In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, ice crystals, or other particulates, suspended in the atmosphere of a body or similar space.
Clouds are seen in the Earth's homosphere, which includes the troposphere, stratosphere, and mesosphere.
Nephology is the science of clouds, which is undertaken in the cloud physics branch of meteorology. The World Meteorological Organization uses two methods of naming clouds in their respective layers of the homosphere, Latin and common name.
Genus types in the troposphere, the atmospheric layer closest to Earth's surface, have Latin names because of the universal adoption of Luke Howard's nomenclature that was formally proposed in 1802. It became the basis of a modern international system that divides clouds into five physical forms which can be further divided or classified into altitude levels to derive ten basic genera. The five main forms are stratus cloud sheets or veils, cumuliform heaps, stratocumuliform bands, rolls, or ripples, cumulonimbiform towers often with fibrous tops, and cirrus cloud wisps or patches. Low-level clouds do not have any altitude-related prefixes. However mid-level stratiform and stratocumuliform types are given the prefix alto- while high-level variants of these same two forms carry the prefix cirro-. In the case of stratocumuliform clouds, the prefix strato- is applied to the low-level genus type but is dropped from the mid- and high-level variants to avoid double-prefixing with alto- and cirro-. Genus types with sufficient vertical extent to occupy more than one level do not carry any altitude-related prefixes. They are classified formally as low- or mid-level depending on the altitude at which each initially forms, and are also more informally characterized as multi-level or vertical. Most of the ten genera derived by this method of classification can be subdivided into species and further subdivided into varieties. Very low stratiform clouds that extend down to the Earth's surface are given the common names fog and mist but have no Latin names.
In the stratosphere and mesosphere, clouds also have common names for their main types. They may have the appearance of veils or sheets, wisps, or bands or ripples, but not heaps or towers as in the troposphere. They are seen infrequently, mostly in the polar regions of Earth. Clouds have been observed in the atmospheres of other and moons in the Solar System and beyond. However, due to their different temperature characteristics, they are often composed of other substances such as methane, ammonia, and sulfuric acid, as well as water.
Tropospheric clouds can have a direct effect on climate change on Earth. They may reflect incoming rays from the Sun which can contribute to a cooling effect where and when these clouds occur, or trap longer wave radiation that reflects up from the Earth's surface which can cause a warming effect. The altitude, form, and thickness of the clouds are the main factors that affect the local heating or cooling of the Earth and the atmosphere. Clouds that form above the troposphere are too scarce and too thin to have any influence on climate change. Clouds are the main uncertainty in climate sensitivity.
Etymology
The origin of the term "cloud" can be found in the
Old English words clud or
clod, meaning a hill or a mass of stone. Around the beginning of the 13th century, the word came to be used as a metaphor for rain clouds, because of the similarity in appearance between a mass of rock and cumulus heap cloud. Over time, the metaphoric usage of the word supplanted the Old English
weolcan, which had been the literal term for clouds in general.
Homospheric nomenclatures and cross-classification
The table that follows is very broad in scope like the cloud genera template upon which it is partly based. There are some variations in styles of nomenclature between the classification scheme used for the troposphere (strict Latin except for surface-based aerosols) and the method used for the higher levels of the homosphere (common terms, some informally derived from Latin). However, these two schemes, both of which are authorized and used by the World Meteorological Organization, share a cross-classification of physical forms and altitude levels to derive the 10 tropospheric genera,
the fog and mist that forms at surface level, and several additional major types above the troposphere. The cumulus genus includes four species that indicate vertical size which can affect the altitude levels.
History of cloud science
Ancient cloud studies were not made in isolation, but were observed in combination with other
weather elements and even other natural sciences. Around 340 BC, Greek philosopher
Aristotle wrote
Meteorologica, a work which represented the sum of knowledge of the time about natural science, including weather and climate. For the first time, precipitation and the clouds from which precipitation fell were called meteors, which originate from the Greek word
meteoros, meaning 'high in the sky'. From that word came the modern term
meteorology, the study of clouds and weather.
Meteorologica was based on intuition and simple observation, but not on what is now considered the scientific method. Nevertheless, it was the first known work that attempted to treat a broad range of meteorological topics in a systematic way, especially the hydrological cycle.
After centuries of speculative theories about the formation and behavior of clouds, the first truly scientific studies were undertaken by
Luke Howard in England and Jean-Baptiste Lamarck in France. Howard was a methodical observer with a strong grounding in the Latin language, and used his background to formally classify the various tropospheric cloud types during 1802. He believed that scientific observations of the changing cloud forms in the sky could unlock the key to weather forecasting.
Lamarck had worked independently on cloud classification the same year and had come up with a different naming scheme that failed to make an impression even in his home country of France because it used unusually descriptive and informal French names and phrases for cloud types. His system of nomenclature included 12 categories of clouds, with such names as (translated from French) hazy clouds, dappled clouds, and broom-like clouds. By contrast, Howard used universally accepted Latin, which caught on quickly after it was published in 1803.
An elaboration of Howard's system was eventually formally adopted by the International Meteorological Conference in 1891.
Formation
Terrestrial clouds can be found throughout most of the homosphere, which includes the troposphere, stratosphere, and mesosphere. Within these layers of the
atmosphere, air can become saturated as a result of being cooled to its
dew point or by having moisture added from an adjacent source.
In the latter case, saturation occurs when the dew point is raised to the ambient air temperature.
Adiabatic cooling
Adiabatic cooling occurs when one or more of three possible lifting agents – convective, cyclonic/frontal, or orographic – cause a parcel of air containing invisible water vapor to rise and cool to its dew point, the temperature at which the air becomes saturated. The main mechanism behind this process is adiabatic cooling.
As the air is cooled to its dew point and becomes saturated, water vapor normally condenses to form cloud drops. This condensation normally occurs on cloud condensation nuclei such as
salt or dust particles that are small enough to be held aloft by normal circulation of the air.
One agent is the convective upward motion of air caused by daytime solar heating at surface level.
Frontal and cyclonic lift occur in the troposphere when stable air is forced aloft at weather fronts and around centers of low pressure by a process called convergence zone.
A third source of lift is wind circulation forcing air over a physical barrier such as a mountain (orographic lift).[Pidwirny, M. (2006). "Cloud Formation Processes" , chapter 8 in Fundamentals of Physical Geography, 2nd ed.]
Clouds formed by any of these lifting agents are initially seen in the troposphere where these agents are most active. However, water vapor that has been lifted to the top of troposphere can be carried even higher by gravity waves where further condensation can result in the formation of clouds in the stratosphere and mesosphere.[ About NLCs, Polar Mesospheric Clouds, from Atmospheric optics]
Non-adiabatic cooling
Along with adiabatic cooling that requires a lifting agent, three major nonadiabatic mechanisms exist for lowering the temperature of the air to its dew point. Conductive, radiational, and evaporative cooling require no lifting mechanism and can cause condensation at surface level resulting in the formation of
fog.
[Ackerman, p. 109]
Adding moisture to the air
Several main sources of water vapor can be added to the air as a way of achieving saturation without any cooling process:
evaporation from surface water or moist ground,
precipitation or
virga,
and
transpiration from plants.
Tropospheric classification
Classification in the troposphere is based on a hierarchy of categories with physical forms and altitude levels at the top.
These are cross-classified into a total of ten genus types, most of which can be divided into species and further subdivided into varieties which are at the bottom of the hierarchy.
Clouds in the troposphere assume five physical forms based on structure and process of formation. These forms are commonly used for the purpose of satellite analysis.
-
Nonconvective Stratus cloud appear in stable airmass conditions and, in general, have flat, sheet-like structures that can form at any altitude in the troposphere.
-
Cirrus cloud in the troposphere are of the genus cirrus and have the appearance of detached or semi-merged filaments. They form at high tropospheric altitudes in air that is mostly stable with little or no convective activity, although denser patches may occasionally show buildups caused by limited high-level convection where the air is partly unstable.
-
Stratocumuliform clouds both cumuliform and stratiform characteristics in the form of rolls, ripples, or elements.
-
Cumulus cloud generally appear in isolated heaps or tufts.
-
Cumulonimbus clouds are largest free-convective clouds, which has a towering vertical extent. They occur in highly unstable air
Levels and genera
Tropospheric clouds form in any of three levels (formerly called ) based on altitude range above the Earth's surface. The grouping of clouds into levels is commonly done for the purposes of
, surface weather observations,
and
.
[JetStream (2008). How to read weather maps. National Weather Service. Retrieved on 16 May 2007.] The base-height range for each level varies depending on the latitudinal geographical zone.
Each altitude level comprises two or three genus-types differentiated mainly by physical form.
The standard levels and genus-types are summarised below in approximate descending order of the altitude at which each is normally based.
High-level
High clouds form at altitudes of in the
, in the temperate regions, and in the
tropics.
All cirriform clouds are classified as high, thus constitute a single genus
cirrus (Ci). Stratocumuliform and stratiform clouds in the high altitude range carry the prefix
cirro-, yielding the respective genus names
cirrocumulus (Cc) and
cirrostratus (Cs). If limited-resolution satellite images of high clouds are analyzed without supporting data from direct human observations, distinguishing between individual forms or genus types becomes impossible, and they are collectively identified as
high-type (or informally as
cirrus-type, though not all high clouds are of the cirrus form or genus).
-
Genus Cirrus cloud (Ci) – these are mostly fibrous wisps of delicate, white, cirriform, ice crystal clouds that show up clearly against the blue sky.
-
Genus cirrocumulus (Cc) – this is a pure white high stratocumuliform layer of limited convection. It is composed of ice crystals or supercooled water droplets appearing as small unshaded round masses or flakes in groups or lines with ripples like sand on a beach.
-
Genus cirrostratus (Cs) – cirrostratus is a thin nonconvective stratiform ice crystal veil that typically gives rise to halos caused by refraction of the sunlight. The Sun and Moon are visible in clear outline.
Mid-level
Nonvertical clouds in the middle level are prefixed by
alto-, yielding the genus names
altocumulus (Ac) for stratocumuliform types and
altostratus (As) for stratiform types. These clouds can form as low as above surface at any latitude, but may be based as high as near the poles, at midlatitudes, and in the tropics.
As with high clouds, the main genus types are easily identified by the human eye, but distinguishing between them using satellite photography alone is not possible. When the supporting data of human observations are not available, these clouds are usually collectively identified as
middle-type on satellite images.
-
Genus altocumulus (Ac) – This is a midlevel cloud layer of limited convection that is usually appears in the form of irregular patches or more extensive sheets arranged in groups, lines, or waves.
-
Genus altostratus (As) – Altostratus is a midlevel opaque or translucent nonconvective veil of gray/blue-gray cloud that often forms along warm fronts and around low-pressure areas. Altostratus is usually composed of water droplets, but may be mixed with ice crystals at higher altitudes. Widespread opaque altostratus can produce light continuous or intermittent precipitation.
Low-level
Low clouds are found from near the surface up to .
Genus types in this level either have no prefix or carry one that refers to a characteristic other than altitude. Clouds that form in the low level of the troposphere are generally of larger structure than those that form in the middle and high levels, so they can usually be identified by their forms and genus types using satellite photography alone.
-
Genus stratocumulus (Sc) – This genus type is a stratocumuliform cloud layer of limited convection, usually in the form of irregular patches or more extensive sheets similar to altocumulus but having larger elements with deeper-gray shading.
-
Species cumulus humilis – These are small detached fair-weather cumuliform clouds that have nearly horizontal bases and flattened tops, and do not produce rain showers.
-
Genus Stratus cloud (St) – This is a flat or sometimes ragged nonconvective stratiform type that sometimes resembles elevated fog.
Multi-level or moderate vertical
These clouds have low- to mid-level bases that form anywhere from near the surface to about and tops that can extend into the mid-altitude range and sometimes higher in the case of nimbostratus.
-
Genus nimbostratus (Ns) – This is a diffuse, dark gray, multi-level stratiform layer with great horizontal extent and usually moderate to deep vertical development that looks feebly illuminated from the inside.
[Ackerman, p. 118] This thick cloud layer lacks any towering structure of its own, but may be accompanied by embedded towering cumuliform or cumulonimbiform types.
-
Species cumulus mediocris – These cumuliform clouds of free convection have clear-cut, medium-gray, flat bases and white, domed tops in the form of small sproutings and generally do not produce precipitation.
Towering vertical
These very large cumuliform and cumulonimbiform types have cloud bases in the same low- to mid-level range as the multi-level and moderate vertical types, but the tops nearly always extend into the high levels. Unlike less vertically developed clouds, they are required to be identified by their standard names or abbreviations in all aviation observations (METARS) and forecasts (TAFS) to warn pilots of possible severe weather and turbulence.
-
Species cumulus congestus – Increasing airmass instability can cause free-convective cumulus to grow very tall to the extent that the vertical height from base to top is greater than the base-width of the cloud. The cloud base takes on a darker gray coloration and the top commonly resembles a cauliflower. This cloud type can produce moderate to heavy showers
-
Genus cumulonimbus (Cb) – This genus type is a heavy, towering, cumulonimbiform mass of free-convective cloud with a dark-gray to nearly black base and a very high top in the form of a mountain or huge tower.
[Ted Fujita (1985). "The Downburst, microburst and macroburst". SMRP Research Paper 210.] and .
Species
Genus types are commonly divided into subtypes called
species that indicate specific structural details which can vary according to the stability and windshear characteristics of the atmosphere at any given time and location. Despite this hierarchy, a particular species may be a subtype of more than one genus, especially if the genera are of the same physical form and are differentiated from each other mainly by altitude or level. There are a few species, each of which can be associated with genera of more than one physical form.
The species types are grouped below according to the physical forms and genera with which each is normally associated. The forms, genera, and species are listed from left to right in approximate ascending order of instability or convective activity.
{| class="wikitable"
!Forms and levels !! Stratiform
non-convective !! Cirriform
mostly nonconvective !! Stratocumuliform
limited-convective !! Cumuliform
free-convective !! Cumulonimbiform
strong convective
|-
!High-level
| Cirrostratus||Cirrus cloud
non-convective limited convective || Cirrocumulus|| ||
|-
!Mid-level
| Altostratus || || Altocumulus||
|-
!Low-level
| Stratus cloud|| || Stratocumulus|| Cumulus ||
|-
!Multi-level or moderate vertical
| Nimbostratus || || || Cumulus ||
|-
!Towering vertical
| || || || Cumulus
