Classification and Characteristics of Cloud Types

Low-Level Clouds

• Cumulus (Cu): Clouds that have a substantially horizontal base, well-defined contours, and a white color when illuminated by the sun. They cause rain in the form of showers and consist mainly of water droplets, though they may contain ice crystals at the top. Variations: humilis (vertical development), mediocris (rounded top), and fractocumulus (fragmented due to turbulence).
• Congestus: Features protruding edges on top and considerable vertical extent. It indicates profound instability and is favored by cyclonic flow at altitude.
• Cumulonimbus (Cb): A cloud with great vertical development in the form of a mountain, often only visible from afar due to its size. The top usually has a characteristic anvil shape. It is a darker cloud formed by large drops of water and hail, potentially containing ice crystals. It is associated with strong storms, thunder, and lightning.
• Stratocumulus (Sc): Gray or white clouds made up of water droplets, associated with rain. Variations: cumulusgenitus, Vesperal.
• Stratus (St): A gray, uniform cloud that causes rain. It has a strong base and usually covers the sun or the moon.

Mid-Level Clouds

These clouds are located at the intermediate stage: 2 to 8 km in tropical latitudes, 2 to 7 km in temperate regions, and 2 to 4 km in polar regions.

• Nimbostratus (Ns): Clouds of great extension and diffuse base, formed by raindrops, crystals, or flakes of ice; they appear quite dark.
• Altostratus (As): Resembles a gray or sometimes blue sheet; it always has fine patches that allow the sun to be seen. It is formed by rain drops and ice crystals.
• Altocumulus (Ac): A gray (sometimes white) cloud that has its own shadow and appears as rolls or fibrous, diffuse slides. They rarely contain ice crystals, and it is possible to see pieces of clear sky through them. Variations: lenticularis, radiatus, cumulusgenitus, opacus, plocus, or castellatus.

High-Level Clouds

These clouds form at high altitudes: 6 to 18 km in the tropics, 5 to 14 km in temperate regions, and 3 to 8 km in polar regions.

• Cirrus (Ci): Clouds with a silky shine, isolated and formed by ice crystals that seem to converge on the horizon. They can form from the evolution of a cumulonimbus anvil. Variations: filosus or fibratus, uncinus, spissatus, nothus, or densus.
• Cirrocumulus (Cc): Clouds composed almost exclusively of ice crystals grouped in semi-transparent granules. Variations: stratiformis, lenticularis, castellatus.
• Cirrostratus (Cs): Clouds that look like a transparent veil, giving the sky a milky appearance. They consist of ice crystals. Variations: fibratus, nebulosus.