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.
Stratus clouds form when weak vertical currents lift a layer of air off the ground and it depressurizes, following the lapse rate. This causes the relative humidity to increase due to 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
Stratus nebulosus clouds appear as a featureless or nebulous veil or layer of stratus clouds with no distinctive features or structure.[6] They are found at low altitudes, and are a good sign of atmospheric stability, which indicates continuous stable weather. Stratus nebulosus may produce light rain and drizzle or flakes of snow. Stratus fractus clouds on the other hand, appear with an irregular shape, and forms with a clearly fragmented or ragged appearance.[6] They mostly appear under the precipitation of major rain-bearing clouds; these are nimbostratus and cumulonimbus clouds, and are classified as pannus clouds. Stratus fractus can also form beside mountain slopes, without the presence of 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]
Stratus undulatus clouds during a rainy day.
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 nimbostratus clouds (stratus nimbostratogenitus) and on cumulonimbus clouds (stratus cumulonimbogenitus). Stratus fractus clouds can also form under the base of precipitation-bearing clouds and are classified as pannus clouds. Stratus clouds may also form from formation mechanisms that are not typical for the cloud type, for example, Stratus homogenitus, which are stratus formed by human activity, Stratus cataractagenitus, which are formed from the spray of 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 stratocumulus spreading out under an inversion, indicating a continuation of prolonged cloudy weather with drizzle for several hours and then an improvement as it breaks into stratocumulus. Stratus clouds can persist for days in anticyclone conditions. It is common for a stratus to form on a weak warm front, rather than the usual 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]
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]
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]
Cirrus is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition onto rocky or metallic dust particles at high altitudes. Globally, they form anywhere between 4,000 and 20,000 meters above sea level, with the higher elevations usually in the tropics and the lower elevations in more polar regions.
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may compose the droplets and crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture from an adjacent source to raise the dew point to the ambient temperature.
Cumulus clouds are clouds that have flat bases and are often described as puffy, cotton-like, or fluffy in appearance. Their name derives from the Latin cumulus, meaning "heap" or "pile". Cumulus clouds are low-level clouds, generally less than 2,000 m (6,600 ft) in altitude unless they are the more vertical cumulus congestus form. Cumulus clouds may appear by themselves, in lines, or in clusters.
Altostratus is a middle-altitude cloud genus made up of water droplets, ice crystals, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapor to condense. Altostratus clouds are usually gray or blueish featureless sheets, although some variants have wavy or banded bases. The sun can be seen through thinner altostratus clouds, but thicker layers can be quite opaque.
A stratocumulus cloud, occasionally called a cumulostratus, belongs to a genus-type of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumulus, and the whole being at a lower height, usually below 2,000 metres (6,600 ft). Weak convective currents create shallow cloud layers because of drier, stable air above preventing continued vertical development. Historically, in English, this type of cloud has been referred to as a twain cloud for being a combination of two types of clouds.
A nimbostratus cloud is a multilevel, amorphous, nearly uniform, and often dark-grey cloud that usually produces continuous rain, snow, or sleet, but no lightning or thunder.
Cirrostratus is a high-altitude, very thin, generally uniform stratiform genus-type of cloud. It is made out of ice-crystals, which are pieces of frozen water. It is difficult to detect and it can make halos. These are made when the cloud takes the form of thin cirrostratus nebulosus. The cloud has a fibrous texture with no halos if it is thicker cirrostratus fibratus. On the approach of a frontal system, the cirrostratus often begins as nebulous and turns to fibratus. If the cirrostratus begins as fragmented of clouds in the sky it often means the front is weak. Cirrostratus is usually located above 5.5 km (18,000 ft). Its presence indicates a large amount of moisture in the upper troposphere. Clouds resembling cirrostratus occasionally form in polar regions of the lower stratosphere. Polar stratospheric clouds can take on this appearance when composed of tiny supercooled droplets of water or nitric acid.
Stratus may refer to:
A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold fronts, and move more slowly than the cold fronts which usually follow because cold air is denser and less easy to remove from the Earth's surface. This also forces temperature differences across warm fronts to be broader in scale. Clouds ahead of the warm front are mostly stratiform, and rainfall generally increases as the front approaches. Fog can also occur preceding a warm frontal passage. Clearing and warming is usually rapid after frontal passage. If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage thundershowers may continue. On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.
Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric clouds. These aerosols are found in the troposphere, stratosphere, and mesosphere, which collectively make up the greatest part of the homosphere. Clouds consist of microscopic droplets of liquid water, tiny crystals of ice, or both, along with microscopic particles of dust, smoke, or other matter, known as condensation nuclei. Cloud droplets initially form by the condensation of water vapor onto condensation nuclei when the supersaturation of air exceeds a critical value according to Köhler theory. Cloud condensation nuclei are necessary for cloud droplets formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the amount of supersaturation needed for condensation to occur is so large, that it does not happen naturally. Raoult's law describes how the vapor pressure is dependent on the amount of solute in a solution. At high concentrations, when the cloud droplets are small, the supersaturation required is smaller than without the presence of a nucleus.
June Gloom is a mainly Southern California term for a weather pattern that results in cloudy, overcast skies with cool temperatures during the late spring and early summer. While it is most common in the month of June, it can occur in surrounding months, giving rise to other colloquialisms, such as “Graypril,” "May Gray," "No-Sky July," and "Fogust." Low-altitude stratus clouds form over the cool water of the California Current, and spread overnight into the coastal regions of California.
Drizzle is a light precipitation which consists of liquid water drops that are smaller than those of rain – generally smaller than 0.5 mm (0.02 in) in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates from drizzle are on the order of a millimetre (0.04 in) per day or less at the ground. Owing to the small size of drizzle drops, under many circumstances drizzle largely evaporates before reaching the surface, and so may be undetected by observers on the ground. The METAR code for drizzle is DZ and for freezing drizzle is FZDZ.
Stratiform may refer to:
Fractus clouds, also called fractostratus or fractocumulus, are small, ragged cloud fragments that are usually found under an ambient cloud base. They form or have broken off from a larger cloud, and are generally sheared by strong winds, giving them a jagged, shredded appearance. Fractus have irregular patterns, appearing much like torn pieces of cotton candy. They change constantly, often forming and dissipating rapidly. They do not have clearly defined bases. Sometimes they are persistent and form very near the surface. Common kinds include scud and cloud tags.
Freezing drizzle is drizzle that freezes on contact with the ground or an object at or near the surface. Its METAR code is FZDZ.
In meteorology, the different types of precipitation often include the character, formation, or phase of the precipitation which is falling to ground level. There are three distinct ways that precipitation can occur. Convective precipitation is generally more intense, and of shorter duration, than stratiform precipitation. Orographic precipitation occurs when moist air is forced upwards over rising terrain and condenses on the slope, such as a mountain.
A homogenitus, anthropogenic or artificial cloud is a cloud induced by human activity. Although most clouds covering the sky have a purely natural origin, since the beginning of the Industrial Revolution, the use of fossil fuels and water vapor and other gases emitted by nuclear, thermal and geothermal power plants yield significant alterations of the local weather conditions. These new atmospheric conditions can thus enhance cloud formation.
A cloud étage is a meteorological term used to delimit any one of three main altitude levels in the troposphere where certain cloud types usually form. The term is derived from the French word which means floor or storey, as in the floor of a multi-storey building. With the exception of the low étage, the altitude range of each level varies according to latitude from Earth's equator to the arctic and antarctic regions at the poles.
Stratus nebulosus is a species of low-level stratus cloud. It is one of only two species that is associated with stratus clouds, other being fractus. Translated from Latin meaning nebulous, their cloud abbreviations can be respectively written as ‘St neb’. For a cloud to be classified as nebulosus, there has to be zero sign of detail in the cloud.
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