In most instances, cirrus and cumulus clouds both mean fair weather. However, distinct differences exist between these two types of clouds, including diversity in height and shape. The Latin root words from which their names come indicate their form – cirro translates into curl of hair, while cumulo means heap, according to the National Weather Service.
Cirrus clouds form at 18,000 feet and higher, well above where normal cumulus clouds develop. Cumulus clouds are low-level clouds, forming in most instances below 6,000 feet above the earth's surface. However, in specific instances, cumulus clouds can rise well into the atmosphere -- even reaching heights above where cirrus clouds form.
Cirrus clouds are thin and wispy, the result of buffeting by fast westerly winds high in the atmosphere, according to USA Today. A common nickname for cirrus clouds is “mare’s tails,” due to looking like a horse’s tail affected by a high wind. Cumulus clouds are puffy, with flat bases and very lumpy tops. Large areas of blue sky usually appear between cumulus clouds, especially in summer. When enough moisture is present and the atmosphere is unstable, cumulus clouds can rise to great heights, forming flat-based thunderheads that resemble giant anvils.
Because of the extremely low temperatures in the part of the atmosphere where they form, cirrus clouds are composed of ice crystals. Cumulus clouds, which develop in a much warmer layer of the atmosphere, form when water vapor condenses and rises on warm air.
Cirrus clouds are sometimes a forerunner to a thunderstorm, as the cumulus clouds that mature into a high-level thunderhead reach into the part of the atmosphere where water vapor becomes ice crystals. Cirrus clouds that typically occur with a thunderstorm form as the thunderstorm weakens and begins to break apart. Cumulus clouds are fair-weather clouds as well, except when they reach high levels. When they do, they're capable of delivering heavy precipitation.