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No two clouds are exactly alike, and they are always changing their shape. Heights and temperatures are the reason we have different types of clouds. Clouds are com­posed of different particles, depending on their height and temperature. The highest clouds are called “noctilucent” clouds. They are about as high as 30 to 50 miles. The next highest are called “nacreous” clouds. They are 12 to 18 miles high. They are very thin, with beauti­ful color composed of dust or water drops, and they can only be seen after sunset, at night, or before sunrise.