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The highest clouds in the atmosphere are noctilucent clouds. Best seen in the lower and higher latitudes, these beautiful, tenuous phenomena form at altitudes of around 80 km (50 miles), above 99.9% of the atmosphere. They can be seen occasionally after the Sun has set when, due to their high altitude, they are still illuminated by the Sun's rays. They are believed to form a mixture of ice crystals and dust from meteors.