A Roll Cloud Over Missouri
影像版權與提供: Dan Bush (Missouri Skies)
Explanation:
What kind of cloud is this? A roll cloud. These rare long clouds may form near advancing cold fronts. In particular, a downdraft from an advancing storm front can cause moist warm air to rise, cool below its dew point, and so form a cloud. When this happens uniformly along an extended front, a roll cloud may form. Roll clouds may actually have air circulating along the long horizontal axis of the cloud. A roll cloud is not thought to be able to morph into a tornado. Unlike a similar shelf cloud, a roll cloud is completely detached from their parent cumulonimbus cloud. Pictured above, a roll cloud extends far into the distance in the summer of 2005 above Albany, Missouri, USA.
說明:
這是那一種雲呢？答案是捲軸雲 (roll cloud；或稱弧狀雲)，這種罕見的長雲有時會出現在冷鋒前緣。來自往前推進的暴風前緣之下沈下流，會推升前方的溼暖空氣，讓它的溫度低於露點並形成雲。如果這種現象在大範圍冷鋒前緣同時發生時，就會出現這種捲軸雲。在捲軸雲內，氣流有時會沿著捲軸雲的水平軸方向流動。和灘雲 (shelf cloud)最大的差異是，捲軸雲並不會轉變成龍捲風，因為它和造成它的積雨母雲是完全分離的。上面這張2005年夏天在美國．密蘇里州、Albany鎮所拍攝的影像裡，一道捲軸雲向遠方延伸。
資料來源: Scientific American
Department of Physics, NCKU
