Clouds are defined by both the way they look at how high they are in the atmosphere. For example, cirro (meaning "wisp of hair") is a prefix given to high-altitude clouds (above 20,000 feet). Alto(meaning "high") is a prefix given to mid-altitude clouds (between 6,000 and 20,000 feet). There is no prefix for low-altitude clouds. When clouds are by the ground we call them fog.