Cloud

Cloud animation at intervals of 20 seconds.

A cloud is a visible hydrometeor formed by the accumulation of ice crystals and/or microscopic water droplets suspended in the atmosphere, as a consequence of the condensation of water vapor. Usually white in color, it can darken as its optical thickness increases, until it becomes a dark gray almost black.

Clouds have a great influence on the climate system, beyond the usual rainfall. They play a fundamental role in the water cycle, the regulation of the average temperature of the planet or the distribution of solar energy.

The science that studies clouds is nephology, a branch of meteorology focused on cloud formation, composition, density, temperature, evolution, movement, grouping, and classification.

Although clouds are constantly changing in appearance, depending on their shape, dimensions, structure, texture, luminance, and color, there are a limited number of common typical shapes that allow them to be grouped into a general classification system divided into genera, species, and varieties in a similar way to how animals and plants are classified.

Cloud formation

File:Cirrocumulus formation.gif

Formation of a cirrh cluster.

Clouds are formed when atmospheric water vapor condenses on small particles that float in the air called condensation nuclei (dust, salt crystals, bacteria, pollen or ash, among others).

Water vapor, coming from the evaporation or sublimation of terrestrial water masses —and, to a lesser extent, from combustion, respiration and transpiration of living beings, volcanism and other biological and geological processes— mixes with atmospheric air hydrating and heating it. Generally, the resulting warm and humid air mass rises thanks to its lower density (humid air is lighter than dry air and when it heats up it expands, further lowering its density) moving towards areas of lower atmospheric pressure where it expands and, in accordance with the ideal gas law, its temperature decreases. If it cools below the saturation temperature or the dew point, part of the water vapor contained It condenses into very small droplets of water or ice (between 0.004 and 0.1mm) that together form a cloud.

When an air mass condenses in the upper areas of the troposphere, it forms clouds of ice crystals, called cirrus clouds, cirrostratus clouds, or cirrocumulus clouds. While at a lower altitude clouds of raindrops are formed, such as the altostratus and altocumulus clouds that generally accompany warm fronts, as well as the lower altitude strata. Cumulus clouds, on the other hand, accompany cold fronts and tend to grow vertically until they form masses in height, known as cumulonimbus clouds.

This general phenomenon can occur in different ways depending on factors such as pressure, humidity and temperature.

Formation processes

Cloud formed by orographic ascension.

Clouds can be formed mainly by three different processes: orographic ascent; thermal convection or convection produced by a front.

Clouds due to orographic ascent

When an air mass hits a mountain, it rises and cools. If the air current is sufficiently humid, the water will condense and form clouds on the windward slope which are called orographic clouds. Occasionally oscillations and air velocity can also form cap or banderole clouds on the leeward slope. The most common orographic clouds belong to the genera Cumulus, Altocumulus and Stratocumulus.

Cumulus mediocris, moderate vertical development cloud formed by thermal convention.

Thermal convection clouds

A current of warm and humid air rises to higher and colder layers, giving rise to the formation of cumulus clouds. This usually occurs below 3 km altitude. The cloud can grow in height, becoming a cumulonimbus. When the rain falls, the cloud separates into two fragments, because the hot air cannot rise. As the cloud breaks up, the rain stops. Short but very intense storms occur.

Clouds produced by a front

Cloud produced by a front.

The fronts are contact zones between two air masses that have different temperatures and densities. If a mass of hot and humid air, in movement, hits a mass of cold air, horizontal clouds are formed, called nimbostratus (3 km altitude), altostratus (between 3 and 5 km altitude) or cirrus and cirrostratus (12 km above sea level). of altitude). Nimbostratus and altostratus generally produce rain. On the other hand, cirrus clouds indicate good weather if they do not move quickly or change into cirrostratus. Cumulonimbus clouds are formed when a moving cold air mass collides with a warm air mass.

Classification of tropospheric clouds

A few clouds in line passing over Santiago, Chile.

At sunset, these clouds take a reddish color, due to the angle of the sun's rays (see volleyball)

Clouds in the city of Hermosillo, Mexico.

Cirros and loudspeakers.

The classification of tropospheric clouds according to their visual characteristics comes from the World Meteorological Organization (WMO) and is included in the International Cloud Atlas, first published in 1930. Without However, the current international classification system in Latin dates back to 1803, when the amateur meteorologist Luke Howard wrote the essay The Modifications of Clouds, in which he differentiated between three types of clouds (cirrus , cumulus, stratus) and four intermediate categories (Cirro-cumulus, cirro-stratus, cumulo-stratus and cumulo-cirro-stratus or Nimbus).

The current WMO classification divides clouds into genera, species and varieties in a similar way to that used in the classification of animals or plants, and, as in these cases, Latin names are used. The system recognizes ten types of clouds, based on their appearance and the part of the sky in which they form: Cumulonimbus, cumulus, stratus, cirrostratus, altostratus, nimbostratus, cirrus, cirrocumulus, altocumulus, stratocumulus. These genera are subdivided into fifteen species, which describe the shape and internal structure of the cloud, and into varieties, which describe the transparency and distribution of the clouds. In total there are about a hundred combinations.

In addition to the general classification, there are two other cloud classifications: special clouds and clouds in the upper layers of the atmosphere. These clouds are rarely or occasionally observed, and in some cases only in certain parts of the planet.

The International Cloud Atlas currently recognizes ten genera of clouds (basic classifications), which describe where in the sky they form and their approximate appearance.

High clouds usually form above 5,000 meters; medium clouds usually form between 2,000 and 7,000 meters; and low clouds usually form at a maximum height of 2,000 meters.

Most cloud names contain Latin prefixes and suffixes which, when combined, give an indication of cloud type. Some of them are the following:

• Stratus/strato: elongated, flattened, and leveled

• Cumulus/cumulus: heap, heap

• Cirrus/cirrus: feathery, fringed

• Nimbus/nimbus: bringer of rain

• Alto: medium level (although altus means high in Latin)

The 10 genera are subdivided into species, which describe the shape and internal structure of the cloud, and varieties, which describe the transparency and distribution of the clouds. In total there are about 100 combinations.

Tropospheric cloud families arranged by altitude

Cloud view from an airplane

According to their altitude, they are grouped into families named by a capital letter:

• Family D, vertical development: less than 3 km: Types nimbostratus and cumulonimbusand cumulus species mediocrities and congestus.
• Family C, low height: less than 2 km: Types stratus and stratocumulusand cumulus species fractus and humilis.
• Family B, average height: 2 to 5 km: Types hightratus and highcumulus.
• Family A, large height: above 5 km: Types cirrus, cirrostratusand cirrocumulus.

Genres

The official names of the different cloud genera are given in Latin and are here translated into English. The World Meteorological Organization (WMO) distinguishes ten combined types, according to their shape: cirrus, cirrocumulus, cirrostratus, altostratus, altocumulus, stratus, stratocumulus, nimbostratus, cumulus and cumulonimbus. The first eight are stratiform clouds, because they are parallel to the earth's surface; the last two are cumuliform, because they are formed vertically.

Most but not all types can be divided into species, some of which can be further subdivided into varieties. Accessory clouds are special formations sometimes considered to be of a particular type or species.

Classification of clouds by altitude.

Species

Different types of genera are divided into species that indicate specific structural details. However, because the latter types are not always limited by height range, some species may be common to several different genera.

Strocumuliforms and Cirriforms

Species of Castellanus that resemble castle towers when viewed from the side, can be found in any type of partially convective stratocumuliform and circumform clouds. The species floccus is also sometimes seen in clouds of the same categories or shapes when they appear as separate globular tufts.

Cumuliforms and Nimbiforms

With the exception of stratocumulus, an unstable local air mass located at lower levels tends to produce convective cumulus and different types of cumulonimbus clouds, whose species are mainly indicators of the degree of vertical development. A cumulus cloud initially forms as a puff of the species fractus or humilis, showing only slightly vertical development. If the air becomes more unstable, the cloud tends to grow vertically first into the mediocris species, and then into congestus, the tallest cumulus species. With increased instability, the cloud may continue to grow into cumulonimbus calvus (essentially a very high congestus cloud, producing thunder), then ultimately when water droplets at the upper sill are supercooled, turning into ice crystals, giving the capillatus a cirriform appearance.^{[citation needed]}

Varieties

The varieties describe the transparency and the distribution of the visible elements of the clouds. A variety may be common to several genera, and a cloud may have characteristics of more than one variety.

Summary of tropospheric types and subtypes arranged by family, height, category and shape

Vertical development (family D): Nimbiforms, cumuliforms and stratiforms

Large vertical development (sub-family D2)

Within 3 km

Cumulonimbo incus near a mountain.

Clusters.

These clouds can have strong updrafts, rise well above their bases, and form at high altitudes.

• Type Cumulonimbus (associated with large rainfall and storms) (Cb)
  • Species Cumulonimbus calvus (Cb cal)
  • Species Cumulonimbus chapeltus (Cb cap)
    • • Accessory Cloud Cumulonimbus pannus
      • Accessory Cloud Cumulonimbus incus
      • Cumulonimbus cloud accessory
      • Accessory Cloud Cumulonimbus pileus
      • Accessory Cloud Cumulonimbus velum
      • Accessory Cloud Cumulonimbus arcus
      • Accessory Cloud Cumulonimbus tuba
• Type Cumulus (Cu)
  • Species Cumulus congestus (Cu con/TCu)
    • Variety Cumulus congestus radiatus
      • Accessory Cloud Cumulus congestus pannu
      • Accessory Cloud Cumulus congestus pileus
      • Accessory Cloud Cumulus congestus velum
      • Accessory Cloud Cumulus congestus arcus
      • Accessory Cloud Cumulus congestus tuba

Moderate vertical development. Subfamily D1

Within 3 km

• Type Nimbostratus (Ns)

• Accessory Cloud Nimbostratus pannus

• Type Cumulus (Cu)
  • Species Cumulus mediocrities (Cu med)
    • Variety Cumulus mediocris radiatus

Low elevation (family C): Stratiforms, stratocumuliforms and cumuliforms

Less than 2 km away

• Type Stratus (St)
  • Species stratus nebulosis (St neb)
  • Species stratus fractus (St fra)
• Type Stratocumulus (Sc)
  • Species Stratocumulus castellanus (Sc cas)
  • Species Stratocumulus lenticularis (Sc len)
  • Species Stratocumulus stratiformis (Sc str)
    • Variety Stratocumulus stratiformis translucidus
    • Variety Stratocumulus stratiformis perlucidus
    • Variety Stratocumulus stratiformis opacus
• Type Cumulus (Cu)
  • Species Cumulus fractus (Cu fra)
  • Species Cumulus humilis (Cu hum)

Medium height (family B): Stratiforms and stratocumuliforms

From 2 to 5 km

• Type Altostratus (As)

• Variety Altostratus undulatus

• Type Altocumulus (Ac)
  • Species Altocumulus floccus (Ac flo)
  • Species Altocumulus castellanus (Ac cas)
  • Species Altocumulus lenticularis (Ac len)
  • Species Altocumulus stratiformis (Ac str)
    • Variety Altocumulus stratiformis translucidus
    • Variety Altocumulus stratiformis perlucidus
    • Variety Altocumulus stratiformis opacus
    • Variety Altocumulus stratiformis undulatus

Large height (family A): Cirriforms, stratiforms and stratocumuliforms

Cloud views from Puerto Caldera, Costa Rica

From 5 km onwards

• Type Cirrus (Ci)
  • Species Cirrus uncinus (Ci unc)
  • Species Cirrus Spissatus
  • Species Cirrus floccus
  • Especie Cirrus castellanus (Ci cas)
  • Species Cirrus fibertus (Ci fib)
    • Variety Cirrus fibertus intortus
    • Variety Cirrus fibertus radiatus
    • Variety Cirrus fibertus vertebratus
    • Variety Cirrus fibertus duplicatus
• Type Cirrostratus (Cs)
  • Species cirrostratus nebulosus (Cs neb)
  • Species cirrostratus fibertus (Cs fib)
    • Variety cirrostratus fibertus duplicatus
    • Variety cirrostratus fibertus undulatus
• Type Cirrocumulus (Cc)
  • Species Cirrocumulus floccus (Cc flo)
  • Species Cirrocumulus castellanus (Cc cas)
  • Species Cirrocumulus lenticularis (Cc len)
  • Species Cirrocumulus stratiformis (Cc str)
    • Variety Cirrocumulus undulatus
    • Variety Cirrocumulus lacunosus

Tropospheric orographic clouds

In addition to the previous types, there are different types of fog and a group of tropospheric clouds called orographic cloud, and there are:

• lenticular clouds: Stratocumulus/altocumulus/cirrocumulus lenticularis:

• 

  Stratocumulus lenticularis

• 

  Altocumulus lenticularis

• 

  Cirrocumulus lenticularis

• banner clouds: Stratocumulus/altocumulus/cirrocumulus stratiformis^{[chuckles]required]}

Cloud generated by the wake of a plane.

Special clouds

These are clouds that are formed as a result of generally localized factors. These factors can be of natural origin or the result of human activity. Several types of special clouds can be differentiated, such as those produced by fires and volcanic eruptions (flammagenitus), those generated by the contrails of airplanes or industrial processes (homogenitus and homomutatus) or those formed in the vicinity of large waterfalls and forests (cataractagenitus and silvagenitus respectively).

Clouds outside the troposphere

Polar Stratospheric Cloud

From 15 to 25 km

• Naked clouds (Nacreous): Clouds made of frozen water crystals.
• Super-cooled polar stratospheric: clouds containing water and nitric acid, sometimes with sulfuric acid.

Polar Mesospheric Cloud

80 to 85 km

• Noctilucente: type I veils, type II bands, type III waves and type IV cloud tugs.

Effects on the troposphere, climate and climate change

Light rays filtered through the clouds (Barcelona)

Tropospheric clouds have numerous influences on Earth's troposphere and climate. First of all, they are the source of precipitation, greatly influencing the distribution and amount of precipitation. Because of their differential buoyancy relative to the surrounding cloud-free air, clouds can be associated with vertical air motions that can be convective, frontal, or cyclonic. The movement is upward if the clouds are less dense because the condensation of water vapor releases heat, warming the air and thus decreasing its density. This can cause a downward movement because the elevation of the air produces a cooling that increases its density. All of these effects are subtly dependent on the vertical temperature and humidity structure of the atmosphere and result in a significant redistribution of heat that affects Earth's climate.

The complexity and diversity of clouds in the troposphere is one of the main reasons for the difficulty in quantifying the effects of clouds on climate and climate change. On the one hand, white cloud tops promote cooling of the Earth's surface by reflecting short-wave radiation (visible and near-infrared) from the sun, decreasing the amount of solar radiation that is absorbed at the surface, improving the albedo of the Earth. Most of the sunlight that hits the ground is absorbed, heating the surface, which emits upward radiation at longer infrared wavelengths. However, at these wavelengths, cloud water acts as an effective absorber. The water reacts by radiating, also in infrared, both upwards and downwards, and downward longwave radiation causes increased heating at the surface. This is analogous to the greenhouse effect of greenhouse gases and water vapor.

High-level genus types particularly display this duality with both shortwave albedo cooling and longwave greenhouse warming effects. In general, clouds of ice crystals in the upper troposphere (cirrus) tend to favor net warming. However, the cooling effect is dominant with mid- and low-level clouds, especially when they form in extensive layers. NASA measurements indicate that, in general, the effects of mid- and low-level clouds that tend to promote cooling outweigh the warming effects of upper layers and the variable results associated with clouds developed vertically.

As difficult as it is to assess the influences of current clouds on today's climate, it is even more problematic to predict changes in cloud patterns and properties in a warmer future climate and the resulting cloud influences on future climate. In a warmer climate, more water would enter the atmosphere by evaporation at the surface; Since clouds are formed from water vapor, cloudiness would be expected to increase. But in a warmer climate, higher temperatures would tend to evaporate clouds. Both statements are considered accurate and both phenomena, known as cloud feedback, are found in climate model calculations. Generally speaking, if clouds, especially low clouds, increase in a warmer climate, the resulting cooling effect leads to negative feedback in the climate response to increased greenhouse gases. But if the low clouds decrease or if the high clouds increase, the feedback is positive. The different amounts of these feedbacks are the main reason for the differences in the climate sensitivities of current global climate models. As a consequence, much research has focused on the response of low and vertical clouds to a changing climate. However, leading world models produce quite different results, with some showing increasing low clouds and others showing decreases. For these reasons, the role of tropospheric clouds in regulating weather and climate remains a major source of uncertainty. in global warming projections.

On other planets

On planets other than Earth, clouds can be made of other substances.

Clouds on Venus are made up of droplets of sulfuric acid. Mars has clouds of water and carbon dioxide. Titan is covered in a dense hydrocarbon fog, which hides clouds of methane. The giant planets Jupiter and Saturn have upper ammonia clouds and intermediate ammonium hydrosulfide clouds and deep water clouds. Uranus and Neptune possibly possess deep Jovian-like clouds and certainly upper methane clouds.

Informal terms

• Skypunch
• Pyrocumulus
• Airline suit. A stroke of a cloud characterized by being extremely thin, caused by the passage of some type of aircraft operating with turbines (turborectors or turbolelices). (OMM Cirrus)
• Fluctus