4.6: Clouds

Cloud types

Clouds form when the air reaches saturation high in the atmosphere. We learned earlier that in the Troposphere, temperature decreases with altitude. Thus, if warm, moist air near the surface were to rise adiabatically, it would cool as it expanded and reach its dew point, forming a cloud. Depending on the temperature within the cloud, they're composed of liquid droplets or ice crystals. Figure \(\PageIndex{1}\) provides a brief overview of the various cloud types and their common abbreviations.

Figure \(\PageIndex{1}\): The cloud naming convention divides the atmosphere into the following cloud categories based on altitude: Low Clouds (below 6,500 ft) include Stratus (St), Stratocumulus (Sc), and Cumulus (Cu); Middle Clouds (6,500 to 20,000 ft); Altocumulus (Ac), Altostratus (As), Nimbostratus (Ns); and High Clouds (above 20,000 ft): Cirrus (Ci), Cirrocumulus (Cc), Cirrostratus (Cs). Nimbostratus (Ns) and Cumulonimbus (Cb) are rain-producing clouds that may cross the altitude boundaries between the classification levels. (CC BY-SA 3.0; Valentin de Bruyn via Wikimedia Commons).

Clouds can be identified based on their appearance, shape, and altitude. The two primary cloud categories are cumuliform and stratiform clouds.

By Appearance and Shape

Cumuliform clouds develop as a result of vertical motions by atmospheric instability. They are convective clouds, meaning they form in buoyant air parcels undergoing convection —the transfer of heat or mixing within a fluid due to warm air rising and cool air sinking. Examples of cumuliform clouds include cumulus, cumulus congestus, and cumulonimbus.

Stratiform clouds are horizontally layered clouds. They tend to spread over wide areas and take on a sheet-like appearance. They typically form when a layer of air is brought to saturation but is thermodynamically stable, or when a convective cloud meets a stable layer and spreads out in a layered fashion. Examples of stratiform clouds include nimbostratus, stratus, altostratus, cirrostratus, and cirrus.

By Water Phase

Clouds composed of only liquid water droplets are called “warm clouds” and typically have clearly defined edges. Low-altitude clouds are usually warm clouds.

Clouds composed entirely of ice crystals are called “cold clouds” and typically have fuzzy edges. The edges appear fuzzy and not well-defined because it takes longer for the ice phase to transition to the vapor phase than in warm clouds. This timescale results in a larger cloud-to-dry air transition region. High-altitude clouds are always cold.

Clouds composed of liquid water and ice crystals are called “mixed phase clouds”. It isn't easy to tell by eye whether a cloud is mixed-phase. Often, the decision comes down to the height of the cloud, since clouds that extend from the surface to high in the atmosphere likely contain a mixture of liquid droplets and ice crystals.

By Altitude

Clouds at a high altitude (above 7 km) have the prefix “cirro” or “cirrus”. Cirrus clouds are made of ice crystals and often appear in the form of white, delicate filaments, mostly in patches or narrow bands. They may have a fibrous (hair-like) and/or silky sheen appearance as shown in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Cirrus Clouds over Indonesia (CC BY-SA 4.0; Stefy1576 via Wikimedia Commons).

Thin and white, cirrocumulus clouds look like a patchy sheet or layer arranged somewhat regularly into grains or ripples without shading, as shown in Figure \(\PageIndex{3}\).

Figure \(\PageIndex{3}\): Cirrocumulus clouds on a summer afternoon (CC BY-SA 3.0; King of Hearts via Wikimedia Commons).

Cirrostratus clouds appear as transparent, whitish, milky, veil-like clouds with a fibrous (hair-like) or smooth appearance, as shown in Figure \(\PageIndex{4}\). A sheet of cirrostratus is very extensive and can cover the whole sky, while still allowing sunlight through and creating shadows. The sun appears with halos around it because light reflects and refracts off ice crystals.

Figure \(\PageIndex{4}\): Cirrostratus clouds covering the clouds in a veil. (Public Domain; Ten Basic Clouds via NOAA).

Clouds at mid-altitude, between 2 and 7 km, have the prefix “alto”. They are usually made out of liquid droplets, but can be a mixture of liquid droplets and ice crystals. Altocumulus clouds appear as puffy masses, larger than cirrocumulus, as shown in Figure \(\PageIndex{5}\). They are also darker and more shaded than cirrocumulus clouds.

Figure \(\PageIndex{5}\): Altocumulus clouds often create beautiful sunset skies. (CC BY-SA 3.0; Simon Eugster via Wikimedia Commons)

Mid-level clouds can also form layers and are called Altostratus clouds, as shown in Figure \(\PageIndex{6}\). Altostratus clouds do not produce a halo phenomenon, nor are the shadows of objects on the ground visible. The sky is visibly darker, with a faint sun visible through the clouds.

Figure \(\PageIndex{6}\): Sun shines dimly through altostratus cloud layer. (CC BY-SA 4.0; Famartin via Wikimedia Commons)

Low-level clouds generally have the prefix "strato", which means layered. Stratocumulus clouds are gray, patchy, sheet, or layered clouds that almost always have a dark honeycomb appearance with rounded masses or rolls. The individual masses are much larger than altostratus or cirrostratus and much darker, as shown in Figure \(\PageIndex{7}\).

Figure \(\PageIndex{7}\): Stratocumulus clouds appear much darker, with regularly arranged masses. (Public Domain; Ten Basic Clouds via NOAA)

By Characteristics

The prefix “nimbo” or suffix “nimbus” indicates a precipitating cloud. Nimbostratus usually has light to moderate precipitation as shown in Figure \(\PageIndex{8}\). It is thick enough throughout to blot out the sun. Low, ragged clouds called stratus fractus frequently occur beneath this cloud and sometimes merge with its base.

Figure \(\PageIndex{8}\): Nimbostratus clouds cover the whole sky and produce rain. (CC BY-SA 3.0; Togabi via Wikimedia Commons)

A cumulonimbus or thunderstorm cloud is a heavy and dense cloud in the form of a mountain or huge tower. The upper portion is usually smoothed and flattened in the shape of an anvil or vast plume due to strong upper-level winds, as shown in Figure \(\PageIndex{9}\). Some of the strongest updrafts can be found in the core of this cloud. Cumulonimbus clouds produce rain, thunder, lightning, and can sometimes also produce hail and tornadoes.

Figure \(\PageIndex{9}\): A cumulonimbus cloud rising tall with a flattened top. (Public Domain; NOAA via Flickr)

Other

The above clouds represent the primary cloud types; however, many other cloud names and types are used in atmospheric sciences. A few common cloud names are listed below.

Lenticular clouds are stationary lens-shaped clouds with a smooth appearance that usually form over the summit of a mountain or on the lee wave crest. They are also referred to as lee-wave clouds or mountain-wave clouds. As air rises over a mountain, it cools and forms a cloud. As it descends on the other side, it heats up, and the cloud dissipates, leaving the lenticular cloud near the summit, as shown in Figure \(\PageIndex{10}\).

Figure \(\PageIndex{10}\): A lenticular cloud covers the summit crater of Mayon Volcano, Philippines. (CC BY-SA 4.0; Patryk Reba via Wikimedia Commons)

Mammatus clouds are formed from downward convection, typically in the anvil portion of a cumulonimbus. They have a distinctive look that makes them easy to spot, especially at sunset. They are made up of hanging pouches that look a little bit like udders, as shown in Figure \(\PageIndex{11}\).

Figure \(\PageIndex{11}\): Mammatus clouds over the Nepal Himalayas. (CC BY-SA 4.0; Anton Yankovyi via Wikimedia Commons)