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Outflow, in
meteorology Meteorology is the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agricultur ...
, is air that flows outwards from a storm system. It is associated with ridging, or anticyclonic flow. In the low levels of the
troposphere The troposphere is the lowest layer of the atmosphere of Earth. It contains 80% of the total mass of the Atmosphere, planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the ...
, 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 A weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation (meteorology), precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern w ...
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 cyclone A tropical cyclone is a rapidly rotating storm system with a low-pressure area, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its locat ...
s. 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 close, the upper-level outflow from the upwind system can limit the development of the other system.


Thunderstorms

For thunderstorms, outflow tends to indicate the development of a system. Large quantities of outflow at the upper levels 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 ...
indicate its development. Too much outflow in the lower levels of a thunderstorm, however, can choke off the low-level inflow which fuels it. 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.Peter S. Parke and Norvan J. Larson (2005)
Boundary Waters Windstorm.
National Weather Service The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...
Forecast Office,
Duluth, Minnesota Duluth ( ) is a Port, port city in the U.S. state of Minnesota and the county seat of St. Louis County, Minnesota, St. Louis County. Located on Lake Superior in Minnesota's Arrowhead Region, the city is a hub for cargo shipping. The population ...
. Retrieved on 2008-07-30.
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.
Insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...
s and
arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
s are swept along by the prevailing winds, making them good indicators of the presence of outflow boundaries. The signature of the leading edge is also influenced by the
density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...
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.


Tropical cyclones

The development of a significant mesoscale convective complex can send out a large enough outflow boundary to weaken the
cyclone 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 (opposite to an ant ...
as the tropical cyclone center moves into the more stable
air mass In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to ...
behind the leading edge of thunderstorm outflow, or outflow boundary. Moderate vertical
wind shear Wind shear (; also written 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 ...
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. 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. 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. 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).


Local effects

Low-level outflow boundaries from thunderstorms are cooler and more moist than the
air mass In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to ...
the thunderstorm originally formed within due to its wet bulbing by
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 ...
, forming a wedge of denser air which spreads out from the base of the parent thunderstorm. If
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 ...
speeds are high enough, such as during microburst events, dust and sand can be carried into the
troposphere The troposphere is the lowest layer of the atmosphere of Earth. It contains 80% of the total mass of the Atmosphere, planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the ...
, reducing visibility. This type of weather event is known as a
haboob A haboob () is a type of intense dust storm carried by the wind of a weather front or thunderstorm. Haboobs occur regularly in arid, dry land area regions throughout the world, including off-Earth, and can be dangerous. Formation and charac ...
, and is most common in the late spring within
Sudan Sudan, officially the Republic of the Sudan, is a country in Northeast Africa. It borders the Central African Republic to the southwest, Chad to the west, Libya to the northwest, Egypt to the north, the Red Sea to the east, Eritrea and Ethiopi ...
. 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.


References

{{good article Meteorological phenomena Severe weather and convection Synoptic meteorology and weather