Why do airplanes leave a trail of cloud?
If you have witnessed an airplane flying against the deep blue sky, chances are that you may have also witnessed the trail of cloud it has probably left behind. These man-made clouds are nothing but “contrails,” and no, they aren't made of smoke.
Airplane engines release water which combines with the much cooler air. This causes the water to condense, which turns into tiny icy crystals before it can melt away. At times, this vapour trail can be in the sky for some time; and sometimes, not. This depends on the moisture in the air. If the air is cold, the water turns into ice, which leaves a quick trail. And bam, you have a show in the sky!