern Ohio (2016)
Stratocumulus clouds after a cold front
Altocumulus clouds often are a sign of an entering cold front.
Altocumulus undulatus clouds several hours after a cold front
A cold front as it appeared on the National Weather Service Wichita, Kansas WSR-88D on April 3, 2011. The thin blue line labeled "cold front" is the front, with severe thunderstorms seen developing behind the front, which is moving towards the bottom right.
Occluded cyclone example. The triple point is the intersection of the cold, warm, and occluded fronts.

If there is significant instability along the boundary, a narrow line of thunderstorms can form along the frontal zone.