5 rarest cloud formations

If you look up in the sky, you will notice that not all clouds have similar shapes. While some are grey, others are puffy. The shapes are so erratic that people see unicorns, bunnies or even cows. But some cloud formations have their own names based on how high up they are in the atmosphere and their shape. A rare phenomenon occurs that give these clouds their shapes. Known what are these rare cloud formations here.