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The sky is the panorama obtained from observing the
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

universe
from the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
and other celestial bodies' surface. In the field of
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
, the sky is also called the
celestial sphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ma ...

celestial sphere
. This is an abstract sphere,
concentric In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position ...
to the Earth, on which the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
,
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
,
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
s, and
stars A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. ...
appear to be drifting. The celestial sphere is conventionally divided into designated areas called
constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern, typically representing an animal, mythological person or creature, or an inanimate object. The origins of the earliest ...

constellation
s. Usually, the term ''sky'' informally refers to a perspective from the Earth's surface; however, the meaning and usage can vary. An observer on the surface of the Earth can see a small part of the sky, which resembles a dome (sometimes called the ''sky bowl'') appearing flatter during the day than at
night Night (also described as night time, night-time, or nighttime, unconventionally spelled as ''nite'') is the period of :wikt:ambient, ambient darkness from sunset to sunrise during each 24-hour day, when the Sun is below the horizon. The exact ...

night
. In some cases, such as in discussing the weather, the sky refers to only the lower, denser layers of the atmosphere. The
daytime On Earth, daytime is the period of the day during which a given location experiences Daylight, natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's Hemisphere ...

daytime
sky appears blue because air molecules
scatter
scatter
shorter wavelengths of
sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

sunlight
more than longer ones (redder light). The
night sky The term night sky, usually associated with astronomy from Earth, refers to the nighttime appearance of astronomical object, celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when ...

night sky
appears to be a mostly dark surface or region spangled with stars. The Sun and sometimes the Moon are visible in the daytime sky unless obscured by
cloud In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid drop (liquid), droplets, ice crystals, frozen crystals, or other particulates, particles suspended in the atmosphere of a planetary body or similar space. Wate ...

cloud
s. At night, the Moon, planets, and stars are similarly visible in the sky. Some of the natural phenomena seen in the sky are clouds,
rainbow A rainbow is a meteorological Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting Weather forecasting is the application of sc ...

rainbow
s, and
aurora An aurora (plural: auroras or aurorae), sometimes referred to as polar lights (aurora polaris), northern lights (aurora borealis), or southern lights (aurora australis), is a natural light display in the Earth's sky, predominantly seen in high-l ...

aurora
e.
Lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden flow of electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan ...

Lightning
and
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...

precipitation
are also visible in the sky. Certain birds and insects, as well as human inventions like
aircraft An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or by using the Lift (force), dynamic lift of an airfoil, or in ...

aircraft
and
kite A kite is a tether A tether is a cord, fixture, or flexible attachment that characteristically anchors something movable to something fixed; it also maybe used to connect two movable objects, such as an item being towing, towed by its tow. ...

kite
s, can fly in the sky. Due to
human activities Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposable thumbs, hairlessness, and intelligence allowing the use of culture, language and tools. They are the only Extant taxon, ...
,
smog Smog, or smoke fog, is a type of intense air pollution Air pollution is the presence of substances in the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', mea ...

smog
during the day and
light pollution Light pollution is the presence of unwanted, inappropriate, or excessive artificial lighting Lighting or illumination is the deliberate use of light Light or visible light is electromagnetic radiation within the portion of the elect ...

light pollution
during the night are often seen above large cities.


Etymology

The word ''sky'' comes from the
Old Norse Old Norse, Old Nordic, or Old Scandinavian is a stage of development of North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants of Scandinavia Scandinavia; : ''Skades ...
, meaning 'cloud, abode of
God In monotheism, monotheistic thought, God is conceived of as the supreme being, creator deity, creator, and principal object of Faith#Religious views, faith.Richard Swinburne, Swinburne, R.G. "God" in Ted Honderich, Honderich, Ted. (ed)''The Oxfo ...

God
'. The Norse term is also the source of the
Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language English is a West Germanic language of the Indo-European language family The Indo-European languages are a language family A language ...
, which shares the same
Indo-European The Indo-European languages are a language family A language family is a group of language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in relation ...
base as the classical
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

Latin
, meaning 'obscure'. In Old English, the term ''
heaven Heaven or the heavens, is a common religious cosmological or transcendent supernatural The supernatural encompasses supposed phenomena or entities that are not subject to the . This term is attributed to , such as s, s, , and . It also ...

heaven
'' was used to describe the observable expanse above the earth. Throughout mentions in
Middle English Middle English (abbreviated to ME) was a form of the English language English is a West Germanic language of the Indo-European language family The Indo-European languages are a language family A language is a structured sys ...
, it was gradually restricted to its current, religious meaning.


During daytime

Except for direct
sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

sunlight
, most of the
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

light
in the
daytime On Earth, daytime is the period of the day during which a given location experiences Daylight, natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's Hemisphere ...

daytime
sky is caused by
scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light Light or visible light is electromagnetic radiation within the portion of the electromagnetic ...
, which is dominated by a small-particle limit called
Rayleigh scattering Rayleigh scattering ( ), named after the nineteenth-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength o ...

Rayleigh scattering
. The scattering due to molecule-sized particles (as in air) is greater in the directions both
toward Toward; ( gd, Tollard) is a village near Dunoon Dunoon (; gd, Dùn Omhain) is the main town on the Cowal Cowal ( gd, Còmhghall) is a peninsula A peninsula ( la, paeninsula from ' "almost" and ' "island") is a landform surrounded by wa ...
and
away
away
from the source of light than it is in directions
perpendicular In elementary geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relativ ...

perpendicular
to the incident path. Scattering is significant for light at all visible wavelengths, but is stronger at the shorter (bluer) end of the
visible spectrum The visible spectrum is the portion of the that is to the . in this range of s is called ' or simply . A typical will respond to wavelengths from about 380 to about 750 . In terms of frequency, this corresponds to a band in the vicinity of ...
, meaning that the scattered light is bluer than its source: the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
. The remaining direct sunlight, having lost some of its shorter-wavelength components, appears slightly less blue.
Scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light Light or visible light is electromagnetic radiation within the portion of the electromagnetic ...

Scattering
also occurs even more strongly in clouds. Individual water droplets refract white light into a set of colored rings. If a cloud is thick enough, scattering from multiple water droplets will wash out the set of colored rings and create a washed-out white color. The sky can turn a multitude of colors such as red, orange, purple, and yellow (especially near sunset or sunrise) when the light must travel a much longer path (or
optical depth In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...
) through the atmosphere. Scattering effects also partially polarize light from the sky and are most pronounced at an angle 90° from the Sun. Scattered light from the horizon travels through as much as 38 times the
air mass upright=1.25, Different air masses which affect North America as well as other continents, tend to be separated by frontal boundaries In meteorology Meteorology is a branch of the atmospheric sciences Atmospheric science is the study of t ...
as does light from the
zenith The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction (plumb line) opposite to the gravity direction at that location (nadir). The zenith is the "high ...

zenith
, causing a blue
gradient In vector calculus Vector calculus, or vector analysis, is concerned with differentiation Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Prod ...
looking vivid at the zenith and pale near the horizon. Red light is also scattered if there is enough air between the source and the observer, causing to change color as the Sun rises or sets. As the air mass nears infinity, scattered daylight appears whiter and whiter. The Sun is not the only object that may appear less blue in the atmosphere. Distant clouds or snowy mountaintops may appear yellowish. The effect is not very obvious on clear days, but is very pronounced when clouds cover the line of sight, reducing the blue hue from scattered sunlight. At higher altitudes, the sky tends toward darker colors since scattering is reduced due to lower
air density The density of air or atmospheric density, denoted ''ρ'' (Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Sout ...
. An extreme example is the Moon, where no atmospheric scattering occurs, making the black even when the Sun is visible. Sky
luminance Luminance is a photometricPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an ...

luminance
distribution models have been recommended by the
International Commission on Illumination The International Commission on Illumination (usually abbreviated CIE for its French name, Commission internationale de l'éclairage) is the international authority on light Light or visible light is electromagnetic radiation within the port ...
(CIE) for the design of
daylighting Daylighting is the practice of placing windows, skylight A skylight (sometimes called a rooflight) is a light-transmitting structure that forms all or part of the roof space of a building for daylighting purposes. History Open skylights we ...
schemes. Recent developments relate to "all sky models" for modelling sky
luminance Luminance is a photometricPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an ...

luminance
under weather conditions ranging from clear to overcast.


During twilight

The brightness and color of the sky vary greatly over the course of a day, and the primary cause of these properties differs as well. When the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
is well above the
horizon The horizon is the apparent line that separates the surface of a celestial body In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that st ...

horizon
, direct
scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light Light or visible light is electromagnetic radiation within the portion of the electromagnetic ...
of
sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

sunlight
(
Rayleigh scattering Rayleigh scattering ( ), named after the nineteenth-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength o ...

Rayleigh scattering
) is the overwhelmingly dominant source of light. However, during
twilight Twilight is the illumination of the lower atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases s ...

twilight
, the period between
sunset Sunset, also known as sundown, is the daily disappearance of the Sun below the horizon The horizon is the apparent line that separates earth Earth is the third planet from the Sun and the only astronomical object known to harbor ...

sunset
and night or between night and
sunrise 250px, Skyline of Kaohsiung harbour, Taiwan at sunrise. Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning. The term can also refer to the entire process of the solar disk crossing the horizon a ...

sunrise
, the situation is more complex.
Green flash The green flash and green ray are meteorological Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting Weather forecasting is t ...

Green flash
es and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible above the Sun, usually for no more than a second or two, or it may resemble a green ray shooting up from the sunset point. Green flashes are a group of phenomena that stem from different causes, most of which occur when there is a temperature
inversion Inversion or inversions may refer to: Arts * , a French gay magazine (1924/1925) * Inversion (artwork), ''Inversion'' (artwork), a 2005 temporary sculpture in Houston, Texas * Inversion (music), a term with various meanings in music theory and mus ...
(when the temperature increases with altitude rather than the normal decrease in temperature with altitude). Green flashes may be observed from any altitude (even from an aircraft). They are usually seen above an unobstructed horizon, such as over the ocean, but are also seen above clouds and mountains. Green flashes may also be observed at the horizon in association with the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
and bright planets, including
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

Venus
and
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
.
Earth's shadow Earth's shadow (or Earth shadow) is the shadow A shadow is a dark area where light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived by the ...
is the shadow that the planet casts through its atmosphere and into outer space. This atmospheric phenomenon is visible during civil twilight (after sunset and before sunrise). When the weather conditions and the observing site permit a clear view of the horizon, the shadow's fringe appears as a dark or dull bluish band just above the horizon, in the low part of the sky opposite of the (setting or rising) Sun's direction. A related phenomenon is the
Belt of Venus Belt may refer to: Apparel * Belt (clothing) A belt is a flexible band or strap, typically made of leather Leather is a durable and flexible material created by tanning animal rawhide and skins. The most common raw material is catt ...

Belt of Venus
(or antitwilight arch), a pinkish band that is visible above the bluish band of Earth's shadow in the same part of the sky. No defined line divides Earth's shadow and the Belt of Venus; one colored band fades into the other in the sky. Twilight is divided into three stages according to the Sun's depth below the horizon, measured in segments of 6°. After sunset, the
civil twilight 300px, Morning twilight: astronomical, nautical and civil stages at dawn. The apparent disk of the Sun is shown to scale. Twilight on Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life ...
sets in; it ends when the Sun drops more than 6° below the horizon. This is followed by the
nautical twilight 300px, Morning twilight: astronomical, nautical and civil stages at dawn. The apparent disk of the Sun is shown to scale. Twilight on Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life ...
, when the Sun is between 6° and 12° below the horizon (depth between −6° and −12°), after which comes the
astronomical twilight 300px, Morning twilight: astronomical, nautical and civil stages at dawn. The apparent disk of the Sun is shown to scale. Twilight on Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life ...
, defined as the period between −12° and −18°. When the Sun drops more than 18° below the horizon, the sky generally attains its minimum brightness. Several sources can be identified as the source of the intrinsic brightness of the sky, namely
airglow Airglow (also called nightglow) is a faint emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky never to be completely dark, even after the effects of starlight and diffuse ...

airglow
, indirect scattering of sunlight, scattering of
starlight Starlight is the light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths ...

starlight
, and artificial
light pollution Light pollution is the presence of unwanted, inappropriate, or excessive artificial lighting Lighting or illumination is the deliberate use of light Light or visible light is electromagnetic radiation within the portion of the elect ...

light pollution
.


During the night

The term night sky refers to the sky as seen at night. The term is usually associated with
skygazing Image:Astronomy Amateur 3 V2.jpg, 250px, Amateur astronomers watch the night sky during the Perseids, Perseid meteor shower. Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the Naked e ...
and
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
, with reference to views of
celestial bodies In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ...
such as stars, the Moon, and planets that become visible on a clear night after the Sun has set. Natural light sources in a night sky include moonlight, starlight, and airglow, depending on location and timing. The fact that the sky is not completely dark at night can be easily observed. Were the sky (in the absence of moon and city lights) absolutely dark, one would not be able to see the silhouette of an object against the sky. The night sky and studies of it have a historical place in both ancient and modern cultures. In the past, for instance, farmers have used the state of the night sky as a calendar to determine when to plant crops. The ancient belief in
astrology Astrology is a pseudoscience that claims to divination, divine information about human affairs and terrestrial events by studying the movements and relative positions of Celestial objects in astrology, celestial objects. Astrology has be ...
is generally based on the belief that relationships between heavenly bodies influence or convey information about events on Earth. The ''scientific'' study of the night sky and bodies observed within it, meanwhile, takes place in the science of astronomy. Within
visible-light astronomy Visible-light astronomy encompasses a wide variety of observations via telescopes that are sensitive in the range of visible light ( optical telescopes). Visible-light astronomy is part of optical astronomy, and differs from astronomies based on ...
, the visibility of celestial objects in the night sky is affected by light pollution. The presence of the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
in the night sky has historically hindered astronomical observation by increasing the amount of ambient lighting. With the advent of artificial light sources, however, light pollution has been a growing problem for viewing the night sky. Special filters and modifications to light fixtures can help to alleviate this problem, but for the best views, both professional and amateur optical astronomers seek viewing sites located far from major urban areas.


Use in weather forecasting

Along with pressure tendency, the condition of the sky is one of the more important parameters used to forecast weather in mountainous areas. Thickening of cloud cover or the invasion of a higher cloud deck is indicative of rain in the near future. At night, high thin
cirrostratus cloud Cirrostratus is a high-level, very thin, generally uniform ''stratiform'' genus-type of cloud. It is made out of ice-crystals, which are pieces of frozen water. It is difficult to detect and it can make Halo (optical phenomenon), halos. These are m ...
s can lead to
halos Halos ( grc, Ἅλος, also appearing as ὁ Ἅλος or ἡ Ἅλος) or Halus, also Latinised as Alos and Alus, was a town and polis in the region of Achaea Phthiotis in ancient Thessaly, on the west side of the Pagasetic Gulf. It is located ...
around the Moon, which indicate the approach of a
warm front A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass upright=1.25, Different air masses which affect North America as well as other continents, tend to be separated by frontal boundaries In meteo ...

warm front
and its associated rain. Morning fog portends fair conditions and can be associated with a
marine layer A marine layer is an air mass that develops over the surface of a large body of water such as the ocean or large lake in the presence of a temperature inversion , Scotland Scotland ( sco, Scotland, gd, Alba ) is a Countries of the Unite ...
, an indication of a stable atmosphere. Rainy conditions are preceded by wind or clouds which prevent fog formation. The approach of a line of thunderstorms could indicate the approach of a
cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough Trough may refer to: In science * Trough (geology), a long depression less steep than a tren ...

cold front
. Cloud-free skies are indicative of fair weather for the near future. The use of sky cover in weather prediction has led to various
weather lore Weather lore is the body of informal folklore related to the prediction of the weather and its greater meaning. Much like regular folklore, weather lore is passed down through speech and writing from normal people without the use of external mea ...
over the centuries.


Tropical cyclones

Within 36 hours of the passage of a
tropical cyclone A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation Atmospheric circulation is the large-scale movement of Atmosphere of Earth, air and together with oc ...
's center, the pressure begins to fall and a veil of white cirrus clouds approaches from the cyclone's direction. Within 24 hours of the closest approach to the center, low clouds begin to move in, also known as the
bar Bar or BAR may refer to: Food *Bar (establishment) A bar is a long raised narrow table or bench designed for dispensing beer or other alcoholic beverage, alcoholic drinks. They were originally chest high, and a bar, often brass, ran the len ...
of a tropical cyclone, as the barometric pressure begins to fall more rapidly and the winds begin to increase. Within 18 hours of the center's approach, squally weather is common, with sudden increases in wind accompanied by rain showers or thunderstorms. Within six hours of the center's arrival, rain becomes continuous. Within an hour of the center, the rain becomes very heavy and the highest winds within the tropical cyclone are experienced. When the center arrives with a strong tropical cyclone, weather conditions improve and the sun becomes visible as the
eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light L ...
moves overhead. Once the system departs, winds reverse and, along with the rain, suddenly increase. One day after the center's passage, the low overcast is replaced with a higher overcast, and the rain becomes intermittent. By 36 hours after the center's passage, the high overcast breaks and the pressure begins to level off.


Use in transportation

Flight is the process by which an object moves through or beyond the sky (as in the case of spaceflight), whether by generating
aerodynamic lift A fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Phase (matter), phase of matter and include liquids, Gas, gases and Plasma (p ...
, propulsive thrust, aerostatically using buoyancy, or by
ballistic Ballistics may refer to: Science * Ballistics, the science that deals with the motion, behavior, and effects of projectiles. ** Forensic ballistics, the science of analyzing firearm usage in crimes ** Internal ballistics Internal may refer t ...
movement, without any direct mechanical support from the ground. The engineering aspects of flight are studied in
aerospace engineering Aerospace engineering is the primary field of engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipl ...

aerospace engineering
which is subdivided into
aeronautics Aeronautics is the science or art involved with the study, design process, design, and manufacturing of air flight–capable machines, and the techniques of operating aircraft and rockets within the atmosphere. The British Royal Aeronautical So ...

aeronautics
, which is the study of vehicles that travel through the air, and
astronautics Astronautics (or cosmonautics) is the theory and practice of travel beyond Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that toget ...
, the study of vehicles that travel through space, and in
ballistics Ballistics is the field of mechanics Mechanics (Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in South ...
, the study of the flight of projectiles. While human beings have been capable of flight via
hot air balloon A hot-air balloon is a lighter-than-air A lifting gas or lighter than air gas is a gas that has a lower density than normal atmospheric gases and rises above them as a result. It is required for aerostats to create buoyancy, particularly in ...
s since 1783, other species have used flight for significantly longer. Animals, such as birds, bats, and insects are capable of flight.
Spore )'', growing on a thinning, thinned hybrid black poplar ''(populus, Populus x canadensis)''. The last stage of the moss#Life cycle, moss lifecycle is shown, where the sporophytes are visible before dispersion of their spores: the calyptra (1) is ...
s and seeds from plants use flight, via use of the wind, as a method of propagating their species.


Significance in mythology

Many mythologies have deities especially associated with the sky. In
Egyptian religionEgyptian religion may refer to: * Modern Religion in Egypt * Ancient Egyptian religion {{disambig