Outflow, in
meteorology
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...
, 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 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 ...
, 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
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 ...
.
Low-level outflow boundaries can disrupt the center of small
tropical cyclone
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Dep ...
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 in close proximity, 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 acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are some ...
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 agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the ...
Forecast Office, Duluth, Minnesota
, settlement_type = City
, nicknames = Twin Ports (with Superior, Wisconsin, Superior), Zenith City
, motto =
, image_skyline =
, image_caption = Clockwise from top: Downtown Dul ...
. 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 pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pa ...
s and
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
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 substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
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 an ...
as the tropical cyclone center moves into the more stable
air mass behind the leading edge of thunderstorm outflow, or outflow boundary. 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.
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 the thunderstorm originally formed within due to its
wet bulbing by
rain
Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
, forming a wedge of denser air which spreads out from the base of the parent thunderstorm. If
wind
Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ...
speeds are high enough, such as during
microburst events, dust and sand can be carried into the
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 ...
, reducing visibility. This type of weather event is known as a
haboob
A haboob ( ar, هَبوب, lit=blasting/drifting, translit=habūb) is a type of intense dust storm carried on an atmospheric gravity current, also known as a weather front. Haboobs occur regularly in dry land area regions throughout the worl ...
, and is most common in the late spring within
Sudan
Sudan ( or ; ar, السودان, as-Sūdān, officially the Republic of the Sudan ( ar, جمهورية السودان, link=no, Jumhūriyyat as-Sūdān), is a country in Northeast Africa. It shares borders with the Central African Republic t ...
.
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
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Meteorological phenomena
Severe weather and convection
Synoptic meteorology and weather