Downburst
Encyclopedia
Tornado
A tornado is a violent, dangerous, 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. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
, winds in a downburst are directed outwards from the point where it hits land or water. Dry downbursts are associated with thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...
s 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 line
Squall line
A squall line is a line of severe thunderstorms that can form 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....
s 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
Wind shear
Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...
or microburst
Microburst
A microburst is a very localized column of sinking air, producing damaging divergent and straight-line winds at the surface that are similar to, but distinguishable from, tornadoes, which generally have convergent damage. There are two types of microbursts: wet microbursts and dry microbursts...
which is dangerous to aviation.
Definition
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 150 mph (240 km/h), often producing damage similar to, but distinguishable from, that caused by tornadoTornado
A tornado is a violent, dangerous, 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. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
es. 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 impacting 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
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
free or contains virga
Virga
In meteorology, virga is an observable streak or shaft of precipitation that falls from a cloud but evaporates before reaching the ground. At high altitudes the precipitation falls mainly as ice crystals before melting and finally evaporating; this is usually due to compressional heating, because...
are known as dry downbursts; those accompanied with precipitation are known as wet downbursts. Most downbursts are less than 2.5 miles (4 km) in extent: these are called microburst
Microburst
A microburst is a very localized column of sinking air, producing damaging divergent and straight-line winds at the surface that are similar to, but distinguishable from, tornadoes, which generally have convergent damage. There are two types of microbursts: wet microbursts and dry microbursts...
s. Downbursts larger than 2.5 miles (4 km) in extent are sometimes called macrobursts. Downbursts can occur over large areas. In the extreme case, a derecho
Derecho
A derecho is a widespread and long-lived, violent convectively induced straight-line windstorm that is associated with a fast-moving band of severe thunderstorms in the form of a squall line usually taking the form of a bow echo...
can cover a huge area more than 200 miles (320 km) wide and over 1000 miles (1600 km) long, lasting up to 12 hours or more, and is associated with some of the most intense straight-line winds, but the generative process is somewhat different from that of most downbursts.
Straight-line winds
Straight-line winds (also known as thundergusts and hurricanes of the prairie) are very strong winds that can produce damage, demonstrating a lack of a rotational damage pattern. Such rotational damage patterns are associated with cyclonic storms including tornadoTornado
A tornado is a violent, dangerous, 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. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
es and tropical cyclone
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
s. 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 reason these storms are so dangerous is because of the consistent wind that does not let up. The winds can reach 80 m.p.h. or more and can last for periods of twenty minutes or longer. Such straight-line wind events are most common during the spring when instability is highest and weather fronts routinely cross the country. However, straight-line wind events in the form of "derechos" can take place in areas outside of the traditional tornado alley (such as in the northeastern United States
Northeastern United States
The Northeastern United States is a region of the United States as defined by the United States Census Bureau.-Composition:The region comprises nine states: the New England states of Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island and Vermont; and the Mid-Atlantic states of New...
/Great Lakes Region and across southern Canada).
Straight-line winds may be damaging to marine interests. Small ships, cutters and sailboats are at risk from this meteorological phenomenon.
Formation
The formation of a downburst starts with hailHail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
or large rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...
drops falling through drier air. Hailstones melt and raindrops evaporate—this is an endothermic process that demands a lot of energy (in the form of latent heat
Latent heat
Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a process that occurs without a change in temperature. A typical example is a change of state of matter, meaning a phase transition such as the melting of ice or the boiling of water. The term was...
) so the air is cooled. Cooler air has a higher density than the warmer air around it, so it falls as a "cold air balloon" (compare to a hot air balloon
Hot air balloon
The hot air balloon is the oldest successful human-carrying flight technology. It is in a class of aircraft known as balloon aircraft. On November 21, 1783, in Paris, France, the first untethered manned flight was made by Jean-François Pilâtre de Rozier and François Laurent d'Arlandes in a hot air...
, which rises because hot air has a lower density than the surrounding air). As the cold air balloon hits the ground it spreads out and a mesoscale
Mesoscale meteorology
Mesoscale meteorology is the study of weather systems smaller than synoptic scale systems but larger than microscale and storm-scale cumulus systems. Horizontal dimensions generally range from around 5 kilometers to several hundred kilometers...
front
Weather front
A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of 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 burstHeat burst
In meteorology, a heat burst is a rare atmospheric phenomenon characterised by gusty winds and a rapid increase in temperature and decrease in dew point...
, which results from precipitation-evaporated air compressionally heating as it descends from very high altitude, usually on the backside of a dying squall line
Squall line
A squall line is a line of severe thunderstorms that can form 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....
or outflow boundary. Heat bursts are chiefly a nocturnal occurrence, can produce winds of up to 100 mph (160 km/h), are characterized by exceptionally dry air, and can suddenly raise the surface temperature to 100 degrees Fahrenheit (38 degrees Celsius) or more, sometimes persisting 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 shearWind shear
Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...
caused by these events. A number of fatal crashes have been attributed to downbursts.