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Nacreous Clouds – Also known as ‘mother of pearl’ cloud or polar stratospheric clouds (PSCs), they occur at altitudes of 15-30km (10-20 miles), and generally at northern latitudes above 50°. They occur in the stratosphere (the second layer of the atmosphere) at temperatures of -80°C and are composed of nitric acid and ice crystals. Seen shortly after sunset, although beautiful, they assist in the production of chlorine atoms which contribute to the depletion of the ozone layer. |
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Noctilucent Clouds – Latin for ‘night-shining’, NLCs are also known as polar mesospheric clouds, occurring in the mesosphere, the third layer of the atmosphere over 80km (50 miles) above the ground and on the fringes of space. These rare clouds are seen above 50°N or below 50°S in mid summer and are the least understood clouds, forming at temperatures of -125°C (-193°F). They are however becoming increasingly common in recent decades, and it is suggested that the greenhouse gases responsible for global warming are having a corresponding cooling effect on the outer layers of the atmosphere. It may also be due to increases in moisture in the mesosphere from space travel. |
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Banner Cloud – this ‘orographic’ cloud occurs when a mountain forces air moving towards it upwards, forming a cloud layer just above its summit and pulled down by negative pressure on its lee side, as seen here on the east face of the Matterhorn in Switzerland. |
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Kelvin-Helmholtz (K-H) Waves – occur when strong horizontal winds cause a mass of warm air to move faster than a mass of cold air beneath it. They usually occur in cirrus or cirrostratus cloud. |