Cloudspotting

A cloud is a visible mass of droplets, in other words, little drops of water or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae. Clouds are studied in the nephology or cloud physics branch of meteorology.

On Earth the condensing substance is typically water vapor, which forms small droplets or ice crystals, typically in diameter. When surrounded by billions of other droplets or crystals they become visible as clouds. Dense deep clouds exhibit a high reflectance (70% to 95%) throughout the visible range of wavelengths. They thus appear white, at least from the top. Cloud droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with depth into the gases, hence the gray or even sometimes dark appearance at the cloud base. Thin clouds may appear to have acquired the color of their environment or background and clouds illuminated by non-white light, such as during sunrise or sunset, may appear colored accordingly. Clouds look darker in the near-infrared because water absorbs solar radiation at those wavelengths.

Cirrus clouds generally refer to atmospheric clouds that are characterized by thin, wisplike strands, often accompanied by tufts, leading to their common (non-standard) name of 'mares tail'''. Sometimes these clouds are so extensive that they are virtually indistinguishable from one another, forming a sheet of cirrus called cirrostratus. Sometimes convection at high altitudes produces another form of cirrus called cirrocumulus, a pattern of small cloud tufts which include droplets of supercooled water. The term is also used for certain interstellar clouds composed of sub-micrometre sized dust grains.

Many cirrus clouds produce hair like filaments made of the heavier ice crystals that precipitate from them. These "fall streaks", a form of virga, often indicate the difference in the motion of air (wind shear) between the upper part of the cirrus cloud and the air below it. Sometimes the top of the cirrus cloud is moving rapidly above a slower layer of air, or the streak is falling into a faster moving lower layer. The directions of these winds can also vary.

Cirrus uncinus is a type of cirrus cloud. The name cirrus uncinus is derived from Latin, meaning "curly hooks". Also known as mares tails'', these clouds are generally sparse in the sky, and very thin.

The clouds occur at very high altitudes, at a temperature of about Minus 40-50 Celsius. They are generally seen when a warm or occluded front is approaching. They are very high in the troposphere, and generally mean that precipitation, usually rain, is approaching.

Cirrostratus clouds are thin, generally uniform clouds, composed of ice-crystals, capable of forming halos. They are usually located above 5.5 km . When thick enough to be seen, they are whitish, usually with no distinguishing features. When covering the whole sky and sometimes so thin as to be hardly discernible, this may indicate a large amount of moisture in the upper atmosphere.Ludlum, D. (1991), New York: Alfred A. Knopf. ISBN 0-679-40851-7

Cirrostratus clouds sometimes signal the beginning of a warm front and thus may be signs that precipitation might follow in the next 12 to 24 hours.

Category: image - :Wavecloudsduval.jpg|thumb|right|300px|A KH instability rendered visible by clouds over Mount Duval in Australia

Category: Image - :KHI.gif|300px|Numerical Simulation of a temporal Kelvin?Helmholtz instability|thumb|right

Category: Image - :Spatial KH 2D vorticity.gif|300px|Numerical Simulation of a spatial Kelvin?Helmholtz instability|thumb|right

Kelvin?Helmholtz instability can occur when velocity shear is present within a continuous fluid or, when there is sufficient velocity difference across the interface between two fluids. One example is wind blowing over a water surface, where the wind causes the relative motion between the stratified layers (i.e., water and air). The instability will manifest itself in the form of waves being generated on the water surface. The theory can be used to predict the onset of instability and transition to turbulent flow in fluids of different densities moving at various speeds. Hermann von Helmholtz studied the dynamics of two fluids of different densities when a small disturbance such as a wave is introduced at the boundary connecting the fluids.

Cirrocumulus clouds are high-altitude clouds that usually occur at an altitude of 5 km to 12 km. Like other cumulus clouds, cirrocumulus clouds signify convection. Unlike other cirrus clouds, cirrocumulus include a small amount of liquid water droplets, although these are in a supercooled state. Ice crystals are the predominant component, and typically, the ice crystals cause the supercooled water drops in the cloud to rapidly freeze, transforming the cirrocumulus into cirrostratus. This process can also produce precipitation in the form of a virga consisting of ice or snow. Thus cirrocumulus clouds are usually short-lived.The cloud collectors handbook Gavin Pretor-Pinney, ISBN 978 0 340 91943 9 page 21

Properly, the term cirrocumulus refers to each cloud, but is typically also used to refer to an entire patch of cirrocumulus. When used in this way, each cirrocumulus element is referred to as a "cloudlet".

Contrails (short for "condensation trails") or vapour trails are basically artificial clouds, visible trails of condensed water vapour, made by the exhaust of aircraft engines. As the hot exhaust gases cool in the surrounding air they may precipitate a cloud of microscopic water droplets. If the air is cold enough, this trail will comprise tiny ice crystals.

The wingtip vortices which trail from the wingtips and wing flaps of aircraft are sometimes partly visible due to condensation in the cores of the vortices. Each vortex is a mass of spinning air and the air pressure at the centre of the vortex is very low. These wingtip vortices are not the same as contrails.

Depending on atmospheric conditions, contrails may be visible for only a few seconds or minutes, or may persist for many hours.Contrails, Cirrus Trends, and Climate - joint paper by Patrick Minnis, Atmospheric Sciences, NASA Langley Research Center; J Kirk Ayers, Rabinda Palikonda and Dung Phan, Analytical Services and Materials

Category: File - :Qantas and contrails.jpg|thumb|750px|center|Contrails from a Qantas jet, Australia

Category: File - :Sarychev peak eruption.jpg|thumb|right|Pileus forming over the ash cloud from the eruption of Sarychev Peak

A pileus (Latin for cap) is a small, horizontal cloud that can appear above a cumulus or cumulonimbus cloud, giving the parent cloud a characteristic "hoodlike" appearance. Pilei tend to change shape rapidly. They are formed by strong updrafts acting upon moist air at lower altitudes, causing the air to cool to its dewpoint. As such, they are usually indicators of severe weather, and a pileus found atop a cumulus cloud often foreshadows transformation into a cumulonimbus cloud, as it indicates a strong updraft within the cloud.

Clouds that are attached to pileus are often given the suffix "pileus" or "with pileus". For example, a cumulonimbus cloud with a pileus attached to it would be called "cumulonimbus with pileus".

Pilei can also form above ash clouds from erupting volcanoes (see the image to the right).

Altostratus is a cloud belonging to a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. The sun can be seen shining through them, and they frequently cover the whole sky. They are similar to lower altitude stratus clouds.

Altostratus is caused by a large air mass that is lifted then condensed, usually by an incoming frontal system and can be found over widespread areas. Altostratus clouds are potentially dangerous, because they can cause ice accretion on aircraft. Their altitude is from 8,000-20,000 feet (2,400-6,100 m). They are made from ice crystals.

Altostratus clouds can produce very light precipitation. However, the precipitation may be virga, especially in dry places. They are also at a medium altitude, but they can sometimes bring the altitude lower to produce rain and/or snow due to the weight of the cloud. Once the cloud seems to be lower, the sun can no longer be seen.

The altostratus undulatus is a type of low altocumulus cloud with signature undulations within it. These undulations may be visible (usually as "wavy bases"), but frequently they are indiscernible to the naked eye. These formations will generally appear in the early stages of destabilizing return flows, especially over the southern plains of the United States, when the surface temperature is still relatively cool. The wavy strips of clouds are generally near an inversion surface.

Also referred to as billow clouds, wind row clouds, or wave clouds, variations of the undulatus can be elements that have merged or single elements that have stretched through the sky. They often run parallel, but can also appear to interweave across the sky, especially if dual wave systems are seen (also referred to as biundulatus). Gravity waves aligned normally to the direction of the wind can show wide, near-parallel cloud strips.

Category: Image - :CloudRipples gobeirne.jpg|thumb|none|400px|Undulated cloud in Christchurch, New Zealand.

Altocumulus (Alto, "high", cumulus, "heaped") is a cloud belonging to a class characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. Like other cumulus clouds, altocumulus signifies convection. It is usually white or gray, and often occurs in sheets or patches with wavy, rounded masses or rolls. Altocumulus often are seen preceding a cold front, and their presence on a warm, humid, summer morning frequently signals the development of thunderstorms later in the day. Alto means high and these clouds may cause rain if they are higher up.

One form of altocumulus, altocumulus lenticularis (Lenticular cloud) is frequently reported as "Unidentified flying objects". The altocumulus approach can sometimes be disturbing, for the cloud has a dark and somewhat frightening appearance. It is about 4,000 feet to 20,000 feet (1,200 to 6,100 meters) above ground level.

image:skyAltocumulusClouds.jpg|Altocumulus in a clear mackerel pattern

Image:Mackerelskylincolnshire.jpg|Mackerel sky over Lincolnshire, UK.

image:Virga 05.jpg|Altocumulus virga.

Image:Markeral-Bold2-Color.jpg|Mackerel Sky

Image:Altocumulus cloud.jpg|Altocumulus cloud, Tel-Aviv, Israel

The altocumulus undulatus is a mid-level cloud (about 8000 - 20,000 ft or 2400 - 6100 m), usually white or grey with layers or patches containing undulations that resemble "waves" or "ripples" in water. Elements within the cloud (such as the edges of the undulations) are generally darker than those in cirrocumulus and smaller than those in stratocumulus. These clouds may appear both as patches or as covering the sky. The width of these clouds is generally less than 300 feet (91.44 meters) thick. The presence of altocumulus undulatus may indicate precipitation within the next 20 hours or simply an overcast day.

As with other altocumulus, the undulatus variety can form in all seasons, announcing an approaching system within the general area (about 100 - 200 mi or 160 - 322 km). They result from wind shear (an abrupt speed or directional shifting in the wind, acutely changing with height). Lines one might see indicate the direction of the shear. In the southern United States, these formations may be the result of tropical or subtropical system outflow in a northern direction. They may also come from a southwestern monsoon making its way across the Western United States and Great Plains.

Altocumulus Castellanus (ACCAS) is named for its tower-like projections that billow upwards from the base of the cloud. The base of the cloud can form as low as 2,000 metres (6,500 feet), or as high as 6,000 metres (20,000 feet).

Castellanus clouds are evidence of mid-atmospheric instability and a high mid-altitude lapse rate. They may be a harbinger of bad weather and, if surface-based convection can connect to the mid-tropospheric unstable layer, continued development of Castellanus clouds can produce cumulonimbus clouds.

Altocumulus Castellanus clouds are typically accompanied by moderate turbulence as well as potential icing conditions. For these reasons, flight through Altocumulus Castellanus clouds is often best avoided by aircraft.

The appearance of Altocumulus Castellanus early in a sunny day may indicate a high probability the formation of thunderstorms in the afternoon, as they may develop into Cumulonimbus cloud storm clouds.

Lenticular clouds are stationary lens-shaped clouds that form at high altitudes, normally aligned perpendicular to the wind direction. Lenticular clouds can be separated into altocumulus standing lenticularis (ACSL), stratocumulus standing lenticular (SCSL), and cirrocumulus standing lenticular (CCSL).

Cumulus humilis is what is commonly referred to as "fair weather cumulus". In hot countries and over mountainous terrain these clouds occur at up to 6000 meters altitude, though elsewhere they are typically found lower.

They are formed by rising warm air that has been heated by the ground, which in turn has been heated by the sun. They have a limited depth (technically known as showing no significant vertical development). This indicates that the temperature in the atmosphere above them either drops off very slowly or not at all with altitude (see Lapse rate). While cumulus humilis may be accompanied by other cloud types, when they appear in a clear sky (see picture), they are an indicator of pleasant weather for the next several hours. Though at times, these clouds collapse into stratocumulus clouds, and cover much of the sky.

Air below the cloud base can be quite turbulent, giving occupants of light aircraft an uncomfortable ride. To avoid turbulence where such clouds are present, pilots may climb above the cloud tops. However, glider pilots actively seek out the rising air to gain altitude.

A Nimbostratus cloud is characterized by a formless cloud layer that is almost uniformly dark gray. "Nimbo" is from the Latin word "nimbus", meaning rain. It is a stratiform cloud that produces rain, developing cloud bases between the surface and 10000 ft (3000 m). Nimbostratus usually has a thickness of 2000 meters. In rare cases, Nimbostratus can be very thin and accompanied by a separate layer of altostratus divided by a cloudless layer. Though found worldwide, nimbostratus is found more commonly in the middle latitudes.

A stratus cloud (St) is a cloud belonging to a class characterized by horizontal layering with a uniform base, as opposed to convective clouds that are as tall or taller than wide (these are termed cumulus clouds). More specifically, the term stratus is used to describe flat, hazy, featureless clouds of low altitude varying in color from dark gray to nearly white. A "cloudy day" usually features a sky filled with stratus clouds obscuring the disk of the sun. These clouds are essentially above-ground fog formed either through the lifting of morning fog or when cold air moves at low altitudes over a region.

Cumulonimbus (Cb) is a type of cloud that is tall, dense, and involved in thunderstorms and other intense weather. Cumulonimbus means "column rain" in Latin. It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold front in a squall line. They create lightning through the heart of the cloud. Cumulonimbus clouds form from cumulus clouds (namely from cumulus congestus) and can further develop into a supercell, a severe thunderstorm with special features.

A cumulonimbus incus (Latin incus, "anvil") is a cumulonimbus cloud which has reached the level of stratospheric stability and has formed the characteristic flat, anvil-top shape. It can cause a supercell and then a tornado.

A cumulonimbus incus is a sub-form of cumulonimbus capillatus.

Image:Anvil cumulus feb 2007.jpg|Anvil Cumulonimbus incus, February 2007

Image:Cumulonimbus incus cloud.jpg|Cumulonimbus incus

Image:Cumulonimbus incus cloud Jan2008.jpg|Cumulonimbus incus with little precipitation observed

Image:Cumulonimbus-incus_mykonos.jpg|Single-cell Cumulonimbus incus - Mykonos / Greece, June 2009

A stratocumulus cloud belongs to a class of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumuli, and the whole being at a lower altitude, usually below 2,400 m (8,000 ft). Weak convective currents create shallow cloud layers because of drier, stable air above preventing continued vertical development.

Category: File - :Stratocumulus lenticularis.jpg|thumb|right|300px|Stratocumulus lenticularis clouds in Jackson, WY

Vast areas of subtropical and polar oceans are covered with massive sheets of stratocumuli. These may organize into distinctive patterns which are currently under active study. In subtropics, they cover the edges of the horse latitude climatological highs, and reduce the amount of solar energy absorbed in the ocean. When these drift over land the summer heat or winter cold is reduced. 'Dull weather' is a common expression incorporated with overcast stratocumulus days. If the air over land is moist and hot enough they may develop to various cumulus clouds, or, more commonly, the sheets of thick stratocumuli may have a nimbostratus look on them. The distinction here is the amount of rain produced. On drier areas they quickly dissipate over land, resembling cumulus humilis.

Most often,stratocumuli produce no precipitation, and when they do, it is generally only light rain or snow. However, these clouds are often seen at either the front or tail end of worse weather, so may indicate storms to come, in the form of thunderheads or gusty winds.

These are same in appearance to altocumuli and are often mistaken for such. A simple test to distinguish these is to compare the size of individual masses or rolls: when pointing your hand in the direction of the cloud, if the cloud is about the size of your thumb, it is altocumulus; if it is the size of your entire hand, it is stratocumulus.

Mammatus, also known as mammatocumulus, meaning "Mammary cloud" or "Breast cloud" http://weatherwing.com/Cumulonimbus-Mammatus.htmlhttp://www.erh.noaa.gov/er/pit/mam98.htm is a meteorological term applied to a cellular pattern of pouches hanging underneath the base of a cloud. The name "mammatus" is derived from the Latin mamma (udder or breast), due to the clouds' characteristic shape, as some believe there is a resemblance between the shape of these clouds and the breast of a woman.

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Category: File - :Cumulus-1.JPG|thumb|left|Cumulus congestus clouds.

Cumulus congestus clouds (also towering cumulus) are characteristic of unstable areas of the atmosphere which are undergoing convection. They are often characterized by sharp outlines and great vertical development. Because cumulus congestus is produced by strong updrafts, it is typically taller than it is wide, and cloud tops can reach ,See or higher in the tropics. Cumulus congestus clouds are formed by the development of cumulus mediocris generally, though they can also be formed from altocumulus castellanus or stratocumulus castellanus as well. The congestus species of cloud can only be found in the genus cumulus.

Cumulus congestus will mature into cumulonimbus calvus under conditions of sufficient instability. This transformation can be seen by the presence of smooth, fibrous, or striated aspects assumed by the cloud's upper part. This cloud type produces precipitation, often in abundance.

A pyrocumulus, or, literally, fire cloud, is a dense cumuliform cloud associated with fire or volcanic activity.Pyrocumulus entry in the AMS Glossary

A pyrocumulus is similar dynamically in some ways to a firestorm, and the two phenomena may occur in conjunction with each other. However, one may occur without the other.

Polar stratospheric clouds (PSCs), also known as nacreous clouds, are clouds in the winter polar stratosphere at altitudes of 15,000-25,000 meters (50,000-80,000 ft). They are implicated in the formation of ozone holes;United States Environmental Protection Agency — see [http://www.epa.gov/ozone/science/hole/whyant.html#psc]

their effects on ozone depletion arise because they support chemical reactions that produce active chlorine which catalyzes ozone destruction, and also because they remove gaseous nitric acid, perturbing nitrogen and chlorine cycles in a way which increases ozone destruction.

World Meteorological Organization Scientific Assessment of Ozone Depletion (2002), particularly section 3.2.2 (pages 3.21, i.e. 195 of the PDF file, and following).