Clouds are a visible mass of liquid droplets, frozen crystals, or a mixture of both floating in the free air. While on Earth, clouds are generally formed of water droplets and ice crystals, clouds also form on other planets and moons, depending on their atmospheric conditions. Venus has clouds composed largely of sulfuric acid in its carbon dioxide atmosphere, Mars has clouds of both water and carbon dioxide ice particles, and clouds on the gas giants are primarily formed of methane and ammonia in their hydrogen-dominant atmosphere.

On Earth, clouds form when invisible water vapor in the atmosphere condenses into visible droplets. Many different types of clouds exist, and they play an important part in the Earth's weather and climate. At night, clouds reflect heat back to the Earth's surface and keep it warm. During the day, clouds produce shade, keeping the Earth cooler. Cloud formation and the condensation of water vapor lead to precipitation. As clouds gather more water and the size of droplets/crystals increases, gravity causes them to fall back to Earth as precipitation. The branch of meteorology regarding the study of clouds is known as nephology.

Clouds generally form in the troposphere, the layer of the atmosphere closest to the surface of the Earth. For water vapor to condense into water droplets or ice crystals, the parcel of air must be saturated; i.e., it is unable to hold all the water it contains in vapor form. Saturation can occur in one of two ways:

1. Increasing the water content in the air, e.g., through evaporation.
2. Cooling the air such that it reaches its dew point?the temperature at which condensation occurs. There is a maximum amount of water vapor air can hold at a given temperature. Generally, the warmer the air, the more water vapor it can hold. Therefore, reducing its temperature decreases its ability to hold water vapor, and condensation will occur.

Graph showing the phase of water with respect to temperature and pressure.

Cooling air is typically what causes clouds to form, with the temperature drop occurring when air rises. As air moves up through the atmosphere, the pressure drops, and this allows it to expand and therefore cool, a process known as adiabatic cooling. The cooling rate as air rises depends on the water content or humidity of the air, with higher humidity causing air to retain heat and cool more slowly. Generally, the temperature drops 1 ^o C for every 100 meters that air rises. With lower temperatures and pressure, the vertical ascent reduces the amount of water vapor it can hold, increasing condensation. The height at which the dew point is reached and clouds start forming is called the condensation level.

There are five factors that cause air to rise, leading to cloud formation:

1. Surface heating?The sun heats the ground, which heats the air causing it to rise.
2. Topography?Air is forced up due to mountains or hills.
3. Weather fronts?Warm air meets cold denser air and is forced upwards.
4. Convergence?Streams of air flowing towards each other converge, forcing air to rise.
5. Turbulence?Sudden change in wind speed high in the atmosphere creates turbulent circulations in the air (eddies), which can draw air from the surface up into the atmosphere.

Typical water droplets formed during cloud formation have a diameter of about a hundredth of a millimeter, and each cubic meter of air contains roughly 100 million droplets. These small droplets can remain in liquid form until temperatures drop to around -30 ^o C; these are known as supercooled droplets. At higher altitudes where temperatures drop below this point, clouds contain ice crystals about a tenth of a millimeter long.

For clouds to form, water vapor needs something to condense onto. Cloud condensation nuclei (CCN) are small particles floating in the atmosphere that enable condensation when the air is saturated. CCNs are a subset of hygroscopic (water-attracting) aerosol particles that nucleate water droplets. CCNs must be small particles that do not settle too fast. Most are soluble, although insoluble CCNs also produce cloud formation.

Most CCN particles originate from emissions on the Earth's surface. Primary aerosols are emitted directly from the source, whereas secondary aerosols are gaseous emissions converted to aerosol particles after reactions in the atmosphere. CCNs are produced by both natural and human-made (anthropogenic) processes.

Natural CCN sources include the following:

• Seaspray
• Volcanoes
• Forests and forest fires
• Gas to particle conversion of naturally occurring gases (e.g. sulfur dioxide)

Anthropogenic CCN sources include the following:

• Industry
• Power plants
• Fires to clear cropland
• Transportation
• Gas to particle conversion of anthropogenic gases

Clouds also form when more water vapor is added to the air. This is particularly prevalent as air accumulates more moisture when passing over lakes. The lake effect refers to cold, dry air flowing across relatively warm lakes, gathering heat and moisture from evaporated lake water. This water vapor condenses into streamers of fog rising from the surface, much of which condenses to form clouds. This process commonly generates storms that produce significant snowfall downwind.

Also referred to as homogenitus or artificial, anthropogenic clouds are clouds induced by human activity. Large-scale industrial facilities such as nuclear, thermal, and geothermal power plants can significantly alter local weather conditions, creating atmospheric conditions that enhance cloud formation.

Cloud spotting guide from the Met Office.

The classification of clouds in use today is based on a system first proposed by Luke Howard, an English amateur meteorologist in 1802. Howard's system split clouds into three main types:

• Stratus
• Cumulus
• Cirrus

The naming of cloud classifications comes from combining Latin prefixes and suffixes:

• Stratus/strato?flat/layered and smooth
• Cumulus/cumulo?heaped up/puffy
• Cirrus/cirro?high up/wispy
• Alto?medium level
• Nimbus/Nimbo?rain-bearing cloud

The International Cloud Atlas recognizes ten basic cloud genera defined according to where in the atmosphere they form (high-, mid-, and low-level clouds) and their approximate appearance. These ten genera are subdivided into species , describing their shape and internal structure, and varieties , which describe the transparency and arrangement of the clouds. Overall, there are roughly 100 combinations. Clouds that get to the ground or close to the Earth’s surface are called fog.

High-level clouds occur above roughly 20,000 feet. They are given the prefix cirro , which means "curl of hair." They primarily appear thin, streaky, and white (although at low sun angles they can appear in an array of colors), due to the cold tropospheric temperatures at these levels. High-level clouds are primarily composed of ice crystals.

Cirrus clouds.

Detached clouds take a wispy, feathery form that is composed entirely of ice crystals. They often are the first sign of an approaching warm front or upper-level jet streak. Cirrus clouds do not produce precipitation that reaches the ground. Before sunrise and after sunset, cirrus clouds are often colored bright yellow or red.

Cirrostratus clouds.

Cirrocumulus clouds.

Cirrostratus form a widespread, veil-like layer (similar to stratus clouds in low levels). When sunlight or moonlight passes through the hexagonal-shaped ice crystals of cirrostratus clouds, the light is dispersed or refracted. As a warm front approaches, cirrus clouds tend to thicken into cirrostratus, which may, in turn, thicken and lower into altostratus, stratus, and even nimbostratus.

Thin, white patch, sheet, or layered clouds are composed of very small elements in the form of more or less regularly arranged grains or ripples. They generally represent a degraded state of cirrus and cirrostratus. Cirrocumulus clouds are formed from smaller clouds called cloudlets. They are generally a sign of fair weather. Like cirrus, cirrocumulus clouds are formed by ice crystals.

Mid-level clouds appear between 6,500 and 20,000 feet and are given the prefix alto . Depending on altitude, time of year, and the vertical temperature structure of the troposphere, these clouds can be composed of liquid water droplets, ice crystals, or a combination of the two, including supercooled droplets (liquid droplets with temperatures below freezing).

Altocumulus clouds.

Altocumulus clouds are mid-level clouds with "cumulo" characteristics?heap-like clouds with convective elements. Similar to cirrocumulus, altocumulus clouds can align in rows or streets with axes indicating areas of ascending moist air and clear zones, suggesting descending drier air. They are white and/or gray in color, and when a thin semitransparent patch of altocumulus passes in front of the sun or moon, a corona appears.

Altostratus clouds.

Altostratus are clouds with a flat and uniform type texture in the mid-level of the atmosphere. Frequently, altocumulus clouds indicate the approach of a warm front, and they may thicken and lower into stratus, then nimbostratus clouds producing rain or snow. Although altostratus clouds rarely produce significant precipitation at the surface, occasionally light showers may occur from a thick alto-stratus deck.

Low-level clouds do not have a set prefix, although their names are derived from strato (layered) or cumulo (heap), depending on their characteristics. Low-level clouds occur below 6500 feet and consist of liquid water droplets or sometimes supercooled droplets. During cold winter storms, they can also hold ice crystals (snow). Low-level clouds can be separated into those that develop horizontally, stratus, and those that develop vertically, cumulus.

Stratus clouds.

Stratus have a generally gray cloud layer with a uniform base, which may, if thick enough, produce drizzle, ice prisms, or snow grains. When the sun is visible through this cloud, its outline is clearly discernible. Sometimes stratus clouds appear as ragged sheets and occasionally produce a halo phenomenon at very low temperatures.

Stratocumulus clouds.

Stratocumulus are hybrid clouds of layered stratus and cellular cumulus (i.e., individual cloud elements) characteristic of cumulo type clouds, clumped together in a continuous distribution, characteristic of strato type clouds. Stratocumulus also can be thought of as a layer of cloud clumps with thick and thin areas. These clouds appear frequently in the atmosphere, either ahead of or behind a frontal system.

Cumulus clouds.

Cumulus clouds are detached, generally dense clouds with sharp outlines that develop vertically in the form of rising mounds, domes, or towers with bulging upper parts that are sometimes described as resembling a cauliflower. Over-land cumulus clouds develop on clear sky days due to diurnal convection, and they appear in the morning, before dissolving in the evening. The sunlit top parts of cumulus clouds are white while their bases can appear relatively dark.

Cumulonimbus clouds.

Cumulonimbus clouds are thunderstorm clouds that are heavy and dense in the form of a mountain or huge tower. The upper portion is usually smoothed, fibrous, or striated and nearly always flattened in the shape of an anvil or vast plume. Cumulonimbus clouds also produce hail and tornadoes. Often under the base of cumulonimbus clouds, low ragged clouds appear that may or may not merge with the base. These produce precipitation, sometime in the form of virga.

Nimbostratus clouds.

Nimbostratus are generally thick, dense stratus or stratocumulus clouds that produce steady rain or snow. Nimbostratus clouds result from thickening altostratus clouds?dark, gray clouds that are thick enough to block sunlight. Low, ragged clouds frequently occur beneath nimbostratus clouds, which sometimes merge with the base. Although nimbostratus clouds can extend high into the atmosphere, during precipitation, the cloud base lowers. This causes confusion as to whether nimbostratus clouds should be classified as mid- or low-level clouds.

Aside from altitude (genera), there are other ways of describing clouds based on different characteristics. Most genera are subdivided into species based on shape and internal structure. Therefore, a cloud is identified by its Latin genera name followed by a specific species name. Cloud species include the following:

• Fibratus?detached clouds or a thin cloud veil that consists of nearly straight or more or less irregularly curved filaments that do not terminate in hooks or tufts. This term applies mainly to cirrus and cirrostratus
• Uncinus?cirrus without grey parts, often shaped like a comma, terminating at the top in a hook, or in a tuft, the upper part of which is not in the form of a rounded protuberance.
• Spissatus?cirrus in patches, sufficiently dense to appear greyish when viewed towards the sun. It can also veil the sun, obscure its outline, or even hide it. Cirrus spissatus often originates from the upper part of a cumulonimbus cloud.
• Castellanus?clouds presenting, at least in some portion of their upper part, cumuliform protuberances in the form of turrets or towers, some of which can be taller than they are wide and are connected by a common base that seems to be arranged in lines. The castellanus character is especially evident when the clouds are seen from the side. This term applies to cirrus, cirrocumulus, attocumulus, and stratocumulus.
• Floccus?a species in which each cloud unit is a small tuft with a cumuliform appearance, the lower part of which is more or less ragged and often accompanied by virga. This term applies to cirrus, cirrocumulus, altocumulus and stratocumulus.
• Stratiformis? spread out clouds in an extensive horizontal sheet or layer. This term applies to altocumulus, stratocumulus, and, occasionally, cirrocumulus.
• Nebulosus?a nebulous or ill-defined veil or layer of clouds showing no distinct details. This term applies mainly to cirrostratus and stratus.
• Lenticularis?clouds having the shape of lenses or almonds, often elongated and usually with well-defined outlines. They occasionally show irisations. Such clouds appear most often in cloud formations of orographic origin (mountains), but may also occur in regions without marked orography. This term applies mainly to cirrocumulus, altocumulus, and stratocumulus.
• Fractus?clouds in the form of irregular shreds, which have a clearly ragged appearance. The term only applies to stratus and cumulus.
• Humilis?cumulus clouds characterized by only a small vertical extent and appearing generally as if flattened.
• Mediocris?cumulus of moderate vertical extent, with small protuberances and sproutings at their tops.
• Congestus?strongly sprouting cumulus with generally sharp outlines and often great vertical extent. The bulging upper part of cumulus congestus frequently resembles a cauliflower.
• Volutus?a long, typically low, horizontal, detached, tube-shaped cloud mass, often appearing to roll slowly about a horizontal axis. This species applies mostly to stratocumulus and occasionally altocumulus.
• Calvus?cumulonimbus in which at least some protuberances of the upper part are beginning to lose their cumuliform outlines but in which no cirriform parts can be distinguished. Protuberances and sproutings tend to form a whitish mass, with vertical striations (grooves or channels in cloud formations, arranged parallel to the flow of air and therefore depicting the airflow).
• Capillatus?cumulonimbus characterized by the presence, mostly in its upper portion, of distinct cirriform parts of clearly fibrous or striated structure. Frequently have the form of an anvil, a plume, or a vast, more or less disorderly mass of hair. Cumulonimbus capillatus is usually accompanied by a shower or by a thunderstorm, often with squalls and sometimes with hail; it frequently produces well-defined virga.

Cloud varieties provide additional characteristics, such as relative transparency or a particular arrangement of its elements. Cloud varieties include the following:

• Translucidus?translucent layers attached to clouds.
• Opacus?like translucidus, opacus represents thick layers that can block the sun.
• Radiatus?varieties that form parallel bands and strips close to each other.
• Inortus?interlaced and entangled clouds.
• Duplicatus?clouds with multiple layers that often merge to create a single and bigger cloud of a different form.
• Undulatus?clouds with different waves.
• Perlucidus?similar to translucidus, perlucidus clouds have different layers that are almost translucent.
• Vertebratus?clouds with a fishbone appearance.
• Lacunosus?clouds with several edges. This cloud variety also looks perforated.

The table below shows the species and varieties associated with each of the ten main cloud genera:

Clouds sometimes have other features attached or merging with them:

• Incus?upper portion of a cumulonimbus spread out in the shape of an anvil with a smooth, fibrous, or striated appearance
• Mamma?hanging protuberances, like udders, on the undersurface of a cloud. Occurs mostly with cirrus, cirrocumulus, altocumulus, altostratus, stratocumulus, and cumulonimbus.
• Virga?vertical or inclined trails of precipitation (fallstreaks) attached to the undersurface of a cloud that does not reach the Earth’s surface. Occurs mostly with cirrocumulus, altocumulus, altostratus, nimbostratus, stratocumulus, cumulus, and Cumulonimbus
• Praecipitatio?precipitation (rain, drizzle, snow, ice pellets, hail, etc.) falling from a cloud and reaching the Earth’s surface. Mostly encountered with altostratus, nimbostratus, stratocumulus, stratus, cumulus, and cumulonimbus.
• Arcus?dense, horizontal roll with tattered edges, situated on the lower front part of certain clouds and having, when extensive, the appearance of a dark arch. Occurs with cumulonimbus and, less often, with cumulus.
• Tuba?cloud column or inverted cloud cone, protruding from a cloud base. It constitutes the cloudy manifestation of a more or less intense vortex. Occurs with cumulonimbus and, less often, with cumulus.
• Asperitas?well-defined, wave-like structures in the underside of the cloud. More chaotic and with less horizontal organization than the variety undulatus. Asperitas is characterized by localized waves in the cloud base, either smooth or dappled with smaller features, sometimes descending into sharp points, as if viewing a roughened sea surface from below. Varying levels of illumination and thickness of the cloud can lead to dramatic visual effects. Occurs mostly with stratocumulus and altocumulus.
• Fluctus?a relatively short-lived wave formation, usually on the top surface of the cloud, in the form of curls or breaking waves (Kelvin-Helmholtz waves). Occurs mostly with cirrus, altocumulus, stratocumulus, stratus, and occasionally cumulus.
• Cavum?a well-defined generally circular (sometimes linear) hole in a thin layer of supercooled water droplet cloud. Virga or wisps of cirrus typically fall from the central part of the hole, which generally grows larger with time. Cavum is typically a circular feature when viewed from directly beneath, but may appear oval-shaped when viewed from a distance. When resulting directly from the interaction of an aircraft with the cloud, it is generally linear (in the form of a dissipation trail). Virga typically falls from the progressively widening dissipation trail. Occurs in altocumulus and cirrocumulus and rarely stratocumulus.
• Murus?a localized, persistent, and often abrupt lowering of cloud from the base of a cumulonimbus from which tuba (spouts) sometimes form. Usually associated with a supercell or severe multicell storm; typically develop in the rain-free portion of a cumulonimbus, indicating areas of strong updraft. Commonly known as a "wall cloud."
• Cauda?a horizontal, tail-shaped cloud at low levels extending from the main precipitation region of a supercell cumulonimbus to the murus (wall cloud). It is typically attached to the wall cloud, and the bases of both are typically at the same height. Cloud motion is away from the precipitation area and towards the murus, with rapid upward motion often observed near the junction of the tail and wall clouds. Commonly known as a "tail cloud."

Precipitation occurs when cloud particles grow large enough to fall and reach the ground before evaporating or sublimating. Condensation in the atmosphere is limited to producing droplets of roughly one-hundredth of an inch in diameter. This limit is the reason the vast majority of clouds do not produce rain. Raindrops range in size from 1/50 inch to 1/5 inch in diameter, and above this they tend to break up while falling. Droplet size due to condensation alone is caused by the following:

• Droplet formation uses water vapor, decreasing supersaturation, making the cloud approach an equilibrium state of saturation
• As droplets grow, the mass of water vapor becoming liquid grows and the resulting latent heat released in the condensation process warms the droplet decreasing the vapor pressure difference between it and the surrounding vapor.

For droplet growth to raindrop size to take place, one or more of the precipitation processes is required. Much of the precipitation originating in high and mid-latitudes begins as snow at higher altitudes, melting to rain as it falls. The occurrence and intensity of precipitation depend on the availability of water vapor and on the concomitant mechanisms for nucleating and growing the particles. Sublimation (the direct transition of gas to a solid) can cause ice crystals to grow large enough to fall from the base of the cloud. However, sublimation can only produce crystals large enough for very light snow, or rain if the crystals melt while falling to the ground. Moderate or heavy precipitation requires additional processes; the two main ones are the ice-crystal process and coalescence.

Clouds containing both ice crystals and supercooled liquid cloud droplets can undergo the ice-crystal process. The saturation vapor pressure differs with respect to liquid water and ice. At the same temperature, the saturation vapor pressure with respect to ice is lower than that of supercooled water. Therefore, if a cloud containing supercooled water is saturated with respect to water, it is supersaturated with respect to ice. The difference between vapor pressure over water and ice causes vapor molecules to be attracted to ice crystals, leading to significant ice crystal growth. This growth compounds the effect by reducing the humidity within the cloud causing existing liquid water to evaporate increasing the supply of water vapor crystallizing into ice. Ice crystals grow at the expense of water droplets until they reach a large enough size to fall out of the cloud as snowflakes, which can melt into raindrops as they fall.

Coalescence accounts for rain falling from clouds with temperatures above freezing. The process of condensation within clouds produces droplets of varying sizes that move at different speeds. This helps facilitate collisions where the droplets grow by fusing or coalescing. As droplets increase in size, they become more effective in the collecting process. Once they grow to the size that gravity causes them to drop from the cloud, they gather more drops on their path down through the cloud. The process of coalescence can also occur in clouds below freezing, with snowflakes coalescing with other snowflakes or supercooled water droplets to form snow pellets.

Studies based on almost a decade of satellite data estimate roughly 67 percent of Earth's surface is covered by clouds. Clouds are more prevalent over the oceans, and research shows less than 10 percent of the sky is completely cloud-free at any one time. The map below shows the global cloud fraction (CF) based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite. CF is a measure of the amount of clouds above a certain location, given as the fraction of the atmosphere volume or projected area occupied by clouds. The map shows the average of all observations taken between July 2002 and April 2015.

Cloud fraction map based on MODIS data between July 2002 - April 2015.

There are three broad bands that are most likely to be under cloud cover?a narrow band near the equator and two much wider strips in the mid-latitudes. The heavily clouded band near the equator is the result of the large-scale circulation patterns (Hadley cells) present in the tropics. Hadley cells are caused by cool air sinking near the 30 ^o latitude lines on either side of the equator and warm air rising near the equator. As warm, moist air converges at low altitudes near the equator, it rises and cools causing water vapor to condense into clouds. This area produces a regular band of thunderstorms known as the inter-tropical convergence zone (ITCZ).

The significant number of clouds forming in the middle latitude bands (60 ^o north and south of the equator) is caused by the edges of the polar and mid-latitude (Ferrel) circulation cells colliding and pushing air upwards. This ascending air fuels the large-scale frontal systems dominating weather patterns in the mid-latitude region. The less cloudy regions between 15 ^o and 30 ^o degrees north and south of the equator are dominated by descending air that inhibits cloud formation, this also accounts for the prevalence of deserts at these latitudes.

Other features present in the distribution of clouds are the following:

• The tendency for clouds to form off the west coasts of continents (particularly visible for Africa, and North and South America), due to ocean water getting pushed westward because of the Earth's rotation. Upwelling, when cooler water from deeper in the ocean rises in these locations, creates a layer of cold air encouraging condensation and low-forming clouds.
• Landscape effects, such as mountain ranges, forcing air upwards causing rain to form on the wind-facing slopes. Air moving over the top of the range generally has little moisture left, creating rain shadows that produce deserts. Examples include the Tibetan plateau (north of the Himalayan mountains) and Death Valley (East of the Sierra Nevada range in California). The rain shadow caused by the Andes mountains also produces the dryness of the coastal Atacama desert in South America.

The relationship between clouds and the wider climate is defined by a complex feedback system. Clouds affect the climate, modulating the solar radiation and water balance of the Earth, but in turn, the climate affects cloud formation and movement.

• Clouds reflect incoming sunlight cooling Earth's surface.
• Clouds warm Earth's surface by absorbing heat emitted from the surface and re-radiating it back down toward the surface.
• Clouds also warm or cool Earth's atmosphere by absorbing heat emitted from the surface and radiating it to space.
• Clouds warm and dry Earth's atmosphere and supply water to the surface through precipitation.
• Clouds are themselves created by the motions of the atmosphere that are caused by the warming or cooling of radiation and precipitation.

It is not well understood what the effect of climate change will be on weather patterns and whether the resulting cloud changes would diminish warming (negative feedback) or enhance warming (positive feedback).

Venus has a thick atmosphere of predominantly carbon dioxide. The planet is shrouded by yellowish clouds of sulfuric acid trapping heat and causing a runaway greenhouse effect. These clouds start at an altitude of 28 miles to 43 miles (45 to 70 kilometers). High in the clouds of Venus, the conditions are cooler and the pressure is similar to Earth's surface. This has led some scientists to theorize microbes might exist there. Phosphine a possible indicator of microbial life has also been observed in Venus' clouds.

Clouds are rare in the thin and dry atmosphere of Mars. They are most commonly found at the planet's equator in the coldest time of year when Mars is furthest away from the Sun. Martian clouds can be formed of water ice and carbon dioxide ice particles. The image below shows iridescent or "mother of pearl" clouds where the colors are caused by cloud particles of almost identical size. The image shows five frames stiched together from a wider panorama taken by Curiosity's Mastcam on March 5th, 2021.

"Mother of pearl" clouds observed by Curiosity rover on March 5, 2021.

Jupiter and Saturn exhibit cloud tops that can be relatively identified and cover the majority of the surface. Uranus and Neptune are largely cloud-free. Clouds on the outer planets are formed from a collection of different condensable materials (volatiles) making up different layers. Cloud layers moving through a gas giant's atmosphere are formed of volatiles that condense at increasing pressure and temperature. The highest clouds on Uranus and Neptune are composed of crystals of methane ice, which cannot form on Jupiter or Saturn due to the higher temperature.