Stratus cloud

Stratus cloud
Stratus cloud
	Stratus nebulosus opacus cloud
Abbreviation	St
Symbol
Genus	Stratus (layered)
Species	Fractus; Nebulosus;
Variety	Opacus; Translucidus; Undulatus;
Altitude	0-2,000 m; (0-7,000 ft)
Precipitation	Common Drizzle, freezing drizzle, Snow or snow grains

Stratus clouds are low-level

clouds characterized by horizontal layering with a uniform base, as opposed to convective or cumuliform clouds formed by rising thermals. The term stratus describes flat, hazy, featureless clouds at low altitudes varying in color from dark gray to nearly white.^[2] The word stratus comes from the Latin prefix strato-, meaning "layer".^[3] Stratus clouds may produce a light drizzle or a small amount of snow. These clouds are essentially above-ground fog

formed either through the lifting of morning fog or through cold air moving at low altitudes. Some call these clouds "high fog" for their fog-like form.

Formation

Stratus clouds form when weak vertical currents lift a layer of air off the ground and it depressurizes, following the

adiabatic cooling.^[4] This occurs in environments where atmospheric stability is abundant.^[5]

Description

Stratus clouds look like featureless gray to white sheets of cloud.[4] They can be composed of water droplets, supercooled water droplets, or ice crystals depending upon the ambient temperature.

Subforms

Species

nimbus clouds

(clouds that precipitate), and their color can be from dark grey to almost white.

Opacity-based varieties

Stratus fractus are not divided into varieties, but stratus nebulosus on the other hand, are divided into two. The Stratus opacus variety appears as a nebulous or milky sheet of the nebulosus species, but are opaque enough to block the sun from view.^[6] Stratus Translucidus is another variety of the nebulosus species. These clouds are considered more thin than the opacus variety because this cloud is rather translucent, allowing the position of the Sun or Moon to be observed from Earth's surface.

Pattern-based variety

Stratus clouds only have one pattern-based variety. This is the stratus undulatus variety. Mild undulations can be observed from this cloud, only associated by the nebulosus species. Though rare, this cloud formation is caused by disturbances on the gentle wind shear. Stratus undulatus clouds are more common on stratus stratocumulomutatus clouds where the wind is stronger as height increases.^[6]

Genitus mother clouds

Stratus cumulogenitus clouds occur when the base of cumulus clouds spreads, creating a nebulous sheet of stratiform clouds. This can also occur on

waterfalls, and Stratus silvagenitus, which are formed by evaporation or evapotranspiration

occurring in a forest.

Mutatus mother cloud

Stratus only has one mutatus mother cloud. Stratus stratocumulomutatus clouds occur when

stratocumulus opacus patches fuse to create a stratiform layer.^[6]

Accessory clouds and supplementary feature

Stratus clouds do not produce accessory clouds, but a supplementary feature praecipitatio is derived from Latin, which means "precipitation". Stratus clouds are generally too low to produce virga, or rain shears that evaporate before reaching the ground, although higher stratus clouds can produce it.

Forecast

A stratus cloud can form from

nimbostratus

.

Effects on climate

According to Sednev, Menon, and McFarquhar, Arctic stratus and other low-level clouds form roughly 50% of the annual cloud cover in Arctic regions, causing a large effect on the energy emissions and absorptions through radiation.^[7]

Relation to other clouds

Cirrostratus clouds

Cirrostratus clouds, a very high ice-crystal form of stratiform clouds, can appear as a milky sheen in the sky^[8] or as a striated sheet.^[9] They are sometimes similar to altostratus and are distinguishable from the latter because the Sun or Moon is always clearly visible through transparent cirrostratus, in contrast to altostratus which tends to be opaque or translucent.^[10] Cirrostratus come in two species, fibratus and nebulosus.^[8] The ice crystals in these clouds vary depending upon the height in the cloud. Towards the bottom, at temperatures of around −35 °C (−31 °F) to −45 °C (−49 °F), the crystals tend to be long, solid, hexagonal columns. Towards the top of the cloud, at temperatures of around −47 °C (−53 °F) to −52 °C (−62 °F), the predominant crystal types are thick, hexagonal plates and short, solid, hexagonal columns.^[11]^[12] These clouds commonly produce halos, and sometimes the halo is the only indication that such clouds are present.^[13] They are formed by warm, moist air being lifted slowly to a very high altitude.^[14] When a warm front approaches, cirrostratus clouds become thicker and descend forming altostratus clouds,^[3] and rain usually begins 12 to 24 hours later.^[13]

Altostratus clouds

Nimbostratus clouds

Stratocumulus clouds

A stratocumulus cloud is another type of a cumuliform or stratiform cloud. Like stratus clouds, they form at low levels;^[3] but like cumulus clouds (and unlike stratus clouds), they form via convection. Unlike cumulus clouds, their growth is almost completely retarded by a strong inversion, causing them to flatten out like stratus clouds and giving them a layered appearance. These clouds are extremely common, covering on average around twenty-three percent of the Earth's oceans and twelve percent of the Earth's continents. They are less common in tropical areas and commonly form after cold fronts. Additionally, stratocumulus clouds reflect a large amount of the incoming sunlight, producing a net cooling effect.^[15] Stratocumulus clouds can produce drizzle, which stabilizes the cloud by warming it and reducing turbulent mixing.^[16]