Here an aircraft has caused tiny water droplets in sub-zero cloud to evaporate and produce the same mass of much larger ice crystals. The large crystals scatter less light and so give the appearance of a gap in the cloud.
Below zero Celsius liquid water is unstable compared to solid ice. Given the chance, the water undergoes a phase transition – it freezes. But, in cloud, the chance may not be there. There is little disturbance, water drops are well separated and most importantly there may be few dust or salt particles to act as nuclei on which ice can start to crystallize. Instead, the cloud droplets remain liquid. They are supercooled – and metastable.
But supercooled water and ice cannot co-exist in equilibrium. If any ice happens to form there is a very fast change and all the water in that part of the cloud gets transformed to ice. Vapour pressure is the key. Supercooled water has a greater vapour pressure than ice. If any ice is present there is a local drop in the pressure of water vapour. This causes droplets to evaporate. This supplies vapour for more ice formation and so on.. In a short time the water droplets have evaporated and been replaced by a few much larger ice crystals.
An aircraft can produce the necessary disturbance via its pressure waves and supply crystallization nuclei from the engine exhausts.
Rather than long trails, holes can also form giving ‘hole-punch’ clouds from which trail virga of ice crystals.
This transformation of water droplets to ice crystals thence to snow which melts to large drops is one route by which rain forms – the Wegener-Bergeron-Findeisen process.