The highest cloud in Earth's atmosphere is the noctilucent cloud, also known as a night-shining cloud. These clouds are composed of ice crystals and form at an incredibly high altitude of around 82 kilometers (50 miles), which is about 70 kilometers (43 miles) higher than virtually all other clouds. Noctilucent clouds are only visible when illuminated by sunlight from below after the sun has set below the horizon.
While cirrus clouds are considered to be the highest clouds in the troposphere, they are much lower than noctilucent clouds. Noctilucent clouds reside in the mesosphere, a layer of the atmosphere above the troposphere and stratosphere.
