飛機在空中為何產生長長白線在空, 很久不散?

因為飛機的引擎會排放大量熱廢氣　依ｄ熱廢氣會產生紅外幅射　因此帶有大量能量
而且在空中　除了空氣粒子外　還有水分子　水分子受熱而蒸發　所以白線其實就係水蒸氣　很久不散的原因是因為大量水分子蒸發了　一時三刻無辨法比高空的風完全吹走

It seems that the answer from koontm306 is more correct.

The exhaust air from jet engines is hot. When this hot exhaust air meets the cold surrounding air at high altitude, the water vapour in the hot air condenses and forms traces of water droplets behind the engines. Thus white [lines] are seen following a jet plane flying on the sky when look on the ground..