Distrail

 Another rare atmospheric phenomenon, the dissipation trail, or "distrail" is essentially the inverse of the aircraft contrail (condensation trail). As an airplane flies through a layer of clouds, if temperature and moisture conditions are just right, its exhaust gases will evaporate the cloud's water droplets cutting a blue sky trail. I had never seen one of these before. This shot was taken over the Porto coastline and luckily my camera was within reach.