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. They go through three stages in their life cycle: the downburst, outburst, and cushion stages. The scale and suddenness of a microburst makes it a great danger to aircraft due to the low-level wind shear caused by its gust front, with several fatal crashes having been attributed to the phenomenon over the past several decades.
A microburst often has high winds that can knock over fully grown trees. They usually last for a duration of a couple of seconds to several minutes.A distinction can be made between a wet microburst which consists of precipitation and a dry microburst which consists of virga.[3] They generally are formed by precipitation-cooled air rushing to the surface, but they perhaps also could be powered from the high speed winds of the jet stream deflected to the surface in a thunderstorm (see downburst). Microbursts are recognized as capable of generating wind speeds higher than 75 m/s (168 mph; 270 km/h).
There are a handful of factors that cause microbursts to develop, including mid-level dry air entrainment, cooling beneath the thunderstorm cloud base, sublimation (occurs when the cloud base is above the freezing level), and the existence of rain and/or hail within the thunderstorm (i.e. precipitation loading). Some microbursts are driven by a combination of these factors while others may only be driven by one factor. Due to this, microbursts can be subdivided into three primary types -- wet, dry, and hybrid. Cooling beneath the thunderstorm cloud base and sublimation are the primary forcing mechanisms with dry microbursts. Dry microbursts typically occur with very little precipitation at the surface or aloft, hence the dry type. Wet microbursts, on the other hand, are primarily driven by entrainment of mid-level dry air and precipitation loading. Hybrid microbursts possess characteristics of both wet and dry microbursts. They are forced in the mid-levels by dry air entrainment and/or precipitation loading and in the low-levels by cooling beneath the cloud base and/or sublimation.
Downbursts are a particular hazard to aircraft at low level, especially on take-off or landing. An aircraft approaching a downburst will first encounter a strong headwind, which will lead to an increase in indicated airspeed. When trying to fly a set airspeed on approach, a pilot might therefore be tempted to reduce power. This would be very dangerous because, as the aircraft passes thorough the downburst, the wind becomes a tailwind and the indicated airspeed and lift drops. The significant downward force of air in the downburst may be enough to force the aircraft into the ground or at least cause it to lose a significant amount of height. The subsequent loss of performance, as the aircraft encounters tailwinds, may cause further loss of height and be enough to cause the aircraft to stall. Once caught in a downburst, escape is only possible by flying straight ahead; whichever way an aircraft turns, it will encounter the tail winds and the associated performance impact. If the aircraft is in a turn at that point then the stalling speed will be higher, possibly making the situation worse.
A microburst often has high winds that can knock over fully grown trees. They usually last for a duration of a couple of seconds to several minutes.A distinction can be made between a wet microburst which consists of precipitation and a dry microburst which consists of virga.[3] They generally are formed by precipitation-cooled air rushing to the surface, but they perhaps also could be powered from the high speed winds of the jet stream deflected to the surface in a thunderstorm (see downburst). Microbursts are recognized as capable of generating wind speeds higher than 75 m/s (168 mph; 270 km/h).
There are a handful of factors that cause microbursts to develop, including mid-level dry air entrainment, cooling beneath the thunderstorm cloud base, sublimation (occurs when the cloud base is above the freezing level), and the existence of rain and/or hail within the thunderstorm (i.e. precipitation loading). Some microbursts are driven by a combination of these factors while others may only be driven by one factor. Due to this, microbursts can be subdivided into three primary types -- wet, dry, and hybrid. Cooling beneath the thunderstorm cloud base and sublimation are the primary forcing mechanisms with dry microbursts. Dry microbursts typically occur with very little precipitation at the surface or aloft, hence the dry type. Wet microbursts, on the other hand, are primarily driven by entrainment of mid-level dry air and precipitation loading. Hybrid microbursts possess characteristics of both wet and dry microbursts. They are forced in the mid-levels by dry air entrainment and/or precipitation loading and in the low-levels by cooling beneath the cloud base and/or sublimation.
Downbursts are a particular hazard to aircraft at low level, especially on take-off or landing. An aircraft approaching a downburst will first encounter a strong headwind, which will lead to an increase in indicated airspeed. When trying to fly a set airspeed on approach, a pilot might therefore be tempted to reduce power. This would be very dangerous because, as the aircraft passes thorough the downburst, the wind becomes a tailwind and the indicated airspeed and lift drops. The significant downward force of air in the downburst may be enough to force the aircraft into the ground or at least cause it to lose a significant amount of height. The subsequent loss of performance, as the aircraft encounters tailwinds, may cause further loss of height and be enough to cause the aircraft to stall. Once caught in a downburst, escape is only possible by flying straight ahead; whichever way an aircraft turns, it will encounter the tail winds and the associated performance impact. If the aircraft is in a turn at that point then the stalling speed will be higher, possibly making the situation worse.
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