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今天夕陽西下前竟然有異星訪客蒞臨？一次還七顆飛碟呢！

原來是莢狀雲 (Lenticular cloud)，引自維基百科，形成原因如下：

當穩定的潮濕空氣在山區或山脈間流動時，一系列大型的背風波就可能在順風方向形成。如果溫度在波峰處下降或低於露點，空氣中的水分就會凝結，形成莢狀雲。由於潮濕的空氣會流向谷底形成槽波，雲就會蒸發成水蒸氣。在某些情況下，一連串的莢狀雲會像洪峰一樣連續的波，形成所謂的「波雲」。「波系統」造成大規模的空氣立式流動，並可能產生足夠的水蒸氣凝結造成降水。這種雲經常被人們誤認成為不明飛行物，因為這些雲平滑的外觀十分類似飛碟。有時，莢狀雲的邊緣會出現亮麗的顏色（被稱為華）。
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We got some extraterrestrial visitors today before dusk. Seven UFOs all at once!

It is lenticular cloud, and according to Wikipedia, the formation is:

As air travels along the surface of the Earth, obstructions are often encountered. These include both natural features of the Earth, such as mountains or hills, and man-made structures, such as buildings and other structures. These disrupt the flow of air into "eddies", or areas of turbulence influenced by these obstructions.

When moist, stable air flows over a larger eddie, like those caused by mountains, a series of large-scale standing waves form on the leeward side of the mountain. If the temperature at the crest of the wave drops below the local dew point, moisture in the air may condense to form lenticular clouds. Under certain conditions, long strings of lenticular clouds can form near the crest of each successive wave, creating a formation known as a "wave cloud." These wave systems can produce large updrafts, occasionally enough for water vapour to condense and produce precipitation.

Lenticular clouds have been mistaken for UFOs (or "visual cover" for UFOs), particularly in the shape of a "flying saucer", because these clouds have a characteristic lens appearance and smooth saucer-like shape; also, because lenticular clouds generally do not form over low-lying or flat terrain, many people may have never seen one before and don't know that they can exist. Bright colours (called iridescence) are sometimes seen along the edge of lenticular clouds.