Downburst

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Downburst seen from the ARMOR Doppler Weather Radar in Huntsville, Alabama in 2012. Note the winds in green going towards the radar, and the winds in red going away from the radar. ARMOR Downburst.png
Downburst seen from the ARMOR Doppler Weather Radar in Huntsville, Alabama in 2012. Note the winds in green going towards the radar, and the winds in red going away from the radar.

A downburst is a strong ground-level wind system that emanates from a point source above and blows radially, that is, in straight lines in all directions from the point of contact at ground level. Often producing damaging winds, it may be confused with a tornado, where high-velocity winds circle a central area, and air moves inward and upward; by contrast, in a downburst, winds are directed downward and then outward from the surface landing point.

Wind Flow of gases or air on a large scale

Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the Sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space. Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the regions in which they occur, and their effect. The strongest observed winds on a planet in the Solar System occur on Neptune and Saturn. Winds have various aspects, an important one being its velocity ; another the density of the gas involved; another its energy content or wind energy. Wind is also a great source of transportation for seeds and small birds; with time things can travel thousands of miles in the wind.

Polar coordinate system two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction

In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction.

Tornado violently rotating column of air that is in contact with both the earths surface and a cumulonimbus cloud in the air

A tornado is a rapidly rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. The windstorm is often referred to as a twister, whirlwind or cyclone, although the word cyclone is used in meteorology to name a weather system with a low-pressure area in the center around which winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern. Tornadoes come in many shapes and sizes, and they are often visible in the form of a condensation funnel originating from the base of a cumulonimbus cloud, with a cloud of rotating debris and dust beneath it. Most tornadoes have wind speeds less than 110 miles per hour (180 km/h), are about 250 feet (80 m) across, and travel a few miles before dissipating. The most extreme tornadoes can attain wind speeds of more than 300 miles per hour (480 km/h), are more than two miles (3 km) in diameter, and stay on the ground for dozens of miles.

Contents

Downbursts are created by an area of significantly rain-cooled air that, after reaching ground level, spreads out in all directions producing strong winds. Dry downbursts are associated with thunderstorms with very little rain, while wet downbursts are created by thunderstorms with high amounts of rainfall. Microbursts and macrobursts are downbursts at very small and larger scales, respectively. Another variety, the heat burst, is created by vertical currents on the backside of old outflow boundaries and squall lines where rainfall is lacking. Heat bursts generate significantly higher temperatures due to the lack of rain-cooled air in their formation. Downbursts create vertical wind shear or microbursts, which is dangerous to aviation.

Thunderstorm type of weather

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.

Microburst

A microburst is an intense small-scale downdraft produced by a thunderstorm or rain shower. There are two types of microbursts: wet microbursts and dry microbursts. They go through three stages in their cycle, the downburst, outburst, and cushion stages also called "Suriano's Stroke". A microburst can be particularly dangerous to aircraft, especially during landing, due to the wind shear caused by its gust front. Several fatal and historic crashes have been attributed to the phenomenon over the past several decades, and flight crew training goes to great lengths on how to properly recover from a microburst/wind shear event.

In meteorology, a heat burst is a rare atmospheric phenomenon characterized by gusty winds along with a rapid increase in temperature and decrease in dew point (moisture). Heat bursts typically occur during night-time and are associated with decaying thunderstorms.

Definition

The curl phase soon after an intense microburst hit the surface Microburst - NOAA.jpg
The curl phase soon after an intense microburst hit the surface
Downburst damages in a straight line. (Source NOAA) Downburst damage.jpg
Downburst damages in a straight line. (Source NOAA)

A downburst is created by a column of sinking air that after hitting ground level, spreads out in all directions and is capable of producing damaging straight-line winds of over 240 km/h (150 mph), often producing damage similar to, but distinguishable from, that caused by tornadoes. This is because the physical properties of a downburst are completely different from those of a tornado. Downburst damage will radiate from a central point as the descending column spreads out when hitting the surface, whereas tornado damage tends towards convergent damage consistent with rotating winds. To differentiate between tornado damage and damage from a downburst, the term straight-line winds is applied to damage from microbursts.

Downbursts are particularly strong downdrafts from thunderstorms. Downbursts in air that is precipitation free or contains virga are known as dry downbursts; [1] those accompanied with precipitation are known as wet downbursts. Most downbursts are less than 4 km (2.5 mi) in extent: these are called microbursts. [2] Downbursts larger than 4 km (2.5 mi) in extent are sometimes called macrobursts. [2] Downbursts can occur over large areas. In the extreme case, a derecho can cover a huge area more than 320 km (200 mi) wide and over 1,600 km (1,000 mi) long, lasting up to 12 hours or more, and is associated with some of the most intense straight-line winds, [3] but the generative process is somewhat different from that of most downbursts.

Virga clouds supplementary feature; precipitation that doesnt reach the ground

In meteorology, a virga is an observable streak or shaft of precipitation falling from a cloud that evaporates or sublimates before reaching the ground. A shaft of precipitation that does not evaporate before reaching the ground is a precipitation shaft. At high altitudes the precipitation falls mainly as ice crystals before melting and finally evaporating; this is often due to compressional heating, because the air pressure increases closer to the ground. It is very common in deserts and temperate climates. In North America, it is commonly seen in the Western United States and the Canadian Prairies. It is also very common in the Middle East, Australia, and North Africa.

Derecho Type of wind storm

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

Straight-line winds

Straight-line winds (also known as plough winds, thundergusts and hurricanes of the prairie) are very strong winds that can produce damage, demonstrating a lack of the rotational damage pattern associated with tornadoes. [4] Straight-line winds are common with the gust front of a thunderstorm or originate with a downburst from a thunderstorm. These events can cause considerable damage, even in the absence of a tornado. The winds can gust to 210 km/h (130 mph) [5] and winds of 93 km/h (58 mph) or more can last for more than twenty minutes. [6] In the United States, such straight-line wind events are most common during the spring when instability is highest and weather fronts routinely cross the country.[ citation needed ] Straight-line wind events in the form of derechos can take place throughout the eastern half of the U.S. [7]

Straight-line winds may be damaging to marine interests. Small ships, cutters and sailboats are at risk from this meteorological phenomenon.[ citation needed ]

Formation

The formation of a downburst starts with hail or large raindrops falling through drier air. Hailstones melt and raindrops evaporate, pulling latent heat from surrounding air and cooling it considerably. Cooler air has a higher density than the warmer air around it, so it sinks to the ground. As the cold air hits the ground it spreads out and a mesoscale front can be observed as a gust front. Areas under and immediately adjacent to the downburst are the areas which receive the highest winds and rainfall, if any is present. Also, because the rain-cooled air is descending from the middle troposphere, a significant drop in temperatures is noticed. Due to interaction with the ground, the downburst quickly loses strength as it fans out and forms the distinctive "curl shape" that is commonly seen at the periphery of the microburst (see image). Downbursts usually last only a few minutes and then dissipate, except in the case of squall lines and derecho events. However, despite their short lifespan, microbursts are a serious hazard to aviation and property and can result in substantial damage to the area.

Heat bursts

A special, and much rarer, kind of downburst is a heat burst, which results from precipitation-evaporated air compressionally heating as it descends from very high altitude, usually on the backside of a dying squall line or outflow boundary. [8] Heat bursts are chiefly a nocturnal occurrence, can produce winds over 160 km/h (100 mph), are characterized by exceptionally dry air, can suddenly raise the surface temperature to 38 °C (100 °F) or more, and sometimes persist for several hours.

Danger to aviation

Downbursts, particularly microbursts, are exceedingly dangerous to aircraft which are taking off or landing due to the strong vertical wind shear caused by these events. A number of fatal crashes have been attributed to downbursts. [9]

See also

Related Research Articles

Squall sudden, sharp increase in the sustained winds over a short time interval

A squall is a sudden, sharp increase in wind speed lasting minutes, contrary to a wind gust lasting 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

A squall line or quasi-linear convective system (QLCS) is a line of thunderstorms forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. It contains heavy precipitation, hail, frequent lightning, strong straight-line winds, and possibly tornadoes and waterspouts. Strong straight-line winds can occur where the squall line is in 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 which develop within the summer season are known as derechos, and they move quite fast through large sections of territory. On the back edge of the rainband associated with mature squall lines, a wake low can be present, sometimes associated with a heat burst.

The Late-May 1998 tornado outbreak and derecho was a historic tornado outbreak and derecho that began on the afternoon of May 30 and extended throughout May 31, 1998, across a large portion of the northern half of the United States and southern Ontario from southeastern Montana east and southeastward to the Atlantic Ocean. The initial tornado outbreak, including the devastating Spencer tornado, hit southeast South Dakota on the evening of May 30. The Spencer tornado was the most destructive and the second-deadliest tornado in South Dakota history. Eleven people were killed; 7 by tornadoes and 6 by the derecho. Over two million people lost electrical power, some for up to 10 days.

Outflow boundary

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.

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 form near weather fronts. The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

Bow echo

A bow echo is the characteristic radar return from a mesoscale convective system that is shaped like an archer’s bow. These systems can produce severe straight-line winds and occasionally tornadoes, causing major damage. They can also become derechos.

The Canton, Illinois Tornadoes of 1975 is a destructive summer tornado event which occurred as part of a significant severe thunderstorm outbreak concentrated from eastern Iowa across northern and central Illinois on the afternoon and evening of July 23, 1975.

Severe weather

Severe weather refers to any dangerous meteorological phenomena with the potential to cause damage, serious social disruption, or loss of human life. Types of severe weather phenomena vary, depending on the latitude, altitude, topography, and atmospheric conditions. High winds, hail, excessive precipitation, and wildfires are forms and effects of severe weather, as are thunderstorms, downbursts, tornadoes, waterspouts, tropical cyclones, and extratropical cyclones. Regional and seasonal severe weather phenomena include blizzards (snowstorms), ice storms, and duststorms.

Atmospheric convection

Atmospheric convection is the result of a parcel-environment instability, or temperature difference layer in the atmosphere. Different lapse rates within dry and moist air masses lead to instability. Mixing of air during the day which expands the height of the planetary boundary layer leads to increased winds, cumulus cloud development, and decreased surface dew points. Moist convection leads to thunderstorm development, which is often responsible for severe weather throughout the world. Special threats from thunderstorms include hail, downbursts, and tornadoes.

Wake low

A wake low, or wake depression, is a mesoscale low-pressure area which trails the mesoscale high following a squall line. Due to the subsiding warm air associated with the systems formation, clearing skies are associated with the wake low. Once difficult to detect in surface weather observations due to their broad spacing, the formation of mesoscale weather station networks, or mesonets, has increased their detection. Severe weather, in the form of high winds, can be generated by the wake low when the pressure difference between the mesohigh preceding it and the wake low is intense enough. When the squall line is in the process of decay, heat bursts can be generated near the wake low. Once new thunderstorm activity along the squall line concludes, the wake low associated with it weakens in tandem.

The May 2009 derecho series was an unusually strong sequence of derecho events and tornadoes beginning on May 2, 2009 and continuing through May 8, which primarily affected the Southern United States. At least seven people were killed by the storms. An associated tornado outbreak also resulted in nearly 100 tornadoes, some strong, with most strong tornadoes, most damage, and all of the deaths on May 8. In total, nine people were killed, dozens were injured and at least $70 million in damage occurred, $58 million on May 8.

June 12–13, 2013 derecho series

From June 12 to June 13, 2013, two derechos occurred across different areas of the Eastern United States. The initial derecho formed on the afternoon of June 12 and tracked across a large section of the Midwestern United States, the central Appalachians, and the Mid-Atlantic states before moving into the Atlantic Ocean during the morning of June 13. A second, more widespread and intense derecho occurred on June 13 across the Southeastern United States, resulting in major wind damage across North Carolina, Virginia, and Maryland, among other states. They resulted in at least three deaths and caused extensive damage – resulting from both tornadoes and straight-line winds – from Iowa to South Carolina. 28 tornadoes touched down in Iowa, Illinois, Ohio, Georgia, North Carolina, Maryland, Virginia, and Tennessee. One of the tornadoes in Iowa was rated as a high-end EF3, destroying a restaurant and two houses. One person was injured by another tornado, rated EF2, in Carroll County, Illinois, and nine people were injured by and EF1 in Cherokee County, Georgia.

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

Glossary of meteorology Wikimedia list article

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

References

  1. Fernando Caracena, Ronald L. Holle, and Charles A. Doswell III. Microbursts: A Handbook for Visual Identification. Retrieved on 9 July 2008.
  2. 1 2 Glossary of Meteorology. Macroburst. Retrieved on 30 July 2008.
  3. Peter S. Parke and Norvan J. Larson.Boundary Waters Windstorm. Retrieved on 30 July 2008.
  4. Glossary of Meteorology. Straight-line wind. Archived 15 April 2008 at the Wayback Machine Retrieved on 1 August 2008.
  5. http://www.spc.noaa.gov/misc/AbtDerechos/derechofacts.htm#strength
  6. http://www.spc.noaa.gov/misc/AbtDerechos/casepages/jun291998page.htm
  7. http://www.spc.noaa.gov/misc/AbtDerechos/derechofacts.htm#climatology
  8. "Oklahoma "heat burst" sends temperatures soaring". USA Today|1999-07-08. 8 July 1999. Archived from the original on 25 December 1996. Retrieved 9 May 2007.
  9. NASA Langley Air Force Base. Making the Skies Safer From Windshear. Retrieved on 29 March 2010.