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Cloud

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

   Cumulonimbus capillatus incus floating over Swifts Creek, Victoria in
   Australia
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   Cumulonimbus capillatus incus floating over Swifts Creek, Victoria in
   Australia

   A cloud is a visible mass of condensed droplets or frozen crystals
   suspended in the atmosphere above the surface of the Earth or another
   planetary body. The branch of meteorology in which clouds are studied
   is nephology.

   On Earth, the condensing substance is water vapor, which forms small
   droplets of water or ice crystals, typically 0.01 mm in diameter. When
   surrounded with billions of other droplets or crystals, they are
   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 very
   efficiently, so that the intensity of the solar radiation decreases
   with depth into the cloud, hence the grey or even sometimes dark
   appearance of the clouds at their base. Thin clouds may appear to have
   acquired the colour of their environment or background, and clouds
   illuminated by non-white light, such as during sunrise or sunset, may
   be colored accordingly. In the near-infrared range, however, clouds
   would appear very dark because the water that constitutes the cloud
   droplets strongly absorbs solar radiation at these wavelengths.

Cloud formation and properties

   Global scheme of cloud optical thickness
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   Global scheme of cloud optical thickness

   Clouds form when the invisible water vapor in the air condenses into
   visible water droplets or ice crystals. This can happen in three ways:

   1. The air is cooled below its saturation point. This happens when the
   air comes in contact with a cold surface or a surface that is cooling
   by radiation, or the air is cooled by adiabatic expansion (rising).
   This can happen:
     * along warm and cold fronts ( frontal lift)
     * where air flows up the side of a mountain and cools as it rises
       higher into the atmosphere ( orographic lift)
     * by the convection caused by the warming of a surface by insolation
       (diurnal heating)
     * when warm air blows over a colder surface such as a cool body of
       water.

   2. Clouds can be formed when two air masses below saturation point mix.
   Examples are breath on a cold day, aircraft contrails and Arctic sea
   smoke.

   3. The air stays the same temperature but absorbs more water vapor into
   it until it reaches saturation point.

   The water in a typical cloud can have a mass of up to several million
   tonnes. However, the volume of a cloud is correspondingly high, and the
   net density of the relatively warm air holding the droplets is low
   enough that air currents below and within the cloud are capable of
   keeping it suspended. As well, conditions inside a cloud are not
   static: water droplets are constantly forming and re-evaporating. A
   typical cloud droplet has a radius on the order of 1 x 10^-5 m and a
   terminal velocity of about 1-2 cm/s. This gives these droplets plenty
   of time to re-evaporate as they fall into the warmer air beneath the
   cloud.
   Cumulonimbus cloud
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   Cumulonimbus cloud

   Most water droplets are formed when water vapor condenses around a
   condensation nucleus, a tiny particle of smoke, dust, ash, or salt. In
   supersaturated conditions, water droplets may act as condensation
   nuclei.

   Water droplets large enough to fall to the ground are produced in two
   ways. The most important means is through the Bergeron Process,
   theorized by Tor Bergeron, in which supercooled water droplets and ice
   crystals in a cloud interact to produce the rapid growth of ice
   crystals; these crystals precipitate from the cloud and melt as they
   fall. This process typically takes place in clouds with tops cooler
   than -15°C. The second most important process is the collision and wake
   capture process, occurring in clouds with warmer tops, in which the
   collision of rising and falling water droplets produces larger and
   larger droplets, which are eventually heavy enough to overcome air
   currents in the cloud and the updraft beneath it and fall as rain. As a
   droplet falls through the smaller droplets which surround it, it
   produces a "wake" which draws some of the smaller droplets into
   collisions, perpetuating the process. This method of raindrop
   production is the primary mechanism in low stratiform clouds and small
   cumulus clouds in trade winds and tropical regions and produces
   raindrops of several millimeters diameter.
   This wave cloud pattern formed off of the Île Amsterdam in the far
   southern Indian Ocean
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   This wave cloud pattern formed off of the Île Amsterdam in the far
   southern Indian Ocean

   The actual form of cloud created depends on the strength of the uplift
   and on air stability. In unstable conditions convection dominates,
   creating vertically developed clouds. Stable air produces horizontally
   homogeneous clouds. Frontal uplift creates various cloud forms
   depending on the composition of the front ( ana-type or kata-type warm
   or cold front). Orographic uplift also creates variable cloud forms
   depending on air stability, although cap cloud and wave clouds are
   specific to orographic clouds.

"Hot Ice" and "Ice Memory" in cloud formation

   In addition to being the colloquial term sometimes used to describe dry
   ice, hot ice is the name given to a surprising phenomenon in which
   water can be turned into ice at room temperature by supplying an
   electric field of the order of 1 million volts per meter. ( Choi 2005).
   The effect of such electric fields has been suggested as an explanation
   of cloud formation. This theory, however, is highly controversial and
   is not, by any means, widely accepted as being the actual mechanism of
   cloud formation. The first time cloud ice forms around a clay particle,
   it requires a temperature of -10°C, but subsequent freezing around the
   same clay particle requires a temperature of just -5°C, suggesting some
   kind of "ice memory" ( Connolly, P.J, et al, 2005).

Cloud classification

   Cloud classification by altitude of occurrence
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   Cloud classification by altitude of occurrence

   Clouds are divided into two general categories: layered and convective.
   These are named stratus clouds (or stratiform, the Latin stratus means
   layer) and cumulus clouds (or cumuliform; cumulus means piled up).
   These two cloud types are divided into four more groups that
   distinguish the cloud's altitude. Clouds are classified by the cloud
   base height not the cloud top. This system was proposed by Luke Howard
   in 1802 in a presentation to the Askesian Society.

High clouds (Family A)

   These generally form above 16,500 feet (5,000 m), in the cold region of
   the troposphere. However, in Polar regions, they may form as low as
   10,000 ft (3,048 m). They are denoted by the prefix cirro- or cirrus.
   At this altitude, water almost always freezes so clouds are composed of
   ice crystals. The clouds tend to be wispy, and are often transparent.

   Clouds in Family A include:
     * Cirrus (Ci)
     * Cirrus uncinus
     * Cirrus Kelvin-Helmholtz colombia
     * Cirrostratus (Cs)
     * Cirrocumulus (Cc)
     * Pileus
     * Contrail, a long thin cloud which develops as the result of the
       passage of an aircraft at high altitudes.

Middle clouds (Family B)

   Altocumulus mackerel sky
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   Altocumulus mackerel sky

   These develop between 6,500 and 16,500 feet (between 2,000 and 5,000 m)
   and are denoted by the prefix alto-. They are made of water droplets
   and are frequently supercooled.

   Clouds in Family B include:
     * Altostratus (As)
     * Altostratus undulatus
     * Altocumulus (Ac)
     * Altocumulus undulatus
     * Altocumulus mackerel sky
     * Altocumulus castellanus
     * Altocumulus lenticularis

Low clouds (Family C)

   Low clouds
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   Low clouds

   These are found up to 6,500 feet (2,000 m) and include the stratus
   (dense and grey). When stratus clouds contact the ground, they are
   called fog.

   Clouds in Family C include:
     * Stratus (St)
     * Nimbostratus (Ns)
     * Cumulus humilis (Cu)
     * Cumulus mediocris (Cu)
     * Stratocumulus (Sc)

Vertical clouds (Family D)

   Cumulonimbus clouds showing strong updrafts
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   Cumulonimbus clouds showing strong updrafts

   These clouds can have strong up-currents, rise far above their bases
   and form at many heights.

   Clouds in Family D include:
     * Cumulonimbus (associated with heavy precipitation and
       thunderstorms) (Cb)
     * Cumulonimbus incus
     * Cumulonimbus calvus
     * Cumulonimbus with mammatus
     * Cumulus congestus
     * Pyrocumulus

Other clouds

   A few clouds can be found above the troposphere; these include
   noctilucent and polar stratospheric clouds (or nacreous clouds), which
   occur in the mesosphere and stratosphere respectively.

Cloud fields

   A cloud field is simply a group of clouds, but sometimes cloud fields
   can take on certain shapes that have their own characteristics and are
   specially classified. For example, stratocumulus clouds can often be
   found in the following forms:
     * Open cell, which resembles a honeycomb, with clouds around the
       edges and clear, open space in the middle.
     * Closed cell, which is cloudy in the centre and clear on the edges,
       similar to a filled honeycomb.
     * Actinoform, which resembles a leaf or a spoked wheel.

Colors

   An example of various cloud colors
   Enlarge
   An example of various cloud colors
   Colourful cloud formation
   Enlarge
   Colourful cloud formation

   The colour of a cloud tells much about what is going on inside the
   cloud.

   Clouds form when relatively warm air containing water vapor is lighter
   than its surrounding air and this causes it to rise. As it rises it
   cools and the vapor condenses out of the air as micro-droplets. These
   tiny particles of water are relatively densely packed, and sunlight
   cannot penetrate far into the cloud before it is reflected out, giving
   a cloud its characteristic white colour. As a cloud matures, the
   droplets may combine to produce larger droplets, which may themselves
   combine to form droplets large enough to fall as rain. In this process
   of accumulation, the space between droplets becomes larger and larger,
   permitting light to penetrate much farther into the cloud. If the cloud
   is sufficiently large, and the droplets within are spaced far enough
   apart, it may be that a percentage of the light which enters the cloud
   is not reflected back out before it is absorbed (Think of how much
   farther one can see in a heavy rain as opposed to how far one can see
   in a heavy fog). This process of reflection/ absorption is what leads
   to the range of cloud colour from white through grey through black. For
   the same reason, the undersides of large clouds and heavy overcasts
   appear various degrees of grey; little light is being reflected or
   transmitted back to the observer.

   Other colors occur naturally in clouds. Bluish-grey is the result of
   light scattering within the cloud. In the visible spectrum, blue and
   green are at the short end of light's visible wavelengths, while red
   and yellow are at the long end. The short rays are more easily
   scattered by water droplets, and the long rays are more likely to be
   absorbed. The bluish colour is evidence that such scattering is being
   produced by rain-sized droplets in the cloud.

   A more ominous colour is the one seen frequently by severe weather
   observers. A greenish tinge to a cloud is produced when sunlight is
   scattered by ice. A cumulonimbus cloud which shows green is a pretty
   sure sign of imminent heavy rain, hail, strong winds and possible
   tornados.

   Yellowish clouds are rare, but may occur in the late spring through
   early fall months during forest fire season. The yellow colour is due
   to the presence of smoke.

   Red, orange and pink clouds occur almost entirely at sunrise/sunset and
   are the result of the scattering of sunlight by the atmosphere itself.
   The clouds themselves are not that colour; they are merely reflecting
   the long (and unscattered) rays of sunlight which are predominant at
   those hours. The effect is much the same as if one were to shine a red
   spotlight on a white sheet. In combination with large, mature
   thunderheads, this can produce blood-red clouds. The evening before the
   Edmonton, Alberta tornado in 1987, Edmontonians observed such clouds —
   deep black on their dark side and intense red on their sunward side. In
   this case, the adage "red sky at night, sailor's delight" was clearly
   incorrect.

Clouds on other planets

   Within our solar system, any planet with an atmosphere also has clouds.
   Venus' clouds are composed entirely of sulfuric acid droplets. Mars has
   high, thin clouds of water ice. Both Jupiter and Saturn have an outer
   cloud deck composed of ammonia clouds, an intermediate deck of ammonium
   hydrosulfide clouds and an inner deck of water clouds. Uranus and
   Neptune have atmospheres dominated by methane clouds.

   Saturn's moon Titan has clouds which are believed to be composed of
   droplets of liquid methane. The Cassini-Huygens Saturn mission has
   uncovered evidence of a fluid cycle on Titan, including fluvial
   channels on the surface of the moon.

   Retrieved from " http://en.wikipedia.org/wiki/Cloud"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
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