Formation and Classification of Clouds

Clouds are formed when water evaporates from oceans, lakes, and ponds or by evapotranspiration over Earth's land surface and rises up into colder areas of the atmosphere due to convective, oragraphic, or frontal lifting. The water vapor attaches itself to condensation nuclei which could be anything from dust to microscopic particles of salt and debris. Once the vapour has been cooled to saturation, the cloud becomes visible.

Clouds can be divided into three main categories based upon the Latin root words which refer to the process of formation and physical structure of the clouds. The first of the three is Cirrus, which has only one genus and therefore shares the same name. It is in the high altitude range and occurs mostly in the form of filaments. The other two are Stratus that are mostly sheet-like in structure, and Cumulus that appear heaped, rolled, and/or rippled. Stratus and Cumulus occur in the high, medium, and low levels of the troposphere, so each of these categories have several cloud genera.

A total of ten cloud genera are derived by cross-classification of the stratus and cumulus categories into high (prefix cirro-), medium (prefix alto)-, low (no height related prefix), and low to medium (which has its own height criteria) ranges for a subtotal of eight main types. To these are added cirrus which is always high, and cumulonimbus which has its own height range characteristics as a cloud of vertical development.

The essentials of the modern nomenclature system for clouds were proposed by Luke Howard, a British manufacturing chemist and an amateur meteorologist with broad interests in science, in an 1802 presentation to the Askesian Society. Since 1890, clouds are classified and illustrated in cloud atlases.

High-level clouds (base above ca. 20,000 feet)

Cirrus

Abbreviation: Ci

Cirrus Cloud form in the highest and coldest altitude water almost always freezes so clouds are composed of ice crystals. The clouds tend to be wispy, and are often transparent. Isolated cirrus clouds often do not bring rain, however, large amounts of cirrus clouds can indicate an approaching storm system eventually followed by fair weather.

Cirrocumulus

Abbreviation: Cc

Clouds of the genus Cirrocumulus form when moist air at a high altitude reaches saturation, creating ice crystals. Convective instability at the cloud level gives the cloud its cumuliform appearance.

Cirrostratus

Abbreviation: Cs

Clouds of the genus Cirrostratus consist of a continuous, wide layer of cirriform cloud that covers a large area of the sky. It is formed when convectively stable moist air cools to saturation at a high altitude, forming ice crystals. Frontal cirrostratus is a precursor to rain or snow if it thickens into mid level altostratus and eventually nimbostratus as the front moves closer to the observer.

Medium-level clouds (base ca. 6,500 to ca. 20,000 feet)

Altocumulus

Abbreviation: Ac

Clouds of the genus Altocumulus are not always associated with a front but can still bring precipitation, usually in the form of virga which doesn't reach the ground. Like all cumulus types, this genus is an indicator of convective instability at the altitude of its formation.

Altostratus

Abbreviation: As

Clouds of the genus Altostratus form when a large convectively stable airmass is lifted to condensation altitude, usually along a frontal system, and can bring rain or snow. If the precipiation becomes continuous, it may thicken into nimbostratus.

Low-level clouds (base near surface to ca. 6,500 feet)

Stratocumulus

Abbreviation: Sc

Clouds of the genus Stratocumulus are lumpy, often forming in slightly unstable air following a cold front, and they can produce light rain or drizzle.

Stratus

Abbreviation: St

Clouds of the genus Stratus form in low horizontal layers having a ragged or uniform base. Ragged stratus often forms in precipitation while more uniform stratus forms in maritime or other moist stable air mass conditions. The latter often produces drizzle.

Nimbostratus

Abbreviation: Ns

Clouds of the genus Nimbostratus tend to bring constant precipitation and low visibility. This cloud type normally forms above 6,500 feet from altostratus cloud but can thicken into the lower levels during the occurrence of precipitation.

Vertically developed clouds (base near surface to ca. 10,000 feet; tops ca. 40,000 feet or higher)

Cumulus

Abbreviation: Cu

Small clouds of the genus Cumulus are often associated with fair weather. However, they are the product of convective airmass instability and can grow into more storm-like buildups including cumulonimbus. Continued upward growth suggests showers later in the day. These clouds usually form below 6,500 feet but can be based as high as 10,000 feet under conditions of very low relative humidity.

Cumulonimbus

Abbreviation: Cb

Clouds of the genus Cumulonimbus develop from cumulus when the airmass is convectively highly unstable. They generally produce thunderstorms, rain or showers, and sometimes strong outflow winds and/or tornados at ground level.