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Middle level clouds that look like cotton balls. Middle clouds (altostratus, altocumulus) reside at an average altitude of ~10,000 feet. Keep in mind that middle clouds can form a couple of thousand feet above or below the 10,000- foot marker. Middle clouds are composed of water droplets and/or ice crystals.
A foggy, rainy day at a lake. Low clouds (stratus, stratocumulus, nimbostratus) can form anywhere from the ground to an altitude of approximately 6,000 feet. Fog is simply a low cloud in contact with the earth's surface.