High level clouds typically form above 20000 feet (6000 meters) and since the temperatures are so cold at such high elevations, these clouds are primarily composed of ice crystals. They are typically thin and white in appearance, however during sunrise and sunset, these clouds can appear in a magnificent array of colors as unscattered components of sunlight (red, yellow, and orange) are reflected by the underside of the clouds.
The most common variation of high level clouds are cirrus clouds. Cirrus are thin, whispy clouds composed of ice crystals that originate from the freezing of supercooled water droplets and exist where temperatures are below -38 degrees Celsius. Cirrus generally occur in fair weather and move from west to east, pointing in the direction of the prevailing winds at their elevation.