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Agustnado is a brief, shallow surface-basedvortex which forms within thedownburst emanating from athunderstorm.[1] The name is aportmanteau byelision of "gust fronttornado", as gustnadoes form due to non-tornadicstraight-line wind features in thedowndraft (outflow), specifically within the gust front of strong thunderstorms. Gustnadoes tend to be noticed when the vortices loft sufficient debris or form condensationclouds to be visible, although it is thewind that makes the gustnado, similar to tornadoes. As theseeddies very rarely connect from the surface to thecloud base, they are very rarely considered as tornadoes. The gustnado has little in common with tornadoes structurally or dynamically in regard to vertical development,intensity, longevity, orformative process—as classic tornadoes are associated withmesocyclones within theinflow (updraft) of the storm, not the outflow.[2][3]
The average gustnado lasts a few seconds to a few minutes, although there can be several generations and simultaneous swarms.[4] Most have the winds equivalent to an F0 or F1 tornado (up to 180 km/h or 110 mph), and are commonly mistaken for tornadoes. However, unlike tornadoes, the rotating column of air in a gustnadousually does not extend all the way to the base of the thundercloud. Gustnadoes actually have more in common with (minor)whirlwinds. They are not considered true tornadoes (unless they connect the surface to the ambient cloud base in which case they'd become alandspout) by most meteorologists and are not included in tornado statistics in most areas. Sometimes referred to asspin-up tornadoes, that term more correctly describes the rare tornadic gustnado that connects the surface to the ambient clouded base, or more commonly to the relatively brief but true tornadoes that are associated with amesovortex.[5]
The most common setting for a gustnado is along the gust front of a severe thunderstorm (by many definitions, containingwind speeds of at least 93 km/h or 58 mph), along which horizontalshear of the wind may be large. A particularly common location is along the rear-flank gust front of supercell storms. Gustnadoes probably form owing to shear instability associated with the strong horizontal shear; a relative maximum in vertical vorticity must exist in order for shear instability to be present. The bigger question is probably what the dynamical origin(s) of the vertical vorticity is (are), such as the tilting of horizontalvorticity into the vertical or vertical vorticity in the ambient environment that preexists the storm. Along therear-flank gust front ofsupercell storms, vertical vorticity very likely has its origins in the upward tilting of vorticity that can occur within descending air in the presence ofbaroclinity.[6]
While injuries or deaths are rare from gustnadoes, strong ones can cause damage and they are hazardous to drivers. There is some speculation that a gustnado might have been responsible for thecollapse of a stage at the Indiana State Fair on August 13, 2011 which killed 7 people and injured 58.[7]
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