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Overshooting top : ウィキペディア英語版 | Overshooting top
An overshooting top (or penetrating top) is a dome-like protrusion that shoots out of the top of the anvil of a thunderstorm. When an overshooting top is present for 10 minutes or longer, it is a strong indication that the thunderstorm may be severe.〔Chance Hayes, National Weather Service Wichita, Kansas. "Storm Fury on the Plains." Storm Spotter Training. 4H Building, Salina, Kansas. 22 Feb. 2010. Lecture.〕 == Formation == When a thunderstorm forms, clouds build vertically into the atmosphere until the storm's updraft (warm rising air) has reached an equilibrium level (EL); the point where the surrounding air is about the same temperature or even warmer.〔(【引用サイトリンク】 publisher = National Severe Storms Lab (NSSL) )〕 This point of equilibrium is often marked by the tropopause. Rather than continuing to rise into the stratosphere, the vertical cloud growth abruptly stops, and instead clouds spread horizontally, forming an "anvil" shape on top of the thunderstorm.〔 An overshooting top forms when a thunderstorm's updraft, due to momentum from rapid ascent and strength of lifting, protrudes its equilibrium level, forming a dome-like structure on top of the anvil.〔(【引用サイトリンク】 publisher = NSSL )〕 This can occur with any cumulonimbus cloud when instability is high. Whereas anvils form at the equilibrium level, overshooting tops continue to the maximum parcel level (MPL).
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