Cloudy trial behind the plane

首先留意個英文字是
trail - 拖行的痕跡 - 這名詞另一個意思是小徑/山徑 或尾; 作為動詞解追蹤

錯串的
trial - 試驗/嘗試 亦解 考驗 審訊

問題1.的文法亦錯了
1). How does this trails form?
如果does + this 便應是單數trail
How does this trail form?
如果想指出一般會見到條trails便應該寫成
How do these trails form?

Answer:
1. Water condensation or formation in the cold (sub-zero) air from the hot exhaust of the airplane engines.

2. The cloudy trails actually forms only at certain altitude where regions with high air humidty and cold temperature. We can see the trails behind an plane in clear sky at high altitude, but if the sky is not clear or the air is relatively dry, either there is no white trails formed or we do not see it because of low visibility.

Next time when you're on an airplane and happen to see another airplane flying along in a different direction, you may see the white trails even when it is above 10000 meters. However, the water content at that high altitude is generally low (above troposphere of 10km), so it is more likely to see the trails slighly below (7-20km).