Outflow (meteorology)

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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. Outflow boundary of storm approaching Tulsa - rocking animation.gif
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.

Outflow, in meteorology, is air that flows outwards from a storm system. It is associated with ridging, or anticyclonic flow. In the low levels of the troposphere, outflow radiates from thunderstorms in the form of a wedge of rain-cooled air, which is visible as a thin rope-like cloud on weather satellite imagery or a fine line on weather radar imagery. For observers on the ground, a thunderstorm outflow boundary often approaches in otherwise clear skies as a low, thick cloud that brings with it a gust front.


Low-level outflow boundaries can disrupt the center of small tropical cyclones. However, outflow aloft is essential for the strengthening of a tropical cyclone. If this outflow is restricted or undercut, the tropical cyclone weakens. If two tropical cyclones are in close proximity, the upper-level outflow from the upwind system can limit the development of the other system.


The outflow boundary indicated by the presence of this shelf cloud preceded a derecho in Minnesota DangerousShelfCloud.jpg
The outflow boundary indicated by the presence of this shelf cloud preceded a derecho in Minnesota

For thunderstorms, outflow tends to indicate the development of a system. Large quantities of outflow at the upper levels of a thunderstorm indicate its development. Too much outflow in the lower levels of a thunderstorm, however, can choke off the low-level inflow which fuels it. [1] 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. This high-pressure area is formed due to strong descending motion behind the squall line, and could come in the form of a downburst. [2]

The "edge" of the outflow boundary can often be detected by Doppler radar (especially in clear air mode). Convergence occurs along the leading edge of the downdraft. Convergence of dust, aerosols, and bugs at the leading edge will lead to a higher clear air signature. Insects and arthropods are swept along by the prevailing winds, making them good indicators of the presence of outflow boundaries. [3] The signature of the leading edge is also influenced by the density change between the cooler air from the downdraft and the warmer environmental air. This density boundary will increase the number of echo returns from the leading edge. Clouds and new thunderstorms also develop along the outflow's leading edge. This makes it possible to locate the outflow boundary when using precipitation mode on a weather radar. Also, it makes outflow boundaries findable within visible satellite imagery as a thin line of cumuliform clouds which is known as an arcus, or arc, cloud. The image to the right depicts a particularly strong outflow boundary ahead of a line of storms. Often, the outflow boundary will bow in the direction it is moving the quickest. [4]

Tropical cyclones

Structure of a tropical cyclone. The upper level outflow is depicted by cirrus clouds in the upper part of the schematic Hurricane structure graphic.jpg
Structure of a tropical cyclone. The upper level outflow is depicted by cirrus clouds in the upper part of the schematic

The development of a significant mesoscale convective complex can send out a large enough outflow boundary to weaken the cyclone as the tropical cyclone center moves into the more stable air mass behind the leading edge of thunderstorm outflow, or outflow boundary. [5] Moderate vertical wind shear can lead to the initial development of the convective complex and surface low similar to the mid-latitudes, but it must relax to allow tropical cyclogenesis to continue. [6]

While the most obvious motion of clouds is toward the center, tropical cyclones also develop an upper-level (high-altitude) outward flow of clouds. These originate from air that has released its moisture and is expelled at high altitude through the "chimney" of the storm engine. [7] This outflow produces high, thin cirrus clouds that spiral away from the center. The clouds are thin enough for the sun to be visible through them. These high cirrus clouds may be the first signs of an approaching tropical cyclone. [8] As air parcels are lifted within the eye of the storm the vorticity is reduced, causing the outflow from a tropical cyclone to have anticyclonic motion. If two tropical cyclones are in proximity to one another, the outflow from the system downstream (normally to the west) can hinder the development of the system upstream (normally to the east). [9]

Local effects

A sandstorm (Haboob) approaching Al Asad, Iraq, just before nightfall on April 27, 2005. Sandstorm in Al Asad, Iraq.jpg
A sandstorm (Haboob) approaching Al Asad, Iraq, just before nightfall on April 27, 2005.

Low-level outflow boundaries from thunderstorms are cooler and more moist than the air mass the thunderstorm originally formed within due to its wet bulbing by rain, [10] forming a wedge of denser air which spreads out from the base of the parent thunderstorm. If wind speeds are high enough, such as during microburst events, dust and sand can be carried into the troposphere, reducing visibility. [11] This type of weather event is known as a haboob, and is most common in the late spring within Sudan. [12] Upper-level outflow can consist of thick cirrus clouds which would then obscure the sun and reduce solar insolation around the outermost edge of tropical cyclones.

Related Research Articles

Cyclone Large scale air mass that rotates around a strong center of low pressure

In meteorology, a cyclone is a 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. Cyclones are characterized by inward-spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. Mesocyclones, tornadoes, and dust devils lie within smaller mesoscale. Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere. Cyclones have also been seen on extraterrestrial planets, such as Mars, Jupiter, and Neptune. Cyclogenesis is the process of cyclone formation and intensification. Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. These zones contract and form weather fronts as the cyclonic circulation closes and intensifies. Later in their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream.

Thunderstorm Type of weather with lightning and thunder

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 sometimes called thundershowers. Thunderstorms occur in a type of cloud known as a cumulonimbus. They are usually accompanied by strong winds and often produce heavy rain and sometimes snow, sleet, or hail, but some thunderstorms produce little precipitation or no precipitation at all. Thunderstorms may line up in a series or become a rainband, known as a squall line. Strong or severe thunderstorms include some of the most dangerous weather phenomena, including large hail, strong winds, and tornadoes. Some of the most persistent severe thunderstorms, known as supercells, rotate as do cyclones. While most thunderstorms move with the mean wind flow through the layer of the troposphere that they occupy, vertical wind shear sometimes causes a deviation in their course at a right angle to the wind shear direction.

Surface weather analysis Type of weather map

Surface weather analysis is a special type of weather map that provides a view of weather elements over a geographical area at a specified time based on information from ground-based weather stations.

Squall Short, sharp increase in wind speed

A squall is a sudden, sharp increase in wind speed lasting minutes, as opposed to a wind gust, which lasts for only seconds. They are usually associated with active weather, such as rain showers, thunderstorms, or heavy snow. Squalls refer to the increase to the sustained winds over that time interval, as there may be higher gusts during a squall event. They usually occur in a region of strong sinking air or cooling in the mid-atmosphere. These force strong localized upward motions at the leading edge of the region of cooling, which then enhances local downward motions just in its wake.

Squall line Line of thunderstorms along or ahead of a cold front

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. Linear thunderstorm structures often contain heavy precipitation, hail, frequent lightning, strong straight-line winds, and occasionally tornadoes or waterspouts. Particularly strong straight-line winds can occur where the linear structure forms into the shape of a bow echo. Tornadoes can occur along waves within a line echo wave pattern (LEWP), where mesoscale low-pressure areas are present. Some bow echoes can grow to become derechos as they move swiftly across a large area. On the back edge of the rainband associated with mature squall lines, a wake low can be present, on very rare occasions associated with a heat burst.

Synoptic scale meteorology 1000-km-order method of measuring weather systems

The synoptic scale in meteorology is a horizontal length scale of the order of 1000 kilometers or more. This corresponds to a horizontal scale typical of mid-latitude depressions. Most high- and low-pressure areas seen on weather maps are synoptic-scale systems, driven by the location of Rossby waves in their respective hemisphere. Low-pressure areas and their related frontal zones occur on the leading edge of a trough within the Rossby wave pattern, while high-pressure areas form on the back edge of the trough. Most precipitation areas occur near frontal zones. The word synoptic is derived from the Greek word συνοπτικός, meaning seen together.

Wall cloud Cloud formation occurring at the base of a thunderstorm

A wall cloud is a large, localized, persistent, and often abrupt lowering of cloud that develops beneath the surrounding base of a cumulonimbus cloud and from which tornadoes sometimes form. It is typically beneath the rain-free base (RFB) portion of a thunderstorm, and indicates the area of the strongest updraft within a storm. Rotating wall clouds are an indication of a mesocyclone in a thunderstorm; most strong tornadoes form from these. Many wall clouds do rotate; however, some do not.

Cyclogenesis The development or strengthening of cyclonic circulation in the atmosphere

Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere. Cyclogenesis is an umbrella term for at least three different processes, all of which result in the development of some sort of cyclone, and at any size from the microscale to the synoptic scale.

Outflow boundary Mesoscale boundary separating outflow from the surroundign air

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 a drop in temperature and a related pressure jump. Outflow boundaries can persist for 24 hours or more after the thunderstorms that generated them dissipate, and can travel hundreds of kilometers from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary. Outflow boundaries can be seen either as fine lines on weather radar imagery or else as arcs of low clouds on weather satellite imagery. From the ground, outflow boundaries can be co-located with the appearance of roll clouds and shelf clouds.

Index of meteorology articles Wikipedia index

This is a list of meteorology topics. The terms relate to meteorology, the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting.

Weather front Boundary separating two masses of air of different densities

A weather front is a boundary separating air masses of several characteristics such as air density, wind, temperature and humidity. Disturbed and unstable weather often arises from these differences. For instance, cold fronts can bring bands of thunderstorms and cumulonimbus precipitation or be preceded by squall lines, while warm fronts are usually preceded by stratiform precipitation and fog. In summer, subtler humidity gradients known as dry lines can trigger severe weather. Some fronts produce no precipitation and little cloudiness, although there is invariably always a wind shift.

Rainband Cloud and precipitation structure

A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated. Rainbands can be stratiform or convective, and are generated by differences in temperature. When noted on weather radar imagery, this precipitation elongation is referred to as banded structure. Rainbands within tropical cyclones are curved in orientation. Rainbands of tropical cyclones contain showers and thunderstorms that, together with the eyewall and the eye, constitute a hurricane or tropical storm. The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity.

Mesoscale convective system Complex of thunderstorms organized on a larger scale

A mesoscale convective system (MCS) is a 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 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. The type that forms during the warm season over land has been noted across North and South America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

Mesoscale convective complex

A mesoscale convective complex (MCC) is a unique kind of mesoscale convective system which is defined by characteristics observed in infrared satellite imagery. They are long-lived, often form nocturnally, and commonly contain heavy rainfall, wind, hail, lightning, and possibly tornadoes.

Cold front Leading edge of a cooler mass of air

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, at the leading edge of its cold air advection pattern—known as the cyclone's dry "conveyor belt" flow. Temperature differences across the boundary can exceed 30 °C (54 °F) from one side to the other. When enough moisture is present, rain can occur along the boundary. If there is significant instability along the boundary, a narrow line of thunderstorms can form along the frontal zone. If instability is weak, a broad shield of rain can move in behind the front, and evaporative cooling of the rain can increase the temperature difference across the front. Cold fronts are stronger in the fall and spring transition seasons and are weakest during the summer.

Inflow (meteorology) Meteorological term for flow of a fluid into a large collection of itself

Inflow is the flow of a fluid into a large collection of that fluid. Within meteorology, inflow normally refers to the influx of warmth and moisture from air within the Earth's atmosphere into storm systems. Extratropical cyclones are fed by inflow focused along their cold front and warm fronts. Tropical cyclones require a large inflow of warmth and moisture from warm oceans in order to develop significantly, mainly within the lowest 1 kilometre (0.62 mi) of the atmosphere. Once the flow of warm and moist air is cut off from thunderstorms and their associated tornadoes, normally by the thunderstorm's own rain-cooled outflow boundary, the storms begin to dissipate. Rear inflow jets behind squall lines act to erode the broad rain shield behind the squall line, and accelerate its forward motion.

Cold-core low Cyclone with an associated cold pool of air at high altitude

A cold-core low, also known as an upper level low or cold-core cyclone, is a cyclone aloft which has an associated cold pool of air residing at high altitude within the Earth's troposphere, without a frontal structure. It is a low pressure system that strengthens with height in accordance with the thermal wind relationship. If a weak surface circulation forms in response to such a feature at subtropical latitudes of the eastern north Pacific or north Indian oceans, it is called a subtropical cyclone. Cloud cover and rainfall mainly occurs with these systems during the day.

Glossary of tropical cyclone terms Wikipedia glossary

The following is a glossary of tropical cyclone terms.

The following is a glossary of tornado terms. It includes scientific as well as selected informal terminology.

Glossary of meteorology List of definitions of terms and concepts commonly used in meteorology

This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.


  1. National Weather Service (2009-06-25). "O". National Oceanic and Atmospheric Administration . Retrieved 2010-04-09.
  2. Peter S. Parke and Norvan J. Larson (2005). Boundary Waters Windstorm. National Weather Service Forecast Office, Duluth, Minnesota. Retrieved on 2008-07-30.
  3. Diana Yates (2008). "Birds migrate together at night in dispersed flocks, new study indicates". University of Illinois at Urbana - Champaign. Retrieved 2009-04-26.
  4. Glossary of Meteorology (June 2000). Line echo wave pattern. Archived 2008-09-24 at the Wayback Machine American Meteorological Society. ISBN   1-878220-34-9. Retrieved on 2009-05-03.
  5. John A. Knaff and John F. Weaver (September 2000). "A Mesoscale Low-Level Thunderstorm Outflow Boundary Associated with Hurricane Luis". Monthly Weather Review . American Meteorological Society. 128 (9): 3352–3355. Bibcode:2000MWRv..128.3352K. doi:10.1175/1520-0493(2000)128<3352:AMLLTO>2.0.CO;2.
  6. University of Illinois (1999-10-04). Hurricanes. Retrieved 2008-08-17.
  7. National Weather Service (September 2006). "Hurricanes... Unleashing Nature's Fury: A Preparedness Guide" (PDF). National Oceanic and Atmospheric Administration. Archived from the original (PDF) on 2008-02-26. Retrieved 2010-04-09.
  8. Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division (2004-08-13). "Frequently Asked Questions: What's it like to go through a hurricane on the ground? What are the early warning signs of an approaching tropical cyclone?". National Oceanic and Atmospheric Administration . Retrieved 2006-07-26.
  9. Dr. Harold P. Gerrish (1989). "Preliminary Report: Tropical Storm Iris - 16–21 September 1989". National Hurricane Center . Retrieved 2010-04-08.
  10. John F. Weaver (June 1982). "History of Thunderstorm Forecasting: Part VI: The Thunderstorm (to 1900)". Storm Track. 5 (6). Archived from the original on 2010-12-01. Retrieved 2010-04-22.
  11. Western Region Climate Center (2002). H. Desert Research Institute. Retrieved on 2006-10-22.
  12. Glossary of Meteorology (June 2000). "Haboob". American Meteorological Society. Archived from the original on 2011-06-06. Retrieved 2010-03-27.