Clouds are visible masses of liquid water droplets or frozen ice crystals suspended in the atmosphere. The formal classification system groups them based on their appearance and the altitude at which they form. This framework organizes the ten primary cloud types, known as genera, into four distinct families. These families are defined by the specific height zone in the troposphere where the cloud base is typically found.
High-Altitude Cloud Groups
The highest family of clouds forms in the coldest region of the troposphere, generally above 20,000 feet (6,000 meters). Due to the extremely low temperatures at this elevation, these cloud formations are composed almost entirely of minute ice crystals. The Latin prefix cirro-, meaning “curl of hair,” is used to denote clouds in this high-level classification.
This group includes Cirrus, Cirrocumulus, and Cirrostratus clouds. Cirrus clouds are the most common, appearing as delicate, wispy, feathery streaks across the sky. Cirrostratus clouds form a widespread, veil-like layer that can cover the entire sky. When sunlight or moonlight passes through the ice crystals of a Cirrostratus cloud, it often refracts the light to create a noticeable halo effect.
Mid-Altitude Cloud Groups
Clouds in the mid-altitude family have their bases typically situated between 6,500 and 20,000 feet (2,000 to 6,000 meters). The composition of these clouds is often a mix, consisting of liquid water droplets, supercooled water, and ice particles. All clouds in this zone are identified using the prefix alto-.
The two main types are Altocumulus and Altostratus. Altocumulus clouds appear as white or gray patches, sheets, or layers that often feature rounded masses or rolls, frequently aligning in rows across the sky. Altostratus clouds form a uniform, grayish or bluish sheet that can appear featureless. While they possess enough density to often obscure the sun or moon, they do not usually produce significant precipitation.
Low-Altitude Cloud Groups
The low-altitude family encompasses clouds that form from the surface up to 6,500 feet (2,000 meters). These clouds are composed primarily of liquid water droplets, although they may contain ice or snow in colder conditions. This family includes the cloud types designated with the Latin prefix strato-.
Stratus clouds are uniform, gray layers resembling elevated fog, which can occasionally produce light drizzle. Stratocumulus clouds are low, lumpy layers that appear in distinct rolls or patches, but these formations rarely produce significant rain. Nimbostratus is a dark, gray, rain-bearing sheet that covers the entire sky and is characterized by continuous, steady precipitation.
Clouds Spanning Multiple Altitudes
The final family is categorized not by a fixed height layer, but by their powerful vertical development, extending from low bases into mid and high altitudes. These formations are driven by strong convective currents of rising air. The cloud types in this group primarily use the prefix cumulo- or nimbo-.
Cumulus are the familiar puffy, cotton-like clouds with flat bases and dome-shaped tops. Small Cumulus clouds, sometimes called “fair weather” clouds, show little vertical growth, but they can develop into the more complex, towering Cumulonimbus. Cumulonimbus clouds are the most massive and dynamic, possessing great vertical extent. These towering storm systems are associated with heavy precipitation, hail, lightning, and are often identifiable by a characteristic flattened anvil shape at their tops.