Cloudy - Tropospheric Class - Summary of Families, Genera, Species, Varieties, Supplementary Features, Mother Clouds, and Associat - Vertical Cumuliform, Stratiform, and Cumulonimbiform (low To Middle Cloud Base)

Vertical Cumuliform, Stratiform, and Cumulonimbiform (low To Middle Cloud Base)

• Genus cumulus (Cu) - moderate vertical extent:

These cumuliform clouds of free convection generally have clear-cut flat bases and domed tops and are capable of producing showers.

• Species Cumulus mediocris (Cu med): This species achieves moderate vertical development, has medium-grey shading underneath, and can produce scattered showers of light intensity. This larger species is coded C_L2.

• Varieties: Cumulus mediocris is always opaque and therefore has no opacity-based varieties. A single pattern-based variety, radiatus, is sometime seen when the individual clouds are arranged into parallel rows.

• Precipitation-based supplementary features: Cumulus mediocris can produce virga and praecipitatio features.
• Accessory clouds: The pannus supplementary feature is sometimes seen with precipitating Cu mediocris, however the C_L7 reporting code normally used with this feature is overridden by the C_L2 code that identifies cumulus with significant vertical development. Pileus (cap cloud), velum (apron), arcus (roll or shelf cloud) and tuba (vertical column) features are also occasionally seen with cumulus mediocris.
• Genitus mother clouds: Cumulus mediocris may form as a result of a partial transformation of altocumulus or stratocumulus.
• Mutatus mother clouds: This genus and species type may also be the result of a complete transformation of stratocumulus or stratus.

• Genus nimbostratus (Ns):

This is a very thick diffuse dark-grey non-convective stratiform layer that looks feebly illuminated from the inside. It normally forms from altostratus and achieves vertical extent when the base subsides into the low altitude range during precipitation that can reach moderate to heavy intensity. It is coded C_M2 on the SYNOP report.

• Species and varieties: Nimbostratus is very thick, opaque, and featureless, so this genus type is not subdivided into species or varieties.

• Precipitation-based supplementary features: Nimbostratus is a major precipition cloud and produces the virga or praecipitatio features. The latter can achieve heavy intensity due to the cloud's vertical depth.
• Accessory cloud: Pannus frequently forms in precipitation and is coded C_L7.
• Genitus mother clouds: This genus type can form from cumulus and cumulonimbus.
• Mutatus mother clouds: Nimbostratus can form due to the complete transformation of altocumulus, altostratus and stratocumulus.

ICAO towering vertical sub-group:

• Genus cumulus (Cu) - great vertical extent:

Increasing airmass instability can cause free-convective cumulus to grow very tall to the extent that the vertical height from base to top is greater than the base-width of the cloud.

• Species: Cumulus congestus (Cu Con) is the largest of the cumulus species and is designated separately as towering cumulus (Tcu) by the International Civil Aviation Organization : They grow upward to great vertical size, usually with dark-grey bases, and are capable of producing severe turbulence and showers of moderate to heavy intensity. It is coded C_L2. The varieties, supplementary features, and mother clouds assiciated with Cu-congestus or towering-Cu are the same as for cumulus mediocris.

• Non-WMO variant: Pyrocumulus (No official abbreviation) is a free convective cloud associated with volcanic eruptions and large-scale fires. Pyrocumulus is not recognized by the WMO as a distinct genus or species, but is, in essence, cumulus congestus formed under special circumstances that can also cause severe turbulence.

• Genus cumulonimbus (Cb):

This genus type is a heavy towering cumulonimbiform mass of free convective cloud with a dark-grey to nearly black base that is associated with thunderstorms and showers. Thunderstorms can produce a range of severe weather that includes hail, tornadoes, a variety of other localized strong wind events, several types of lightning, and local very heavy downpours of rain that can cause flash floods, although lightning is the only one of these that requires a thunderstorm to be taking place. In general, cumulonimbus require moisture, an unstable air mass, and a lifting force (heat) in order to form. Cumulonimbus typically go through three stages: the developing stage, the mature stage (where the main cloud may reach supercell status in favorable conditions), and the dissipation stage. The average thunderstorm has a 24 km (15 mi) diameter. Depending on the conditions present in the atmosphere, these three stages take an average of 30 minutes to go through.

• Species: Cumulonimbus calvus (Cb cal) has a very high clear-cut domed top similar to towering cumulus and is coded C_L3. The capillatus species (Cb cap) has very high tops that have become fibrous due to the presence of ice crystals. It is coded C_L9 in the SYNOP report.

• Varieties: Cumulonimbus is too large and opaque to show any opacity or pattern-based varieties.

• Precipitation-based supplementary features: This is also a major precipitation cloud and can produce virga or praecipitatio features, of which the latter can reach heavy intensity.
• Accessory clouds: The cloud-based supplementary features normally associated with cumulonimbus are pannus, incus (cirriform anvil top), mamma, pileus, velum, arcus, and tuba. As with precipitating cumulus, the C_L7 coding for pannus is overridden by higher codes, in this case C_L3 or 9 depending on the species of cumulonimbus. The tuba feature can develop into a funnel cloud, water spout, or tornado.
• Genitus mother clouds: Cumulonimbus can develop from altocumulus, altostratus, nimbostratus, stratocumulus, and cumulus.
• Mutatus mother cloud: This genus type can also result from the complete transformation of cumulus undergoing rapid vertical growth.