Water in The Atmosphere

Humidity is the amount of water vapour in the air. Water vapour is the gaseous state of water and is invisible. Humidity indicates the likelihood of precipitation, dew, or fog. There are three main measurements of humidity: absolute, relative and specific. Absolute humidity is the water content of air at a given temperature expressed in gramme per cubic meter. Relative humidity, expressed as a percent, measures the current absolute humidity relative to the maximum (highest point) for that temperature. Specific humidity is a ratio of the water vapour content of the mixture to the total air content on a mass basis.

There is a limit to the amount of water vapour which can be held by the air. This limit changes with temperature. Warm air can hold more water vapour than cold air. Air containing maximum moisture it can hold at the given temperature is said to be saturated. Hence, the amount of water vapour that is needed to achieve saturation increases as the temperature increases. As temperature decreases, the amount of water vapour needed to reach saturation also decreases. As the temperature of a mass of air becomes lower it will eventually reach the point of saturation without adding or losing water mass.

When air reaches saturation, the water which it cannot hold gets precipitated as rain, snow hail etc. The temperature at which the air gets saturated without the addition of water vapour is called dew point.

The humidity is affected by winds and by rainfall. At the same time, humidity affects the energy budget and thereby influences temperatures in two major ways. One, water needs the energy to convert itself into vapour. This energy is absorbed from surroundings causing a cooling effect. This heat is returned to the surroundings when precipitation occurs.

Second, water vapour is the most abundant of all greenhouse gases. Water vapour, like a green lens that allows green light to pass through it but absorbs red light, is a “selective absorber”. Along with other greenhouse gases, water vapour is transparent to most solar energy, as you can literally see. But it absorbs the infrared energy emitted (radiated) upward by the earth’s surface, which is the reason that humid areas experience very little nocturnal cooling but dry desert regions cool considerably at night. This selective absorption causes the greenhouse effect.

Clouds are classified according to how they look and how high the base of the cloud is in the sky. This system was suggested in 1803. There are different sorts of clouds because the air where they form can be still or moving forward or up and down at different speeds. Very thick clouds with large enough water droplets can make rain or snow, and the biggest clouds can make thunder and lightning. There are five basic families of clouds based on how they look.
Cirrus clouds are high and thin. The air is very cold at high levels, so these clouds are made of ice crystals instead of water droplets. Cirrus clouds are sometimes called mares’ tails because they look like the tails of a horse.
Stratus clouds are like flat sheets. They may be low-level clouds (stratus), medium-level (altostratus), high-level (cirrostratus), or thick multi-level clouds that make rain or snow (nimbostratus).

Stratocumulus clouds are in the form of rolls or ripples. They may be low-level clouds (stratocumulus), medium-level (altocumulus), or high-level (cirrocumulus).

Cumulus clouds are puffy and small when they first form. They may grow into heap clouds that have a moderate vertical extent (nothing added to the name) or become towering vertical clouds (towering cumulus).
Cumulonimbus clouds are very large cumulus-type clouds that usually develop cirrus tops and sometimes other features that give them their own unique look.

Based on the height of formation of clouds, they are classified into different types.

High clouds form from 10,000 to 25,000 ft (3,000 to 8,000 m) in cold places, 16,500 to 40,000 ft (5,000 to 12,000 m) in mild regions and 20,000 to 60,000 ft (6,000 to 18,000 m) in the very hot tropics. They are too high and thin to produce rain or snow.

High-level clouds include Cirrus (Ci), Cirrocumulus (Cc), Cirrostratus (Cs)
Medium-level clouds or Middle clouds usually form at 6,500 ft (2,000 m) in colder areas. However, they may form as high as 25,000 ft (8,000 m) in the tropics where it’s very warm all year. Middle clouds are usually made of water droplets but may also have some ice crystals. They occasionally produce rain or snow that usually evaporates before reaching the ground.

Medium-level clouds include Altocumulus (Ac), Altostratus (As)
Low-level clouds are usually seen from near ground level to as high as 6,500 ft (2,000 m). Low clouds are usually made of water droplets and may occasionally produce very light rain, drizzle, or snow.

Low-level clouds include Stratocumulus (Sc) and Stratus (St). When very low stratus cloud touches the ground, it is called fog.

Moderate-vertical clouds are clouds of medium thickness that can form anywhere from near ground level to as high as 10,000 ft (3,000 m). Medium-level Cumulus does not have ‘alto’ added to its name. The tops of these clouds are usually not much higher than 20,000 ft (6,000 m). Vertical clouds often create rain and snow. They are made mostly of water droplets, but when they push up through cold higher levels they may also have ice crystals. They include Cumulus (Cu) and Nimbostratus (Ns)

Towering-vertical clouds are very tall with tops usually higher than 20,000 ft (6,000 m). They can create heavy rain and snow showers. Cumulonimbus, the biggest clouds of all, can also produce thunderstorms. These clouds are mostly made of water droplets, but the tops of very large cumulonimbus clouds are often made mostly of ice crystals.

Rain is when water falls from clouds in droplets that are bigger than 0.5 mm. Droplets of water that are about 0.2mm to 0.45mm big are called drizzle. Rain is a kind of precipitation. Precipitation is any kind of water that falls from clouds in the sky, like rain, hail, sleet and snow.

When the Sun heats the Earth’s surface, the ground heats the air above it. Convection makes the air rise and cool. When it cools to the dew point, clouds form and rain follows. This type of rainfall often causes summer showers and thunderstorms. This type of rainfall is called convectional rainfall.

Relief rain usually occurs along coastal areas where a line of hills runs along the coast. When the wet onshore wind from the sea meets a mountain, hill or any other sort of barrier, it is forced to rise along the slope and cools. When the air temperature falls to its dew point, water vapour condenses to form clouds. When the clouds can no longer hold the water droplets, relief rain begins to fall on the windward slope of the mountain. On the leeward slope, air sinks, it is warmed and further dried by compression. Therefore, the leeward slope is known as the rain shadow. Moist winds blow in from the sea and are forced to rise over the land. The air cools and the water vapour condenses, forming raindrops. Relief rain is also a very dense and cold mixture of precipitation.

Relief or orographic rain is formed when the air is forced to cool when it rises over relief features in the landscape such as hills or mountains. As it rises it cools, condenses and forms rain. The highest annual rainfall totals occur in mountain areas. There is often a rain shadow effect whereby the leeward (downwind) slope receives a relatively small amount of rain.

Frontal rain happens when cooler and warmer, humid air meets in a weather front. The less dense warm air rises and condenses forming clouds. These clouds grow and eventually create rain. In some places in the northern temperate zone, the cold air front tends to come from the North West and the warm air front comes from the south-west.

Other forms of precipitation include snow, sleet, dew, frost, and hail. Fog and mist are not precipitation but suspensions. In that case, the water vapour does not condense sufficiently to precipitate.

Snow forms when water in the atmosphere becomes frozen. When the dew point is below the freezing point of water, water crystallises as snow and falls down. Sleet is a type of precipitation where ice pellets fall down from the sky. When drops of rain fall through a mass of air below freezing point, the drops get solidified to become ice pellets.

Dew is a thin film of water that has condensed on the surface of objects near the ground in the morning or evening. These objects cool in the night. When they cool, the thin layer of air around them cools too. This makes some water vapour condense on the object. Frost is ice that is formed when water vapour freezes onto a surface. It has a white, powdery appearance. It forms on cold surfaces when the temperature of the air is very low.

A piece of hail (called a hailstone) is a lump of ice that falls out of a storm cloud. A hailstone begins as a small water droplet or as a rounded snow pellet in a cloud. The drop grows by collecting many cloud drops. The little drop is blown by a strong wind inside the cloud to where it meets with some extremely cold water drops. These supercooled drops are still liquid water even though the temperature is below freezing. When the little drop mixes with these extremely cold drops, they join, and the little drop has now become a hailstone.

The little hailstone is thrown up inside the cloud, still collecting other cold drops. The hailstone gets bigger and bigger until it goes to the top of the cloud. Then, because there is no more wind, it falls back down through the cloud. While it is falling it gets even bigger as it bangs into more supercooled drops. If it goes down very fast it can hit the Earth at up to 90 mph (144 kph), bouncing like popcorn. If the hailstone hits the dirt, it can actually bury itself.
The patterns of precipitation and wind decide the climate on different places of the earth. Climate means the usual condition of the temperature, humidity, atmospheric pressure, the wind, rainfall, and other meteorological elements in an area of the Earth’s surface for a long time. Climate is different from weather. Weather is the condition of these elements right now, for shorter periods of time that are up to two weeks.

The latitude, ground, and height can change the climate of a location. It is also important to note if oceans or other large bodies of water are nearby. Climates are most commonly classified by temperature and precipitation.
Weather is the day-to-day or hour-to-hour change in the atmosphere. Weather includes wind, lightning, storms, hurricanes, tornadoes, rain, hail, snow, and lots more. Energy from the Sun affects the weather. Climate tells us what kinds of weather usually happen in an area at different times of the year. Changes in weather can affect our mood. We wear different clothes and do different things in different weather conditions. We choose different foods in different seasons, like ice cream in the summer, or hot chocolate in the winter.

Hence geography and climate decide the lifestyle and economic activities in an area and study of geography is essential for understanding the distribution and utilisation of resources.

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Atmospheric Temperature, Pressure and The Wind

Insolation is the solar radiation that reaches the earth’s surface. It is measured by the amount of solar energy received per square centimetre per minute. Insolation affects the temperature at any place on earth. The more the Insolation at a place, the higher is the temperature there. In any given day, the strongest insolation is received at noon.

Factors affecting insolation are Angle of the sun, Distance between the sun and the earth, Duration of daylight and atmospheric conditions preventing sun’s rays from reaching the earth.

Received radiation is unevenly distributed because the Sun heats equatorial regions more than polar regions. Energy is absorbed by the atmosphere, hydrosphere, and lithosphere, and the heat energy is redistributed through evaporation of surface water, convection, rainfall, winds, and ocean circulation. When the incoming solar energy is equal to flow of heat to space, global temperatures will be stable.

To quantify Earth’s heat budget or heat balance, let the insolation received at the top of the atmosphere be 100 units. Called the albedo of Earth, around 35 units are reflected back to space: 27 from the top of clouds, 2 from snow and ice-covered areas, and 6 by other parts of the atmosphere. The 65 remaining units are absorbed: 14 within the atmosphere and 51 by the Earth’s surface. These 51 units are radiated to space in the form of terrestrial radiation: 17 directly radiated to space and 34 absorbed by the atmosphere (19 through latent heat of condensation, 9 via convection and turbulence, and 6 directly absorbed). The 48 units absorbed by the atmosphere (34 units from terrestrial radiation and 14 from insolation) are finally radiated back to space. These 65 units (17 from the ground and 48 from the atmosphere) balance the 65 units absorbed from the sun; thereby demonstrating no net gain of energy by the Earth.

The major atmospheric gases (oxygen and nitrogen) are transparent to incoming sunlight but are also transparent to outgoing thermal (infrared) radiation. However, water vapour, carbon dioxide, methane and other trace gases are opaque to many wavelengths of thermal radiation. The Earth’s surface radiates the net equivalent of 17 percent of the incoming solar energy in the form of thermal infrared. However, the amount that directly escapes to space is only about 12 percent of incoming solar energy. The remaining fraction, 5 to 6 percent, is absorbed by the atmosphere by greenhouse gas molecules. This absorption of heat energy by greenhouse gases leads to increase of Earth’s average temperature. This is why it is important to ensure that the proportion of gases in the atmosphere is constant to prevent an increase in Earth’s temperature beyond inhabitable levels.

We already know that vertical rays of the sun are warmer than slanting rays of the sun. So, the region around the equator will be comparatively warmer than other regions while polar regions will be comparatively colder. Because of this differential heating, the five main latitude regions of the Earth’s surface comprise geographical zones, divided by the major circles of latitude.

The Torrid or Tropical Zone is also known as the Tropics. The zone is bounded on the north by the Tropic of Cancer and on the south by the Tropic of Capricorn; these latitudes mark the northern and southern extremes of regions in which the sun seasonally passes directly overhead. At those two latitudes this happens once a year, but in the region between them, the sun passes overhead twice a year.

In the Northern Hemisphere, in the sun’s apparent northward migration after the March equinox, it passes overhead once, then after the June solstice, at which time it reaches the Tropic of Cancer, it passes over again on its apparent southward journey. After the September equinox, the sun passes into the Southern Hemisphere. It then passes similarly over the southern tropical regions until it reaches the Tropic of Capricorn at the December solstice, and back again as it returns northwards to the Equator.

In the two Temperate Zones, the Sun is never directly overhead, and the climate is mild, generally ranging from warm to cool. The four annual seasons, spring, summer, autumn and winter, occur in these areas. The North Temperate Zone includes Europe, Northern Asia, and North and Central America. The South Temperate Zone includes Southern Australasia, southern South America, and Southern Africa.

The two Frigid Zones, or polar regions, experience the midnight sun and the polar night for part of the year – at the edge of the zone there is one day in the winter when the Sun is invisible, and one day at the summer solstice when the sun remains above the horizon for 24 hours, while in the center of the zone (the pole), the day is literally one year long, with six months of daylight and six months of night. The Frigid Zones are the coldest parts of the earth and is generally covered with ice and snow.

Atmospheric pressure is a force in an area pushed against a surface by the weight of air in Earth’s atmosphere. The earth is covered in a layer of air. However, this layer is not distributed evenly around the globe. At different times, the layer of air is thicker in some places than in others. Where the layer of air is thicker, there is more air. Since there is more air, there is a higher pressure in that spot. Where the layer of air is thinner, there is a lower atmospheric pressure.

Atmospheric temperature is a measure of temperature at different levels of the Earth’s atmosphere. It is governed by many factors, including incoming solar radiation, humidity and altitude. We have already seen the temperature at different latitudes and heights. When the temperature is high the air expands reducing the pressure at that area. Hence, the geographic zones are not only temperature zones, but also pressure zones with tropics which is heated to maximum having a low pressure compared to poles which have a high pressure because of very low temperatures.

The Wind is the movement of air. Short bursts of fast winds are called gusts. Strong winds that go on for about one minute are called squalls. Winds that go on for a long time are called many different things, such as breeze, gale, hurricane, and typhoon. Sunlight and differential heating causes the Earth’s atmospheric circulation. The resulting winds blow over land and sea, producing weather. The wind is named after the direction from which it flows. For example, a wind blowing from the east is called easterly.

If there is a high-pressure system near a low-pressure system, the air will move from the high pressure to the low pressure to try and even out the pressures. A big difference in pressure can make high winds. In some storms, such as hurricanes, typhoons, cyclones, or tornadoes, the pressure differences can cause winds faster than 320 kilometres per hour.

The Wind can also be caused by the rising of hot air or the falling of cool air. When hot air rises, it creates a low pressure underneath it, and air moves in to equalise the pressure. When cold air drops (because it is denser or heavier than warm air), it creates a high pressure and flows out to even out the pressure with the low-pressure around it.
Atmospheric circulation is the large-scale movement of air through the troposphere and the means (with ocean circulation) by which heat is distributed around Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant because it is determined by Earth’s rotation rate and the difference in solar radiation between the equator and poles.

The Earth’s weather is a consequence of its illumination by the Sun and the laws of thermodynamics. The atmospheric circulation can be viewed, from that standpoint, as a heat engine driven by the Sun’s energy, and whose energy sink, ultimately, is the blackness of space. The work produced by that engine causes the motion of the masses of air and in that process, it redistributes the energy absorbed by the Earth’s surface near the tropics to space and incidentally to the latitudes nearer the poles.

In tropics, air is warmed by the Earth’s surface, decreases in density and rises. The rising air creates a low-pressure zone near the equator. Air mass rising on all sides of the equator forces those rising air masses to move poleward. As the air moves poleward, it cools, becomes denser, and descends at about 30th parallel, creating a high-pressure area. The descended air then travels toward the equator along the surface, replacing the air that rose from the equatorial zone, closing the loop. This atmospheric circulation pattern that George Hadley described was an attempt to explain the trade winds and is called Hadley Cell.

The Coriolis effect is a force that is found in a rotating object. Gaspard-Gustave de Coriolis first described the Coriolis effect in 1835. The Coriolis effect can best be seen in hurricanes. In the northern hemisphere or part of the earth, they spin clockwise, in the southern hemisphere they spin the other way. This happens because the earth spins on its tilt.
One example of the Coriolis effect that is the winds in northern hemisphere tilts rightward from their direction of motion and the winds in southern hemisphere tilt leftwards.

The poleward movement of the air in the upper part of the troposphere deviates toward the east due to the Coriolis force. At the ground level, however, the movement of the air toward the equator in the lower troposphere deviates toward the west producing a wind from the east. The winds that flow to the west (from the east, easterly wind) at the ground level in the Hadley cell, are called the Trade Winds.

Though the Hadley cell is described as located at the equator, in the northern hemisphere, it shifts to higher latitudes June and July and toward lower latitudes December and January as it is caused by the Sun’s heating of the surface. The zone where the greatest heating takes place is called the “thermal equator” or Inter-Tropical Convergence Zone(ITCZ). As the southern hemisphere summer is December to March, the movement of the thermal equator to higher southern latitudes takes place then.

The Polar cell, likewise, is a simple system. Though cool and dry relative to equatorial air, the air masses at the 60th parallel are still sufficiently warm and moist to undergo convection and drive a thermal loop. At the 60th parallel, the air rises to the tropopause (about 8 km at this latitude) and moves poleward. As it does so, the upper-level air mass deviates toward the east. When the air reaches the polar areas, it has cooled and is considerably denser than the underlying air. It descends, creating a cold, dry high-pressure area. At the polar surface level, the mass of air is driven toward the 60th parallel, replacing the air that rose there, and the Polar circulation cell is complete. As the air at the surface moves equatorward, it deviates toward the west. Again, the deviations of the air masses is due to conservation of energy, which is also referred to as the Coriolis effect. The air flows at the surface are called the Polar easterlies (easterly from the east).

Part of the air rising at 60° latitude diverges at high altitude toward the poles and creates the polar cell. The rest moves toward the equator where it collides at 30° latitude with the high level air of the Hadley cell. There it subsides and strengthens the high pressure ridges beneath. A large part of the energy that drives the Ferrel cell is provided by the Polar and Hadley cells circulating on either side and that drag the Ferrel cell with it. The Ferrel cell, theorised by William Ferrel (1817–1891), is, therefore, a secondary circulation feature, whose existence depends upon the Hadley and Polar cells on either side of it; it behaves much as an atmospheric ball bearing between the two.

Apart from these planetary winds, there are local winds due to relative heating within the same locality. This is called longitudinal circulation as they are not dependent on parallels, but can be within same latitudinal areas. Longitudinal circulation is a result of the heat capacity of water, it’s absorptivity, and it’s mixing. Water absorbs more heat than does the land, but its temperature does not rise as greatly as does the land. As a result, temperature variations on land are greater than on water. The Hadley, Ferrel, and Polar cells operate at the largest scale of thousands of kilometres (synoptic scale). But, even at mesoscales (a horizontal range of 5 to several hundred kilometres), this effect is noticeable. During the day, air warmed by the relatively hotter land rises, and as it does so it draws a cool breeze from the sea that replaces the risen air. At night, the relatively warmer water and cooler land reverse the process, and a breeze from the land, of air cooled by the land, is carried offshore by night. This described effect is daily (diurnal). They are also called sea breeze and land breeze.

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The Atmosphere

All around the Earth is a large amount of air (the atmosphere). The mass of the Earth pulls the gasses in the air down and does not let them go into outer space due to gravity. Most living things need the air (or parts of the air gripped in the water) to breathe and live. They use the gasses—especially oxygen and carbon dioxide—to make and use sugar and to give themselves energy.

The atmosphere is made up of nitrogen (78.1%) and oxygen (20.9%), with small amounts of argon (0.9%), carbon dioxide (~ 0.035%), water vapour, and other gases. Water vapour accounts for roughly 0.25% of the atmosphere by mass. The concentration of water vapour (a greenhouse gas) varies significantly from around 10 ppm by volume in the coldest portions of the atmosphere to as much as 5% by volume in hot, humid air masses, and concentrations of other atmospheric gases are typically quoted in terms of dry air (without water vapour). The atmosphere protects life on Earth by absorbing (taking) ultraviolet rays from the sun. It makes our days cooler and our nights warmer. Solid particulates, including ash, dust, volcanic ash, etc. are small parts of the atmosphere. They are important in making clouds and fog.

The atmosphere does not end at a specific place. The higher above the Earth something is, the thinner the atmosphere around it is. There is no clear border between the atmosphere and outer space. 75% of the atmosphere is within 11 kilometres (6.8 miles) of the Earth’s surface.

Earth’s atmosphere can be divided into five main layers based on fluctuations in the temperature profile. In general, air pressure and density decrease with altitude in the atmosphere. However, the temperature has a more complicated profile with altitude and may remain relatively constant or even increase with altitude in some regions. Excluding the exosphere, Earth has four primary layers, which are the troposphere, stratosphere, mesosphere, and thermosphere.

The exosphere is the outermost layer of Earth’s atmosphere (i.e. the upper limit of the atmosphere). It extends from the exobase, which is located at the top of the thermosphere at an altitude of about 700 km above sea level, to about 10,000 km (6,200 mi; 33,000,000 ft) where it merges into the solar wind.

This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide closer to the exobase. The atoms and molecules are so far apart that they can travel hundreds of kilometres without colliding with one another. Thus, the exosphere no longer behaves like a gas, and the particles constantly escape into space.

The thermosphere is the second-highest layer of Earth’s atmosphere. It extends from the mesopause (which separates it from the mesosphere) at an altitude of about 80 km (50 mi; 260,000 ft) up to the thermopause at an altitude range of 500–1000 km (310–620 mi; 1,600,000–3,300,000 ft). The height of the thermopause varies considerably due to changes in solar activity. Because the thermopause lies at the lower boundary of the exosphere, it is also referred to as the exobase. The lower part of the thermosphere, from 80 to 550 kilometres (50 to 342 mi) above Earth’s surface, contains the ionosphere.

The temperature of the thermosphere gradually increases with height. Unlike the stratosphere beneath it, wherein a temperature inversion is due to the absorption of radiation by ozone, the inversion in the thermosphere occurs due to the extremely low density of its molecules. The temperature of this layer can rise as high as 1500 °C (2700 °F), though the gas molecules are so far apart that its temperature in the usual sense is not very meaningful. Although the thermosphere has a high proportion of molecules with high energy, it would not feel hot to a human in direct contact, because its density is too low to conduct a significant amount of energy to or from the skin.
The mesosphere is the third highest layer of Earth’s atmosphere, occupying the region above the stratosphere and below the thermosphere. It extends from the stratopause at an altitude of about 50 km (31 mi; 160,000 ft) to the mesopause at 80–85 km (50–53 mi; 260,000–280,000 ft) above sea level.
Temperatures drop with increasing altitude to the mesopause that marks the top of this middle layer of the atmosphere. It is the coldest place on Earth and has an average temperature around −85 °C (−120 °F; 190 K).

The mesosphere is also the layer where most meteors burn up upon atmospheric entrance. It is too high above Earth to be accessible to jet-powered aircraft and balloons, and too low to permit orbital spacecraft. The mesosphere is mainly accessed by sounding rockets and rocket-powered aircraft.

The stratosphere is the second-lowest layer of Earth’s atmosphere. It lies above the troposphere and is separated from it by the tropopause. This layer extends from the top of the troposphere at roughly 12 km (7.5 mi; 39,000 ft) above Earth’s surface to the stratopause at an altitude of about 50 to 55 km (31 to 34 mi; 164,000 to 180,000 ft).

The atmospheric pressure at the top of the stratosphere is roughly 1/1000 the pressure at sea level. It contains the ozone layer, which is the part of Earth’s atmosphere that contains relatively high concentrations of that gas. The stratosphere defines a layer in which temperatures rise with increasing altitude. This rise in temperature is caused by the absorption of ultraviolet radiation (UV) radiation from the Sun by the ozone layer, which restricts turbulence and mixing. Although the temperature may be −60 °C (−76 °F; 210 K) at the tropopause, the top of the stratosphere is much warmer and may be near 0 °C.

The stratospheric temperature profile creates very stable atmospheric conditions, so the stratosphere lacks the weather-producing air turbulence that is so prevalent in the troposphere. Consequently, the stratosphere is almost completely free of clouds and other forms of weather. However, some clouds are occasionally seen in the lower part of this layer of the atmosphere where the air is coldest. This is the highest layer that can be accessed by jet-powered aircraft.

The troposphere is the lowest layer of Earth’s atmosphere. It extends from Earth’s surface to an average height of about 12 km, although this altitude actually varies from about 9 km (30,000 ft) at the poles to 17 km (56,000 ft) at the equator, with some variation due to weather. The troposphere is bounded above by the tropopause, a boundary marked in most places by a temperature inversion (i.e. a layer of relatively warm air above a colder one), and in others by a zone which is isothermal with height.
Although variations do occur, the temperature usually decreases with increasing height in the troposphere because the troposphere is mostly heated through energy transfer from the surface. Thus, the lowest part of the troposphere (i.e. Earth’s surface) is typically the warmest section of the troposphere. The troposphere is denser than all its overlying atmospheric layers because a larger atmospheric weight sits on top of the troposphere and causes it to be most severely compressed and contains roughly 80% of the mass of Earth’s atmosphere. Fifty percent of the total mass of the atmosphere is located in the lower 5.6 km (18,000 ft) of the troposphere.

Almost all atmospheric water vapour or moisture is found in the troposphere, so it is the layer where most of Earth’s weather takes place. It has basically all the weather-associated cloud genus types generated by active wind circulation, although very tall cumulonimbus thunder clouds can penetrate the tropopause from below and rise into the lower part of the stratosphere. Most conventional aviation activity takes place in the troposphere, and it is the only layer that can be accessed by propeller-driven aircraft.

The ozone layer is contained within the stratosphere. In this layer ozone concentrations are about 2 to 8 parts per million, which is much higher than in the lower atmosphere but still very small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from about 15–35 km (9.3–21.7 mi; 49,000–115,000 ft), though the thickness varies seasonally and geographically. About 90% of the ozone in Earth’s atmosphere is contained in the stratosphere.

The ionosphere is a region of the atmosphere that is ionised by solar radiation. It is responsible for auroras. During daytime hours, it stretches from 50 to 1,000 km (31 to 621 mi; 160,000 to 3,280,000 ft) and includes the mesosphere, thermosphere, and parts of the exosphere.

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The Hydrosphere

Over 70% of the Earth is covered by seas full of salty water. This salty water makes up about 97½% of all Earth’s water. The fresh water persons can drink is mostly ice. Only a very small amount is at hand in rivers and under the earth, for humans to drink and use. The air above the Earth stops the water from going away into outer space.
An ocean is a large area of salt water between continents. Oceans are very big and they join smaller seas together. Together, the oceans are like one “ocean”, because all the “oceans” are joined. Oceans (or marine biomes) cover 72% of our planet. The largest ocean is the Pacific Ocean. It covers 1/3 (one-third) of the Earth’s surface.
The smallest ocean is the Arctic Ocean. Different water movements separate the Southern Ocean from the Atlantic, Pacific, and Indian Oceans. The Southern ocean is also called the Antarctic Ocean because it covers the area around Antarctica. Older maps may not use the names the Arctic Ocean and the Southern Ocean.
The deepest ocean is the Pacific ocean. The deepest point is the Mariana Trench, being about 11,000 meters (36,200 feet) deep. The deep ocean is characterized by cold temperatures, high pressure, and complete darkness. Some very unusual organisms live in this part of the ocean. They do not require energy from the sun to survive because they use chemicals from deep inside the Earth.
The ocean floor, also known as sea-bed, also has mountains valleys and plains like that on land. A continental shelf is the part of the continent that is under water. The shelf was part of the land during the ice ages in the glacial periods. Beyond the continental shelf, the ocean floor goes down to much greater depths. The continental shelf is a shallow ocean. It varies in depth, up to 140 meters deep. It varies greatly in its width. At the leading edge of a moving continental plate, there will be little or no shelf. The shelf on a passive edge of a plate will be wide and shallow. The widest shelf is the Siberian shelf in the Arctic Ocean: it is 1500 km (930 miles) in width.
The shelf usually ends at a point of decreasing slope (called the shelf break). The sea floor below the break is the continental slope. The character of the shelf changes dramatically at the shelf break, where the continental slope begins. With a few exceptions, the shelf break is located at a remarkably uniform depth of roughly 140 m (460 ft); this is likely a hallmark of past ice ages when sea level was lower than it is now.
Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources. The structure of the oceans, starting with the continents, begins usually with a continental shelf, continues to the continental slope – which is a steep descent into the ocean, until reaching the abyssal plain – a topographic plain area, the beginning of the seabed, and its main area. The border between the continental slope and the abyssal plain usually has a more gradual descent and is called the continental rise, which is caused by sediment cascading down the continental slope.
The mid-ocean ridge, as its name implies, is a mountainous rise through the middle of all the oceans, between the continents. Typically a rift runs along the edge of this ridge. Along tectonic plate edges, there are typically oceanic trenches – deep valleys, created by the mantle circulation movement from the mid-ocean mountain ridge to the oceanic trench.
An ocean current is a continuous movement of ocean water from one place to another. Ocean currents are created by the wind, water temperature, salt content, and the gravity of the moon. The current’s direction and speed depend on the shoreline and the ocean floor. They can flow for thousands of miles and are found in all the major oceans of the world. Ocean currents can be found on the water surface and deeper down.
Currents on the surface often depend on the wind. They travel clockwise in the northern hemisphere. They travel counterclockwise in the southern hemisphere. They are found up to 400 meters (1,300 ft) below the surface of the ocean. Deeper currents depend on water pressure, temperature, and salt content.

Ocean currents are classified into cold currents and warm currents. Generally, currents flowing from warm tropics to poles are warm currents and currents flowing from cold tropics to tropics are cold. They also regulate the temperature of the area through which it flows and causes variation in climate.
Hydrological cycle or water cycle is the cycle that water goes through on Earth. It includes the movement of water from water bodies to the atmosphere by evaporation and return of water from the atmosphere back by condensation and precipitation.
First, water on the surface of the Earth evaporates and gets collected as water vapor in the sky. This water vapor condenses when gets cooled to makes clouds. The small droplets in clouds coalesce to form larger clouds and the water falls from the sky as rain, snow, sleet, or hail which is called precipitation. This water sinks into the surface and also collects into lakes, oceans, or aquifers. It evaporates again and continues the cycle.

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Erosion & Deposition

As you know, the river is a body of flowing water. It flows down the slope under the effect of gravity, in a valley. The origin of the river is called source and the place where it ends, merging with a large water body is called the mouth.
Force of flowing water causes a dragging effect on river bed causing erosion and deepening of the valleys. Rivers also erodes the banks and widens the valleys. Sometimes soluble components are dissolved in water causing further erosion.
In the upper part of the river, as they are flowing from a great height, the velocity of flow will be higher and there will be a faster erosion of river bed. As a result, there will be the formation of deep and narrow ‘I’ shaped valleys called Gorges. Waterfalls or rapids are formed when a softer rock which is easily eroded comes after a layer of the hard rock layer.
When rivers erode the branches and widen the valleys the bank will be gently sloping leading to ‘v’ shaped valleys.
The river transports the eroded materials along with their flow and the number of materials carried depends on the speed of flow. Some materials are carried by the river in dissolved form while some are carried as suspended particles(suspended load). Coarse and heavy materials are dragged along the river bed as bed load.
In the later course of a river, it slows down and the carrying capacity also comes down. Excess load gets deposited on the riverbed and along the banks. Mainly it raises the river bed and the river becomes shallow and wide. When the deposits reach the level of flow of water the river gets bifurcated into multiple channels and riverine islands may be formed when some of these channels rejoin. Such a channel is called a braided channel.
Rivers deposits heavier particles at an earlier point and lighter particles further downstream. When river bed raises due to deposition of sediments, the river will show a tendency to overflow their banks. Such areas where rivers overflow, depositing rich and fertile soil is called flood plains. As the flow reduces, the amount of deposition also reduces away from the channel leading to accumulation of deposition along the banks. They are called natural levee.
In the flood plains, there can be the formation of broad curves due to flooding and changing course of rivers. This tendency is called meandering tendency. Such a meander loop can become cut off from the river stream by further deposition leading to the formation of an Oxbow Lake.

When the river reaches the mouth, if it is sufficiently loaded with sediments, can deposit them at the mouth causing the formation of triangular land forms, They are called deltas. As the delta gets enlarged the mouth of river shifts further and some area which was earlier under the oceans or seas are reclaimed as land. Because of depositional activity, the river stream may split into multiple channels and they are called distributaries.
The water seeping underground becomes ground water and it also causes some erosion and formation of landforms. This is more clearly seen in limestone region and is called Karst topography.
Water seeping underground fills the gaps and spaces in rock strata and flows down till it reaches a zone of saturation where all the spaces are filled with water. This area is called water table. The depth of water table differs from area to area.
While water seeps underground through the fractures in limestone, it dissolves the stone in due course of time due to carbonisation. This results in large underground caves and passages. Some of them may carry underground streams and some of these streams may reappear on the surface as springs.
As groundwater containing dissolved lime or calcite drips from the roof of the caves, the water evaporates leaving calcite behind. These deposits hanging from the cave ceilings are stalactites. When similar deposition happens upwards from the floor of the cave, they are called stalagmites. Some of them may join together to form pillars inside the caves.
Wind action of gradation is mostly limited to dry arid areas especially deserts. Erosion happens when loosely bounded particles are carried away by the wind causing wind eroded basins. These particles are deposited over other areas when there is an obstacle lying in its path creating sand dunes of varying sizes and shapes. Sand dunes usually have a gentler slope towards the windward side and are convex from that side.
Glaciers are formed on top of high mountains and in subarctic zones. When the glacier is huge, the lowest layers will be slightly plastic and they slowly flow in the direction of the slope. A glacier behaves like a slow moving river of ice.
Glaciers are categorised by their morphology, thermal characteristics, and behaviour. Alpine glaciers, also known as mountain glaciers or cirque glaciers, form on the crests and slopes of mountains. An alpine glacier that fills a valley of a former stream is sometimes called a valley glacier. A large body of glacial ice astride a mountain, mountain range, or volcano is termed an ice cap or ice field. Ice caps have an area less than 50,000 km2 (19,000 sq mi) by definition.
Glacial bodies larger than 50,000 km2 (19,000 sq mi) are called ice sheets or continental glaciers. Several kilometres deep, they obscure the underlying topography. Only nunataks protrude from their surfaces. The only extant ice sheets are the two that cover most of Antarctica and Greenland.
As the glaciers expanded, due to their accumulating weight of snow and ice, they crush and abrade scoured surface rocks and bedrock. The resulting erosional landforms include cirques, glacial horns, arêtes, U-shaped valleys, over deepenings and hanging valleys. Cirque is the starting location for mountain glaciers.U-shaped valleys are created by mountain glaciers. When filled with ocean water so as to create an inlet, these valleys are called fjords. Arête is a spiky high land between two glaciers, if the glacial action erodes through, a spillway forms.

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Rock Cycle & Weathering

All places on Earth are made of or are on top of rocks. A rock is a naturally occurring solid made up of a mixture of one or several minerals, in varying proportions. The minerals in the rocks vary, making different kinds of rock. The Earth’s crust is made of rock. Rock is often covered by soil or water. Rock is beneath the oceans, lakes, and rivers of the earth, and under the polar icecaps.

Minerals are usually solid, inorganic, crystalline substances formed naturally by geological processes. A mineral is a homogeneous naturally occurring substance with a definite but not necessarily fixed chemical composition. Most minerals are solids with an ordered atomic arrangement made of a single chemical element or more usually a compound. There are over 4,000 types of known minerals.
Rocks are classified by their minerals and chemical make-up.Rocks are aggregates of minerals. Geologists divide rocks into three groups: igneous, metamorphic, and sedimentary. Igneous rocks crystallise from magma. Metamorphic rocks form by the deformation and/or recrystallization of pre-existing rock by changes in temperature, pressure, and/or chemistry. Sedimentary rocks form by weathering and erosion of pre-existing rock to make sediment, which is lithified into rock.

Igneous rocks are formed when molten magma cools, either above or below the surface. They are divided into two main categories: plutonic rock and volcanic rock. Plutonic or intrusive rocks are made when magma cools and crystallises slowly within the Earth’s crust (example granite). Volcanic or extrusive rocks result from magma reaching the surface either as lava or ejecta (examples pumice and basalt).
Sedimentary rocks are the most common rocks on Earth. They form at or near the Earth’s surface. Sedimentary rock is formed in layers which were laid down one by one on top of another. Some of the layers are thin, some are thick. Layers are made by deposition of sediment, organic matter, and chemical precipitates. Deposition is followed by squeezing of sediment under its own weight, and cementation. This process is called ‘consolidation’: it turns the sediment into a more or less hard substance.
Metamorphic rocks are secondary rocks formed by rocks transformed under great pressure and high temperatures under earth’s crust. These conditions change the make-up of the original minerals.
The rock cycle is the process by which rocks of one kind changing into rocks of another kind. Three types of rocks can change into the other kinds by physical processes: cooling, melting, heat, weathering/erosion, compacting (squeezing tightly together), cementing, and pressure. Other substances also can become rocks and enter the rock cycle. When heated deep underground, rocks become magma (liquid rock). Above ground, it is called lava. Sediment, the particles from rock erosion and weathering, is the basis for a sedimentary rock of the future and soil may be reconverted to rock by this process.
These processes can occur in different orders, and the cycle goes on forever. Wind and water can create sediment from rocks, and movement of one tectonic plate against another creates enormous heat and pressure which affects rocks greatly. Subduction converts all kinds into magma, which eventually rejoins the cycle as igneous rock.
Weathering is the breaking down of rocks, soil and their minerals through direct contact with the Earth’s atmosphere, waters, or living things. Weathering occurs in situ (in place, with no movement). It leads to erosion. Erosion is where rocks and minerals are moved downhill (usually towards the sea) by water, ice and the wind.
The two main types of weathering are physical and chemical. Sometimes there are also aspects of biology. Physical weathering is important in very cold or very dry environments. Chemical reactions are most intense where the climate is wet and hot. However, both types of weathering occur together, and each tends to accelerate the other. The materials left after the rock breaks down combined with organic material creates soil.
Mechanical or physical weathering means the breakdown of rocks and soils through direct contact with atmospheric conditions such as heat, water, ice and pressure. It is the mechanical disintegration of rocks without a change in chemical components.
Wind processes are called ‘aeolian’. The Wind erodes the Earth’s surface by removing loose, fine-grained particles, called ‘deflation’. Sand carried by the wind wears down surfaces. Regions which have intense and sustained erosion are called deflation zones. Most aeolian deflation zones are composed of desert pavement, a sheet-like surface of rock fragments that remains after wind and water have removed the fine particles. Almost half of Earth’s desert surfaces are stone deflation zones. The rock mantle in desert pavements protects the underlying material from deflation.
Rain is another force that works slowly. The force of raindrops on some rocks makes them wear down. Rain also can make a chemical change in some rocks, because it is usually slightly acidic. The water mixes with the minerals in the rock to break it down.
Changing temperature can make a rock crack. Every day when the sun shines on the rock, its surface is heated. Heat causes the surface to expand (get bigger) a little. The inside of the rock, though, does not heat up as fast as the outside of the rock. The inside of the rock stays cooler. At night, the surface cools down and contracts. The expanding and contracting makes some places on the surface weak, and a crack is made. Also, if water gets into a crack in a rock and the temperature goes below the freezing point, the water will freeze and expand. After some time, the rock may be weak enough to break into pieces.
Ice, which can be miles thick, grinds the surface of the rock below it. The particles are carried with the ice, and if a glacier ends up in the sea, so does all the material carried with it.
Lava or magma can cause weathering as when the molten rock touches rock (either intrusive rock extrusive) it causes the rock to change form to add to the quantity of molten rock causing the rock to have changed form. So the rocks have formed a different crystal structure to before it came in direct contact with the lava or magma.
Chemical weathering is the direct effect of atmospheric or biological chemicals in the breakdown of rocks, soils and minerals. The mineral composition of rock changes due to chemical reaction with water or air. The carbon dioxide cycle is the most important for weathering. CO2 is put into the atmosphere mostly by volcanoes, and it is taken out of the atmosphere by photosynthesis, and by one other process. While it is in the air, CO2 can dissolve in water droplets to form dilute carbonic acid.
When rain hits a rock it does so with mechanical energy and dilute acid. The acid dissolves many types of minerals and rocks though, of course, very slowly. When a mineral like feldspar is dissolved, it lets sodium ions into the sea; chlorine ions come from other minerals. The sea tastes salty because of the elements which have been dissolved out of rocks.
Biological weathering happens when animals mechanically burrow or when plants and trees extend their roots into rock strata breaking it up.
Due to weathering, the particles are loosened up and under the effect of gravity, they are pulled downward. This movement under the influence of gravity is called mass wasting or mass movement.
The mass movement of soil and rock materials gets saturated with water and flows downward with water across a gentle slope, it is called earth flow. Similar flow occurring downwards a steep slope is called mud-flow. Dry soil and rock pieces suddenly flowing downhill across a steep slope is called a landslide.
Weathering is important for soil formation. The soil is a combination of fine rock particles and organic materials called humus. Humus is derived from remains of plants and animals. Soil formation is a slow process taking hundreds of years. Soil formation happens in layers and these layers can be seen distinctly if we dig a pit. The arrangement of horizontal layers of soil is called soil profile.
The topmost layer of soil is called topsoil and contains clay, silt, sand and humus. Most of the plants extend their roots in this layer. The layer below it is called subsoil. It contains coarse clay, sand and minerals. Partially weathered rocks and bedrock are seen below it.
As a consequence of weathering, the rock strata gets disintegrated and transported downwards to get deposited at lower places. Running water, glaciers, winds and sea waves causes this movement and are called agents of gradation. They erode, transport and deposit the earth material along their course of movement. In this process, they continuously change the landscapes and creates new land forms.
Running water in the form of rivulets, streams and rivers is the most important agent of gradation. While waterfalls on the earth’s surface some of them will get soaked into the earth and the remaining flows on the surface of the earth as run-off. Run-off depends on the slope of the land, amount of rainfall and extent of vegetation in the area. The water washes away the topsoil and reduces the fertility. This is called as soil erosion.
During heavy rainfall entire layer of soil is washed off from a large area without plant cover. Such erosion is called sheet erosion. Gully erosion is when rainwater scoops out the soil creating narrow deep channels called gullies while moving down the slope in uneven terrain.

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Landforms & Plate Tectonics

The surface of the Earth is not even. There are high places called mountains, and high flat places called plateaus. There are low places called valleys and canyons and relatively flat areas called plains. For the most part, moving air and water from the sky and seas damages rocks in high places and breaks them into small pieces. The air and water then move these pieces to lower places.
We have already seen that the solid thin lithosphere rests on plastic asthenosphere. The lithosphere is of two types, Oceanic and Continental. Oceanic lithosphere is associated with oceanic crust and exists in the ocean basins. Oceanic lithosphere is typically about 50–100 km thick. Continental lithosphere is associated with continental crust. Continental lithosphere has a range in thickness from about 40 km to perhaps 200 km, of which about 40 km is the crust.
The lithosphere is divided into tectonic plates, which move gradually relative to one another. Plate tectonics is the theory developed to explain large scale motions of the Earth’s lithosphere. This theory builds on older ideas of continental drift and seafloor spreading. Exactly how this happens is still not understood and the driving forces moving the plates continue to be active subjects of research.
Although solid, the asthenosphere can flow like a liquid on long time scales. Large convection currents in the asthenosphere transfer heat to the surface, where plumes of less dense magma break apart the plates at the spreading centres and cause the movement of plates. The deeper mantle below the asthenosphere is more rigid again due to extremely high pressure.
The lithosphere is divided into eight major and many minor plates. These lithospheric plates ride on the asthenosphere at one of three types of plate boundaries convergent boundaries, divergent boundaries or transform fault boundaries. Tectonic plates can create mountains, earthquakes, volcanoes, mid-oceanic ridges and oceanic trenches, depending on which way the plates are moving.
A convergent boundary is where two or more tectonic plates collide with each other causing massive earth movements. The Himalayas were formed by such a collision. Earthquakes and volcanoes are common near convergent boundaries. This is because of pressure, friction, and plate material melting in the mantle.

There can be two types of subduction in a convergent plate boundary. When Oceanic crust moves under, a deep ocean trench forms at the coast and an arc of mountainous volcanoes form inland as seen along the western edge of the Americas. But, when continental crust moves under, the edge of the continental plate folds into a huge mountain range. Behind it is a high plateau. The Himalayas and the Tibetan plateau are a perfect example of this. Folds can be upfolds(anticlines) or downfolds(synclines).

A divergent boundary, also known as a constructive boundary, is two or more plates that move apart from each other because of plate tectonics. When they move apart either water or magma fills the space. If it is magma, when it has cooled it then creates a new plate which then creates new land.
A fault is a fracture, or break, in the Earth’s crust (lithosphere). Some faults are active. Here, sections of rock move past each other. This sometimes makes earthquakes. Faulting occurs when shear stress on a rock overcomes the forces which hold it together. The fracture itself is called a fault plane.
An earthquake (or quakes, tremors) is shaking caused by sudden movements of rocks in the Earth’s crust. They can be extremely violent. Earthquakes are usually quite brief but may repeat. They are the result of a sudden release of energy in the Earth’s crust. This creates seismic waves, which are waves of energy that travel through the Earth. The study of earthquakes is called seismology. Seismology studies the frequency, type and size of earthquakes over a period of time. The magnitude of an earthquake and the intensity of shaking is usually reported on the Richter scale. On the scale, 3 or less is scarcely noticeable, and magnitude 7 (or more) causes damage over a wide area. Earthquakes are caused by tectonic movements in the Earth’s crust. The main cause is that when tectonic plates collide, one rides over the other, causing orogeny (mountain building), earthquakes and volcanoes.
An earthquake under the ocean can cause a tsunami. This can cause just as much death and destruction as the earthquake itself. Landslides can happen, too. Earthquakes are part of the Earth’s rock cycle. Tsunami or a chain of fast moving waves in the ocean caused by powerful earthquakes is a very serious challenge for people’s safety and for earthquake engineering. Those waves can inundate coastal areas, destroy houses and even swept away whole towns.
A volcano is a mountain where lava (hot, liquid rock) comes from a magma chamber under the ground. Most volcanoes have a volcanic crater at the top. The magma which reaches the earth’s surface is called lava and the path through which it comes out is called a vent. When a volcano is active, materials including lava, steam, gaseous sulphur compounds, ash and broken rock pieces come out of it. When there is enough pressure, the volcano erupts. Some volcanic eruptions blow off the top of the volcano. The magma comes out, sometimes quickly and sometimes slowly. Some eruptions come out at a side instead of the top.
The lava and pyroclastic material (clouds of ash, lava fragments and vapour) that comes out from volcanoes can make many different kinds of land shapes. There are different types of volcanoes based on the type of lava and manner in which it flows.OR Volcanoes are classified on the basis of nature of eruption and the form developed at the surface.
Shield volcanoes are the largest of all the volcanoes on the earth. They built out of layers of lava from continual eruptions (without explosions). These volcanoes are formed by fluid low-silica mafic lava. Because the lava is so fluid, it spreads out, often over a wide area. Shield volcanoes do not grow to a great height, and the layers of lava spread out to give the volcano gently sloping sides. Shield volcanoes can produce huge areas of basalt, which is usually what lava is when cooled. The base of the volcano increases in size over successive eruptions, where solidified lava spreads out and accumulates. Some of the world’s largest volcanoes are shield volcanoes.

A cinder cone volcano is a tall, conical volcano. Unlike shield volcanoes, cinder-cone volcanoes have a steep profile. The lava that flows from stratovolcanoes cools and hardens before spreading far as it has high viscosity. The magma forming this lava is often felsic, with high-to-intermediate levels of silica, and less mafic magma. Big felsic lava flows are uncommon but have travelled as far as 15 km.
A series of successive eruptions of different types of lava gives rise to composite volcanoes. These volcanoes are characterised by eruptions of cooler and more viscous lavas than basalt. There will be alternate layers of lava, cinder and ash.
Volcanoes are commonly seen at convergent plate boundaries. When two plates meet, one of them (usually the oceanic plate) goes under the continental plate by the process of subduction. Afterwards, it melts and makes magma (inside the magma chamber), and the pressure builds up until the magma bursts through the Earth’s crust.
The second way is when a tectonic plate moves over a hotspot in the Earth’s crust. The hot spot works its way through the crust until it breaks through.
As volcanoes and earthquakes are results of tectonic movements, they are seen concentrated around plates and there are areas where they are very common like ‘Pacific ring of fire’ around the Pacific Ocean.
Orogeny is the process of mountain-building. It takes place when two tectonic plates come together. Mountains develop while a continental plate is crumpled and thickened and involve a great range of geological processes. This is called folding. Mountains can also be formed when faults occur. A fractured area of crust may dip down leading to the elevated area forming the mountain. Volcanism can also lead to mountain formation by deposition of magma.

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The Structure of the Earth & Lithosphere

The Earth is the largest of the rocky planets moving around the Sun by mass and by size. The Earth’s shape is a spheroid: not quite a sphere because it is slightly squashed on the top and bottom. The shape is called an oblate spheroid. As the Earth spins around itself, the centrifugal force forces the equator out a little and pulls the poles in a little making it a unique type of spheroid. As this shape is unique it is also called ‘geoid’ or Earth-like.
The deepest hole ever dug is only about 12.3 kilometres or 7.6 miles. Still, we know something about the inside of the Earth, because we can learn things from earthquakes and the volcano eruptions. We are able to see how quickly the shock waves move through the Earth in different places and determine the type of materials making up that area.
The interior of the Earth is divided into layers. These layers are both physically and chemically different. The Earth has an outer solid crust, a highly viscous mantle, a liquid outer core, and a solid inner core.
The boundaries between these layers were discovered by seismographs which showed the way vibrations bounced off the layers during earthquakes. Between the Earth’s crust and the mantle is a boundary called the moho. It was the first discovery of a major change in the Earth’s structure as one goes deeper.
The crust is the outermost layer of the Earth. It is made of solid rocks. It is mostly made of the lighter elements, silicon, oxygen, aluminium. Because of this, it is known as sial (silicon = Si; aluminium = Al) or felsic. The thickness of the crust varies under the oceanic and the continental areas. Oceanic crust is thinner as compared to the continental crust. The mean thickness of oceanic crust is 5 km whereas that of the continent is around 30 km. The continental crust is thicker in the areas of major mountain systems. It is as much as 70 km thick in the Himalayan region.
The mantle is the layer of the Earth right below the crust. It is made mostly of oxygen, silicon and the heavier element magnesium. It is known as siam (Si + am for magnesium) or mafic.The mantle extends from Moho’s discontinuity to a depth of 2,900 km. The mantle itself is divided into layers.
The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere(lithos=rock), an irregular layer with a maximum thickness of perhaps 200 km, of which the uppermost mantle is 120 to 50 km thick.
Below the lithosphere, the upper mantle becomes notably more plastic, called the asthenosphere, and is composed of flowing rock in the state of plasticity, about 200 km thick. It behaves as a hot viscous liquid and consists of hot, weak material that can be deformed like silly putty. That means it is capable of gradual flow. The lower mantle is much thicker than the upper mantle. It is made of magma, under great pressure, and so is thicker (higher viscosity) and flows less easily.
The Earth’s core is the part of Earth in the middle of our planet. It has a solid inner core and a liquid outer core.
The outer core of the Earth is a liquid layer about 2,260 kilometres thick. It is made of iron and nickel. This is above the Earth’s solid inner core and below the mantle. Its outer boundary is 2,890 km (1,800 mi) beneath the Earth’s surface. The transition between the inner core and outer core is approximately 5,150 km beneath the Earth’s surface.
The temperature of the outer core ranges from 4400 °C in the outer regions to 6100 °C near the inner core. The outer core is not under enough pressure to be solid, so it is liquid even though it’s mostly made of the same stuff as the inner core. Sulphur and oxygen could also be in the outer core.
The inner core of the Earth, as detected by seismology, is a solid sphere about 1,216 km (760 mi) in radius, or about 70% that of the Moon. It is believed to be an iron–nickel alloy and may have a temperature similar to the Sun’s surface, approximately 5778 K (5505 °C).

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The Realms of the Earth

The earth is unique as it is the only planet that sustains life. This is made possible because of the presence of Land, Water and Air. We classify each of them into separate realms for better understanding and detailed study.
Structurally, the Earth is divided into layers which are both physically and chemically different. The soil which we stand on is the solid layer with different types of rocks and other minerals. This solid part of the earth is called the lithosphere.
Above this solid earth, there is a blanket of gases/ air covering the earth. This blanket contains various gases in differing proportions and is responsible for many of the climatic phenomenon on earth. This layer is called Atmosphere.
As we all know, 3/4th of the earth’s surface is covered by water in the form of oceans, seas, lakes etc. Apart from that water is present under the soil in the form of ground water and as water vapour and clouds in the atmosphere. The whole of this water content of earth is called Hydrosphere.
Life is possible on earth due to the confluence of water, soil and air. The area of earth where all these factors meet and life is present is called Biosphere or Sphere of Life.

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Seasons on The Earth

We have seen in earlier chapter that the sun heats up more when overhead. Similarly, the area of earth where the sun is overhead during the season, gets heated up more than other areas. Because of the axial tilt of the earth, there is an apparent movement of orbital plane in relation to the earth. We feel that the sun migrates from northern hemisphere to southern hemisphere and back in the period of a year. The midday sun is exactly overhead at all latitudes between tropics of Capricorn and cancer, at least once in a year. This area gets maximum heat and is called Torrid Zone.

The sun is never overhead in any place on earth beyond the Tropic of Cancer and Capricorn and the angle of sun’s rays goes on decreasing towards the poles. At areas beyond Arctic Circle and Antarctic Circle, the sun appears just above the horizon and these areas are very cold due to this. These areas are called Frigid Zones. The areas between Torrid Zone and Frigid Zone have a moderate temperature and are called Temperate Zones.

Because of the constant inclination of the earth’s axis in one direction, the Northern hemisphere faces the sun for about half of the year and faces away from the sun for next half of the year.  The part which faces the sun gets more heat and light compared to the other part.  Every point in this hemisphere remains in the sunlight for a longer period of time and hence we say that the days are longer in this hemisphere.

While northern hemisphere faces the sun, southern hemisphere will be facing away and will get less heat and light and will have shorter days and longer nights. This reverses itself in every 6 months.  The hemisphere which faces the sun will be warm and is said to have summer season and the hemisphere which faces away from the sun will be cold and is said to be having winter.  Hence you can say that summer in Northern hemisphere coincides with winter in the Southern hemisphere and vice versa.

There is a season of transition between the two when it is neither too warm nor too hot when the sun is almost around the equator. This season is called autumn or spring.

Spring is the season after winter and before summer. Days become longer and the weather gets warmer in the temperate zone because the Earth tilts towards the Sun. In many parts of the world, plants grow and flowers bloom.

Autumn is the season after summer and before winter. In the United States, this season is also called fall. In many places in the temperate zone, autumn is a time for harvesting most crops. Deciduous trees (trees that lose their leaves every year) lose their leaves, usually after turning yellow, red, or brown.

When it is autumn in the Northern Hemisphere, it is spring in the Southern Hemisphere and vice versa. On the Equator, autumn is very much like spring, with little difference in temperature or in weather.

When the sun reaches the northern-most point it will be overhead at the tropic of cancer, and when it reaches the southern most point it will be overhead the tropic of Capricorn.  The day when the sun is overhead of Tropic of Cancer is called Summer Solstice(Northern Solstice) and the day it is overhead at the tropic of Capricorn it is called Winter Solstice(Southern Solstice ). The day of the solstice is either the “longest day of the year” or the “shortest day of the year” for any place on Earth depending on the hemisphere facing the sun and location of the place.

The Northern Solstice is also called June Solstice as it is usually on June 21. In the Northern Hemisphere the June solstice is called the Summer Solstice (and marks the longest day of the year), while in the Southern Hemisphere it is called the Winter Solstice (and marks the shortest day of the year).

The southern solstice is also called December Solstice and is usually on December 21. At the moment of the December solstice, the Sun is directly overhead some point on the Tropic of Capricorn; this is the furthest south that the subsolar point ever reaches. In the Southern Hemisphere, the December solstice is called the Summer Solstice (and marks the longest day of the year), while in the Northern Hemisphere it is called the Winter Solstice (and marks the shortest day of the year).

Similarly, the day when the sun is directly overhead the equator is called Equinox. There are two equinoxes each year. On these days, the nights are equal in length at all latitudes North and South. The word equinox comes from two Latin words meaning “equal” and “night”. Around the day of the equinox, the length of the day and the length of the night are almost equal. They occur on or around March 21 and September 21.

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