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A mountain is an elevated portion of the Earth’s crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually higher than a hill, typically rising at least 300 metres (980 ft) above the surrounding land. A few mountains are isolated summits, but most occur in mountain ranges.^[1]

Mountains are formed through tectonic forces, erosion, or volcanism,^[1] which act on time scales of up to tens of millions of years.^[2] Once mountain building ceases, mountains are slowly leveled through the action of weathering, through slumping and other forms of mass wasting, as well as through erosion by rivers and glaciers.^[3]

High elevations on mountains produce colder climates than at sea level at similar latitude. These colder climates strongly affect the ecosystems of mountains: different elevations have different plants and animals. Because of the less hospitable terrain and climate, mountains tend to be used less for agriculture and more for resource extraction, such as mining and logging, along with recreation, such as mountain climbing and skiing.

The highest mountain on Earth is Mount Everest in the Himalayas of Asia, whose summit is 8,850 m (29,035 ft) above mean sea level. The highest known mountain on any planet in the Solar System is Olympus Mons on Mars at 21,171 m (69,459 ft). The tallest mountain including submarine terrain is Mauna Kea in Hawaii from its underwater base at 9,330 m (30,610 ft); some scientists consider it to be the tallest on earth.^[3]

Definition

There is no universally accepted definition of a mountain. Elevation, volume, relief, steepness, spacing and continuity have been used as criteria for defining a mountain.^[5] In the Oxford English Dictionary a mountain is defined as “a natural elevation of the earth surface rising more or less abruptly from the surrounding level and attaining an altitude which, relatively to the adjacent elevation, is impressive or notable.”^[5]

Whether a landform is called a mountain may depend on local usage. John Whittow’s Dictionary of Physical Geography^[6] states “Some authorities regard eminences above 600 metres (1,969 ft) as mountains, those below being referred to as hills.”

In the United Kingdom and the Republic of Ireland, a mountain is usually defined as any summit at least 2,000 feet (610 m) high,^{[7][8][9][10][11]} which accords with the official UK government’s definition that a mountain, for the purposes of access, is a summit of 2,000 feet (610 m) or higher.^[12] In addition, some definitions also include a topographical prominence requirement, such as that the mountain rises 300 metres (984 ft) above the surrounding terrain.^[1] At one time, the United States Board on Geographic Names defined a mountain as being 1,000 feet (305 m) or taller,^[13] but has abandoned the definition since the 1970s. Any similar landform lower than this height was considered a hill. However, today, the United States Geological Survey concludes that these terms do not have technical definitions in the US.^[14]

The UN Environmental Programme‘s definition of “mountainous environment” includes any of the following:^[15]: 74

• Class 1: Elevation greater than 4,500 m (14,764 ft).
• Class 2: Elevation between 3,500 and 4,500 m (11,483 and 14,764 ft).
• Class 3: Elevation between 2,500 and 3,500 m (8,202 and 11,483 ft).
• Class 4: Elevation between 1,500 and 2,500 m (4,921 and 8,202 ft), with a slope greater than 2 degrees.
• Class 5: Elevation between 1,000 and 1,500 m (3,281 and 4,921 ft), with a slope greater than 5 degrees or 300 m (984 ft) elevation range within 7 km (4.3 mi).
• Class 6: Elevation between 300 and 1,000 m (984 and 3,281 ft), with a 300 m (984 ft) elevation range within 7 km (4.3 mi).
• Class 7: Isolated inner basins and plateaus less than 25 km² (9.7 sq mi) in area that are completely surrounded by Class 1 to 6 mountains, but do not themselves meet criteria for Class 1 to 6 mountains.

Using these definitions, mountains cover 33% of Eurasia, 19% of South America, 24% of North America, and 14% of Africa.^[15]: 14 As a whole, 24% of the Earth’s land mass is mountainous.^[16]

Geology

Main articles: Mountain formation and List of mountain types

There are three main types of mountains: volcanic, fold, and block.^[17] All three types are formed from plate tectonics: when portions of the Earth’s crust move, crumple, and dive. Compressional forces, isostatic uplift and intrusion of igneous matter forces surface rock upward, creating a landform higher than the surrounding features. The height of the feature makes it either a hill or, if higher and steeper, a mountain. Major mountains tend to occur in long linear arcs, indicating tectonic plate boundaries and activity.

Volcanoes

Main article: Volcano

Volcanoes are formed when a plate is pushed below another plate, or at a mid-ocean ridge or hotspot.^[18] At a depth of around 100 km (60 mi), melting occurs in rock above the slab (due to the addition of water), and forms magma that reaches the surface. When the magma reaches the surface, it often builds a volcanic mountain, such as a shield volcano or a stratovolcano.^[5]: 194 Examples of volcanoes include Mount Fuji in Japan and Mount Pinatubo in the Philippines. The magma does not have to reach the surface in order to create a mountain: magma that solidifies below ground can still form dome mountains, such as Navajo Mountain in the US.^[19]

Fold mountains

Main article: Fold mountains

Fold mountains occur when two plates collide: shortening occurs along thrust faults and the crust is overthickened.^[20] Since the less dense continental crust “floats” on the denser mantle rocks beneath, the weight of any crustal material forced upward to form hills, plateaus or mountains must be balanced by the buoyancy force of a much greater volume forced downward into the mantle. Thus the continental crust is normally much thicker under mountains, compared to lower lying areas.^[21] Rock can fold either symmetrically or asymmetrically. The upfolds are anticlines and the downfolds are synclines: in asymmetric folding there may also be recumbent and overturned folds. The Balkan Mountains^[22] and the Jura Mountains^[23] are examples of fold mountains.

Block mountains

Main article: Block mountains

Block mountains are caused by faults in the crust: a plane where rocks have moved past each other. When rocks on one side of a fault rise relative to the other, it can form a mountain.^[24] The uplifted blocks are block mountains or horsts. The intervening dropped blocks are termed graben: these can be small or form extensive rift valley systems. This kind of landscape can be seen in East Africa,^[25] the Vosges and Rhine valley,^[26] and the Basin and Range Province of Western North America.^[27] These areas often occur when the regional stress is extensional and the crust is thinned.^[27]

Erosion

Main article: Erosion

During and following uplift, mountains are subjected to the agents of erosion (water, wind, ice, and gravity) which gradually wear the uplifted area down. Erosion causes the surface of mountains to be younger than the rocks that form the mountains themselves.^[28]: 160 Glacial processes produce characteristic landforms, such as pyramidal peaks, knife-edge arêtes, and bowl-shaped cirques that can contain lakes.^[29] Plateau mountains, such as the Catskills, are formed from the erosion of an uplifted plateau.^[30]

Climate

Main article: Alpine climate

Climate in the mountains becomes colder at high elevations, due to an interaction between radiation and convection. Sunlight in the visible spectrum hits the ground and heats it. The ground then heats the air at the surface. If radiation were the only way to transfer heat from the ground to space, the greenhouse effect of gases in the atmosphere would keep the ground at roughly 333 K (60 °C; 140 °F), and the temperature would decay exponentially with height.^[31]

However, when air is hot, it tends to expand, which lowers its density. Thus, hot air tends to rise and transfer heat upward. This is the process of convection. Convection comes to equilibrium when a parcel of air at a given altitude has the same density as its surroundings. Air is a poor conductor of heat, so a parcel of air will rise and fall without exchanging heat. This is known as an adiabatic process, which has a characteristic pressure-temperature dependence. As the pressure gets lower, the temperature decreases. The rate of decrease of temperature with elevation is known as the adiabatic lapse rate, which is approximately 9.8 °C per kilometre (or 5.4 °F (3.0 °C) per 1000 feet) of altitude.^[31]

The presence of water in the atmosphere complicates the process of convection. Water vapor contains latent heat of vaporization. As air rises and cools, it eventually becomes saturated and cannot hold its quantity of water vapor. The water vapor condenses to form clouds and releases heat, which changes the lapse rate from the dry adiabatic lapse rate to the moist adiabatic lapse rate (5.5 °C per kilometre or 3 °F (1.7 °C) per 1000 feet)^[32] The actual lapse rate can vary by altitude and by location. Therefore, moving up 100 m (330 ft) on a mountain is roughly equivalent to moving 80 kilometres (45 miles or 0.75° of latitude) towards the nearest pole.^[15]: 15 This relationship is only approximate, however, since local factors such as proximity to oceans (such as the Arctic Ocean) can drastically modify the climate.^[33] As the altitude increases, the main form of precipitation becomes snow and the winds increase.^[15]: 12

The effect of the climate on the ecology at an elevation can be largely captured through a combination of amount of precipitation, and the biotemperature, as described by Leslie Holdridge in 1947.^[34] Biotemperature is the mean temperature; all temperatures below 0 °C (32 °F) are considered to be 0 °C. When the temperature is below 0 °C, plants are dormant, so the exact temperature is unimportant. The peaks of mountains with permanent snow can have a biotemperature below 1.5 °C (34.7 °F).

Climate change

Mountain environments are particularly sensitive to anthropogenic climate change and are currently undergoing alterations unprecedented in last 10,000 years.^[35] The effect of global warming on mountain regions (relative to lowlands) is still an active area of study. Observational studies show that highlands are warming faster than nearby lowlands, but when compared globally, the effect disappears.^[36] Precipitation in highland areas is not increasing as quickly as in lowland areas.^[36] Climate modeling give mixed signals about whether a particular highland area will have increased or decreased precipitation.^[37]

Climate change has started to affect the physical and ecological systems of mountains. In recent decades mountain ice caps and glaciers have experienced accelerating ice loss.^[38] The melting of the glaciers, permafrost and snow has caused underlying surfaces to become increasingly unstable. Landslip hazards have increased in both number and magnitude due to climate change.^[39] Patterns of river discharge will also be significantly affected by climate change, which in turn will have significant impacts on communities that rely on water fed from alpine sources. Nearly half of mountain areas provide essential or supportive water resources for mainly urban populations,^[40] in particular during the dry season and in semiarid areas such as in central Asia.

Alpine ecosystems can be particularly climatically sensitive. Many mid-latitude mountains act as cold climate refugia, with the ecosystems occupying small environmental niches. As well as the direct influence that the change in climate can have on an ecosystem, there is also the indirect one on the soils from changes in stability and soil development.^[41]

Ecology

Main article: Montane ecology

The colder climate on mountains affects the plants and animals residing on mountains. A particular set of plants and animals tend to be adapted to a relatively narrow range of climate. Thus, ecosystems tend to lie along elevation bands of roughly constant climate. This is called altitudinal zonation.^[42] In regions with dry climates, the tendency of mountains to have higher precipitation as well as lower temperatures also provides for varying conditions, which enhances zonation.^[15][43]

Some plants and animals found in altitudinal zones tend to become isolated since the conditions above and below a particular zone will be inhospitable and thus constrain their movements or dispersal. These isolated ecological systems are known as sky islands.^[44]

Altitudinal zones tend to follow a typical pattern. At the highest elevations, trees cannot grow, and whatever life may be present will be of the alpine type, resembling tundra.^[43] Just below the tree line, one may find subalpine forests of needleleaf trees, which can withstand cold, dry conditions.^[45] Below that, montane forests grow. In the temperate portions of the earth, those forests tend to be needleleaf trees, while in the tropics, they can be broadleaf trees growing in a rainforest.

Mountains and humans

See also: List of highest cities in the world

The highest known permanently tolerable altitude is at 5,950 metres (19,520 ft).^[46] At very high altitudes, the decreasing atmospheric pressure means that less oxygen is available for breathing, and there is less protection against solar radiation (UV).^[15] Above 8,000 metres (26,000 ft) elevation, there is not enough oxygen to support human life. This is sometimes referred to as the “death zone“.^[47] The summits of Mount Everest and K2 are in the death zone.

Mountain societies and economies

Mountains are generally less preferable for human habitation than lowlands, because of harsh weather and little level ground suitable for agriculture. While 7% of the land area of Earth is above 2,500 metres (8,200 ft),^[15]: 14 only 140 million people live above that altitude^[48] and only 20-30 million people above 3,000 metres (9,800 ft) elevation.^[49] About half of mountain dwellers live in the Andes, Central Asia, and Africa.^[16]

With limited access to infrastructure, only a handful of human communities exist above 4,000 metres (13,000 ft) of elevation. Many are small and have heavily specialized economies, often relying on industries such as agriculture, mining, and tourism.^[50] An example of such a specialized town is La Rinconada, Peru, a gold-mining town and the highest elevation human habitation at 5,100 metres (16,700 ft).^[51] A counterexample is El Alto, Bolivia, at 4,150 metres (13,620 ft), which has a highly diverse service and manufacturing economy and a population of nearly 1 million.^[52]

Traditional mountain societies rely on agriculture, with higher risk of crop failure than at lower elevations. Minerals often occur in mountains, with mining being an important component of the economics of some mountain-based societies. More recently, tourism has become more important to the economies of mountain communities, with developments focused around attractions such as national parks and ski resorts.^[15]: 17 Approximately 80% of mountain people live below the poverty line.^[16]

Most of the world’s rivers are fed from mountain sources, with snow acting as a storage mechanism for downstream users.^[15]: 22 More than half of humanity depends on mountains for water.^[53][54]

In geopolitics, mountains are often seen as natural boundaries between polities.^[55][56]

Contemporary development studies recognise transportation networks as a key element of economic development, socio-economic well-being and poverty reduction.^[57] However, road network development has not always fulfilled its original intentions and has contributed significantly to environmental degradation and, in some cases, led to the loss of cultural traditions and the marginalisation of indigenous peoples.^[58][59] Compared to roads, the development of air links (helicopters and planes) has had an even more devastating impact. What is more, helicopters used for tourist activities are subject to considerable criticism from a perspective of environmental protection as well as sports ethics.^[60]

Mountaineering

This section is an excerpt from Mountaineering.[edit]

Mountaineering, mountain climbing, or alpinism^[61] is a set of outdoor activities that involves ascending mountains. Mountaineering-related activities include traditional outdoor climbing, skiing, and traversing via ferratas that have become sports in their own right.^{[62][63][64][65]} Indoor climbing, sport climbing, and bouldering are also considered variants of mountaineering by some,^[66][67] but are part of a wide group of mountain sports.Unlike most sports, mountaineering lacks widely applied formal rules, regulations, and governance; mountaineers adhere to a large variety of techniques and philosophies (including grading and guidebooks) when climbing mountains.^[67][68] Numerous local alpine clubs support mountaineers by hosting resources and social activities. A federation of alpine clubs, the International Climbing and Mountaineering Federation (UIAA), is the International Olympic Committee-recognized world organization for mountaineering and climbing.^[69] The consequences of mountaineering on the natural environment can be seen in terms of individual components of the environment (land relief, soil, vegetation, fauna, and landscape) and the location/zone of mountaineering activity (hiking, trekking, or climbing zone).^[70] Mountaineering impacts communities on economic, political, social and cultural levels, often leading to changes in people’s worldviews influenced by globalization, specifically foreign cultures and lifestyles.^[71]

Mountains as sacred places

Main article: Sacred mountains

Mountains often play a significant role in religion. There are for example a number of sacred mountains within Greece such as Mount Olympus which was held to be the home of the gods.^[72] In Japanese culture, the 3,776.24 m (12,389.2 ft) volcano of Mount Fuji is also held to be sacred with tens of thousands of Japanese ascending it each year.^[73] Mount Kailash, in the Tibet Autonomous Region of China, is considered to be sacred in four religions: Hinduism, Bon, Buddhism, and Jainism. In Ireland, pilgrimages are made up the 952 metres (3,123 ft) Mount Brandon by Irish Catholics.^[74] The Himalayan peak of Nanda Devi is associated with the Hindu goddesses Nanda and Sunanda;^[75] it has been off-limits to climbers since 1983. Mount Ararat is a sacred mountain, as it is believed to be the biblical landing place of Noah’s Ark. In Europe and especially in the Alps, summit crosses are often erected on the tops of prominent mountains.^[76]

Superlatives

Main article: List of highest mountains

Heights of mountains are typically measured above sea level. Using this metric, Mount Everest is the highest mountain on Earth, at 8,848 metres (29,029 ft).^[77] There are at least 100 mountains with heights of over 7,200 metres (23,622 ft) above sea level, all of which are located in central and southern Asia. The highest mountains above sea level are generally not the highest above the surrounding terrain. There is no precise definition of surrounding base, but Denali,^[78] Mount Kilimanjaro and Nanga Parbat are possible candidates for the tallest mountain on land by this measure. The bases of mountain islands are below sea level, and given this consideration Mauna Kea (4,207 m (13,802 ft) above sea level) is the world’s tallest mountain and volcano, rising about 10,203 m (33,474 ft) from the Pacific Ocean floor.^[79]

The highest mountains are not generally the most voluminous. Mauna Loa (4,169 m or 13,678 ft) is the largest mountain on Earth in terms of base area (about 2,000 sq mi or 5,200 km²) and volume (about 18,000 cu mi or 75,000 km³).^[80] Mount Kilimanjaro is the largest non-shield volcano in terms of both base area (245 sq mi or 635 km²) and volume (1,150 cu mi or 4,793 km³). Mount Logan is the largest non-volcanic mountain in base area (120 sq mi or 311 km²).

The highest mountains above sea level are also not those with peaks farthest from the centre of the Earth, because the figure of the Earth is not spherical. Sea level closer to the equator is several miles farther from the centre of the Earth. The summit of Chimborazo, Ecuador’s tallest mountain, is usually considered to be the farthest point from the Earth’s centre, although the southern summit of Peru’s tallest mountain, Huascarán, is another contender.^[4] Both have elevations above sea level more than 2 kilometres (6,600 ft) less than that of Everest.

The sky is an unobstructed view upward from the surface of the Earth. It includes the atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from outer space.

In the field of astronomy, the sky is also called the celestial sphere. This is an abstract sphere, concentric to the Earth, on which the Sun, Moon, planets, and stars appear to be drifting. The celestial sphere is conventionally divided into designated areas called constellations.

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.^[1] In some cases, such as in discussing the weather, the sky refers to only the lower, denser layers of the atmosphere.

The daytime sky appears blue because air molecules scatter shorter wavelengths of sunlight more than longer ones (redder light).^[2][3][4][5] The 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 clouds. At night, the Moon, planets, and stars are similarly visible in the sky.

Some of the natural phenomena seen in the sky are clouds, rainbows, and aurorae. Lightning and precipitation are also visible in the sky. Certain birds and insects, as well as human inventions like aircraft and kites, can fly in the sky. Due to human activities, smog during the day and light pollution during the night are often seen above large cities.

Etymology

The word sky comes from the Old Norse sky, meaning ‘cloud, abode of God‘. The Norse term is also the source of the Old English scēo, which shares the same Indo-European base as the classical Latin obscūrus, meaning ‘obscure’.

In Old English, the term heaven was used to describe the observable expanse above the earth. During the period of Middle English, “heaven” began shifting toward its current, religious meaning.^[6]

During daytime

See also: Atmospheric optics and Diffuse sky radiation

Except for direct sunlight, most of the light in the daytime sky is caused by scattering, which is dominated by a small-particle limit called Rayleigh scattering. The scattering due to molecule-sized particles (as in air) is greater in the directions both toward and away from the source of light than it is in directions perpendicular to the incident path.^[7] Scattering is significant for light at all visible wavelengths, but is stronger at the shorter (bluer) end of the visible spectrum, meaning that the scattered light is bluer than its source: the Sun. The remaining direct sunlight, having lost some of its shorter-wavelength components, appears slightly less blue.^[5]

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.^{[clarification needed][8]}

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) 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 as does light from the zenith, causing a blue gradient looking vivid at the zenith and pale near the horizon.^[9] Red light is also scattered if there is enough air between the source and the observer, causing parts of the sky to change color as the Sun rises or sets. As the air mass nears infinity, scattered daylight appears whiter and whiter.^[10]

Apart from the Sun, distant clouds or snowy mountaintops may appear yellow. 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.^[10] At higher altitudes, the sky tends toward darker colors since scattering is reduced due to lower air density. An extreme example is the Moon, where no atmospheric scattering occurs, making the lunar sky black even when the Sun is visible.^[11]

Sky luminance distribution models have been recommended by the International Commission on Illumination (CIE) for the design of daylighting schemes. Recent developments relate to “all sky models” for modelling sky luminance under weather conditions ranging from clear to overcast.^[12]

During twilight

Main articles: Twilight, Dawn, and Dusk

See also: Sky brightness, Earth’s shadow, and Chappuis absorption

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 is well above the horizon, direct scattering of sunlight (Rayleigh scattering) is the overwhelmingly dominant source of light. However, during twilight, the period between sunset and night or between night and sunrise, the situation is more complex.

Green flashes 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,^[14] most of which occur when there is a temperature inversion (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 and bright planets, including Venus and Jupiter.^[15][16]

Earth’s shadow 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 (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.^[17][18]

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 sets in; it ends when the Sun drops more than 6° below the horizon. This is followed by the nautical twilight, when the Sun is between 6° and 12° below the horizon (depth between −6° and −12°), after which comes the astronomical twilight, 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.^[19]

Several sources can be identified as the source of the intrinsic brightness of the sky, namely airglow, indirect scattering of sunlight, scattering of starlight, and artificial light pollution.

During the night

Main article: Night sky

The term night sky refers to the sky as seen at night. The term is usually associated with skygazing and astronomy, with reference to views of celestial bodies 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 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, the visibility of celestial objects in the night sky is affected by light pollution. The presence of the 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

See also: 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 clouds can lead to halos around the Moon, which indicate the approach of a warm front and its associated rain.^[20] Morning fog portends fair conditions and can be associated with a marine layer, an indication of a stable atmosphere.^[21] 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. Cloud-free skies are indicative of fair weather for the near future.^[22] The use of sky cover in weather prediction has led to various weather lore over the centuries.^[23]

Tropical cyclones

Within 36 hours of the passage of a tropical cyclone‘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 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 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.^[24]

Use in transportation

Main article: Flight

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, propulsive thrust, aerostatically using buoyancy, or by ballistic movement, without any direct mechanical support from the ground. The engineering aspects of flight are studied in aerospace engineering which is subdivided into aeronautics, which is the study of vehicles that travel through the air, and astronautics, the study of vehicles that travel through space, and in ballistics, the study of the flight of projectiles. While human beings have been capable of flight via hot air balloons since 1783,^[25] other species have used flight for significantly longer. Animals, such as birds, bats, and insects are capable of flight. Spores and seeds from plants use flight, via use of the wind, as a method of propagating their species.^[26]

Significance in mythology

See also: Astrology and Sky father

Many mythologies have deities especially associated with the sky. In Egyptian religion, the sky was deified as the goddess Nut and as the god Horus. Dyeus is reconstructed as the god of the sky, or the sky personified, in Proto-Indo-European religion, whence Zeus, the god of the sky and thunder in Greek mythology and the Roman god of sky and thunder Jupiter.

In Australian Aboriginal mythology, Altjira (or Arrernte) is the main sky god and also the creator god. In Iroquois mythology, Atahensic was a sky goddess who fell down to the ground during the creation of the Earth. Many cultures have drawn constellations between stars in the sky, using them in association with legends and mythology about their deities.