Clouds Formation

Above the troposphere
A few relatively uncommon clouds can be found above the troposphere where moisture is very scarce. These include polar mesospheric noctilucent clouds and nacreous polar stratospheric clouds. They are composed mostly of ice crystals and occur at high latitudes, mostly within 40 degrees of the poles in the mesosphere and stratosphere respectively. Most clouds above the troposphere have a wispy or fibrous appearance and can be mistakenly identified as high tropospheric cirrus clouds.
• Polar stratospheric
Nacreous clouds occur most typically at altitudes of 15,000–25,000 m (50,000–80,000 ft) during the winter when that part of the atmosphere is coldest and has the best chance of triggering condensation. Also known as mother of pearl clouds, they are typically very thin with a cirriform appearance. Nacreous clouds are sub-classified alpha-numerically based on chemical makeup rather than variations in physical appearance.