Distrails
Distrail is short for "dissipation trail". Where an aircraft passes through a cloud, it can clear a path through it; this is known as a distrail. Because the plane's contrail is not yet visible (contrails usually form above 8,000 meters/26,000 feet, depending on the temperature and other factors) a distrail looks like a tunnel through the cloud if the cloud is very thin.
Distrails are created by the elevated temperature of the exhaust gases absorbing the moisture from the cloud. Clouds exist where the relative humidity is 100% but by increasing the temperature the air can hold more moisture and the relative humidity drops below 100%, even for the same absolute moisture density, causing the visible water droplets in the cloud to be converted back into water vapour.
Read more about this topic: Contrail