External structure of the earth

The word atmosphere is the combination of two Greek words; 'atmos' meaning "vapour" and 'sphaira' meaning "sphere" it is a layer of a gases surrounding a planet Earth or other material body of sufficient mass that is held in place by the gravity of the body. The earth's atmosphere is comprised of mixture of gasses, suspended solids and liquids that surround the Earth. Thus the atmosphere is the blanket of air that envelops the Earth. It is the zone of mixed odorless gas or air. The earth's atmosphere is about 600 to 1000 kilometres thick.

The atmosphere is constituted of both living and non-living components. The living organisms include bacteria in which some are harmful to human life. Non-living components include gaseous and non-gaseous components such as dusts and particles. The atmosphere is composed of various gasses as follows:

1. Nitrogen gas: 78% dilutes oxygen and prevents rapid burning at the earth's surface. Living things need it to make proteins.
2. Oxygen gas: 21% used by all living things, essential for respiration. It is necessary for combustion or burning.
3. Carbon dioxide: 0.03% plants use it to make oxygen. Acts as a blanket and prevents the escape of heat into outer space.
4. Argon: 0.93% used in light bulbs.
5. Water vapor: 0.20 to 4.0% essential for life processes also prevents heat loss from the Earth.
6. Trace gases: found only in a very small amounts, including neon, helium, krypton and xenon.

The usefulness of atmosphere is highly determined by its composition. For instance, water vapour facilitates weather processes, dusts and soot particles act as condensational nuclei or hydroscopic nuclei, while gases support life among others.

The atmosphere is divided into layers based on its temperature. These layers are the troposphere, the stratosphere, the mesosphere and the thermosphere. The uppermost region, beginning about 500 km above the Earth's surface, is called the exosphere.

1. Troposphere The troposphere is the bottom and, immediate layer from the earth's surface. It starts at the earth's surface and extends to an average of 12 kilometers high being 8 kilometres at the poles and 17 kilometres at the equator. The temperature in the troposphere decreases with increase in altitudes by 0.6°C for every 100 metres or 6.5°C for every 1 kilometer (environmental lapse rate). Pressure falls with altitude due to decrease of gravity. The air pressure at the top of the troposphere is only 10% of that at the sea level (0.1 atmospheres). The troposphere is where clouds, precipitation and violent storm form. Here wind speed increases with height. About 99% of all water vapour is concentrated in this layer. It is by far the wettest layer of the atmosphere. Also, it contains three-quarters of the mass of the entire atmosphere. The troposphere is where all weather takes place; it is the region of rising and falling pockets of air. There is a thin buffer zone between the troposphere and the next layer called the tropopause. The tropopause lies above the troposphere. It extends from 12 km to 20 km. Here temperature is constant. The layer is unstable and contains most of the atmospheres water vapour, gasses, clouds dust and pollutants. Troposphere plays greater role on life including formation of precipitation, providing oxygen and hydrogen and carbon dioxide which is essential for photosynthesis. It provides water vapour which regulates temperature by preventing the outgoing radiation from thesun.
2. Stratosphere This is the layer above the tropopause. It extends to about 50 It is characterized by steady increase in temperature caused by concentration of ozone (O3) which absorbs ultraviolet radiation from the Sun. It acts as a protective shield against deteriorates which usually burn out as they enter into gravitational field. Winds are low in the lower troposphere but increase with height. Air is dry, and pressure continues to fall with altitude. There is a recent concern that man-made fluorocarbon is depleting the ozone layer. This has potentially a devastating consequence to life on Earth. The boundary separating stratosphere with the next layer is called the stratopause. It lies above the stratosphere and extends from 49 km to 52 km. The stratosphere contains 90% of ozone layer primarily between the top of the troposphere and about 50 km (31 miles) altitude. It is approximated that about 16–50 km (10–31 miles) above earth's surface, ozone forms a thin invisible shield protecting life from the sun's ultraviolet (UV) radiation. Ozone is a molecule that contains three oxygen atoms. At any given time, ozone molecules are constantly formed and destroyed in the stratosphere. The total amount has remained relatively stable during the decades that it has been measured. Ozone layer absorbs the harmful ultraviolet radiation which is emitted from the sun. Without this layer the life wouldn't be possible since it filters harmful radiation from the sun that can destroy animal tissues. The stratosphere is another layer where the temperature increases with the increase in height. The is a convenient layer for flying of stratosphere plane to avoid turbulence which is common in the troposphere.
3. Mesosphere The mesosphere is a layer above the stratopause. It extends from 52km to 80 km above the surface. In this layer temperature decreases rapidly with increasing height in the absence of water vapor, dusts and clouds to absorb incoming radiation. This is the region with lowest temperature which drops to -90°C and the strongest wind with the speed of 3000 km/hr. The wind is estimated using temperature dates which is usually collected by satellites. Also, this layer contains fairly high concentrations of ion and metal atoms. The shooting stars you observe in the night sky are meteorites that evaporate as it burns up in the mesosphere. Gases here are mixed, not layered by masses, and air is so thin that you cannot breathe. Mesosphere has various importance to human life. Mesosphere can be seen as a protective layer of the earth's surface because meteorites of various sizes are burn up in this layer. It is estimated that meteorites with 20 meters in size, are burnt every year. For this case, the mesosphere is important layer in protecting us from the falling meteor shower, with hundreds of rocks with 20 meters in size that would have reached on earth's surface at supersonic speed. Life could not be possible on earth Also, the mesosphere protects us from solar radiation with varying wavelengths. At the end of this layer is the mesopause. Mesopause lies above mesosphere. This layer shows no change of temperature and is characterized by lowest temperature.
4. Thermosphere Thermosphere broadly corresponds with the ionosphere. This layer lies above mesopause extending from 80 km into space. In this layer temperature ranges between 1000°C and 1500°C. However, it cannot be felt because air molecules are far apart. Also, it is due to presence of atomic ions (oxygen) which absorb a very short wave solar energy by of oxygen and nitrogen. In this absorption zone the air is extremely thin with almost zero amount of air density and almost without existence of gravity. The little air present in the thermosphere mainly consists of helium, atomic nitrogen, and atomic oxygen. For this reason it is not possible to breath in the thermosphere. The thermosphere contributes to the protection of the Earth as it absorbs x-rays and extreme ultraviolet radiation. Also it enables space exploration and easy communication. The charged particles in the thermosphere make it easy for long distance communication through radio, by which radio signs travel in straight line and bounces forth between the earth's surface and the ionosphere. It absorbs the sun's energy and creates moderate temperature.
5. Exosphere This is the highest and top layer of the earth's atmosphere. It starts at 500 kilometers high and marks the edge of space. There are very few molecules layer mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide. This layer is the earth's first line of defense against the harmful sun rays. Also, it is the first layer to come into contact and protect the Earth from meteors, asteroids and cosmic rays. Figure 1.9 (a) and (b) show layers of the earth's atmosphere and its vertical structure.

The atmosphere is shared by every person living on earth. Without the atmosphere we would find it very hard to survive. Humans can last weeks without food, days without water, but only minutes without air. Our daily lives are influenced by the atmosphere. Also, we are protected by the atmosphere. It acts as a huge blanket, keeping the earth warmer than it would be without the atmosphere. This is known as the greenhouse effect, without the atmosphere, the average temperature on Earth would be below freezing. The atmosphere protects living things on Earth from the sun's harmful ultraviolet radiation. A thin layer of gas called ozone high up in the atmosphere at stratosphere filters out these dangerous rays. Also, the atmosphere helps to sustain life on Earth. It provides oxygen for humans and animals to breath, and carbon dioxide for plants. Through the hydrological cycle, plants and animals receive the water needed to survive. Another importance of the atmosphere is to protect us from objects coming towards the Earth from outer space. Everyday meteoroids enter the earth's atmosphere. These are pieces of rock that are travelling through space. When they reach the atmosphere, most of them burn up due to the friction of the air. The ones that don't completely burn up reach the surface of the earth in a much smaller form. When they reach the Earth we call them meteorites. The atmosphere protects us from space junk that would do damage to us, to our homes, and to the Earth itself. It is because of the heat created by the friction of the atmosphere that the space shuttle had to be equipped with a heat shield to prevent it from burning up as it re-entered the earth's atmosphere. The heat caused by friction is dangerous to the astronauts but it is very important to protect us from things coming in from outer space.

Hydrosphere is the area that includes all earth's water in the state of liquid, frozen or floating ice in the upper part or beneath the soil. Most (97%) of water is found in oceans and seas; freshwater accounts for 3% only of the hydrosphere and most of this is found in glaciers, permanent snow covers at the poles, lakes, rivers, groundwater, and in mountainous regions. The rest is in lakes, rivers, and ground water. Hydrosphere is referred to as a major setting of the earth's hydrological cycle. Freshwater amounts are essentially held at a constant level by the hydrologic cycle that is driven by the sun. The hydrological cycle continuously moves water around the planet by exchanging water molecules from the vegetation and oceans to the atmosphere and back around. This movement of water is through the process of evapotranspiration, condensation, precipitation, infiltration, runoff, and subsurface flow of which are the wheels in the hydrologic cycle. When water evaporates, the surroundings are cooled; as it condenses, water releases energy and warms its surroundings. Water is responsible for the formation of landforms through erosion deposition, and the movement of minerals. It further hydrates life on the planet and transfers the energy from terrestrial to aquatic systems. Generally, without these cycles, and water itself, life would cease to exist on the planet Earth.

Biosphere refers to the narrow zone of the Earth in which all life forms exist. Life becomes possible in this zone due to presence of land, air and water which support all forms of life. Biosphere extends vertically into the atmosphere to about 10 kilometers, downward into the ocean to a depth of about 10.4 kilometers. There are three basic components of the biosphere namely; abiotic, biotic and energy. The abiotic component consists of all non living elements which are essential for survival of all living organisms. These elements are lithosphere, atmosphere and hydrosphere which in turn provide mineral nutrients, water and certain gases. The biotic component is composed of plants and animals include human beings, plants, micro-organisms and macro-organisms. Also, energy is another component which is essential for generation and reproduction of all biological lives of this planet. The biosphere is characterized by the continuous cycling of matter and an accompanying flow of solar energy through which certain large molecules and cells are self-reproducing.

In some places on the Earth water exists in a form of solid ice or snow. These frozen places of the Earth are called 'cryosphere.' The word 'cryosphere' comes from the Greek word, 'kryos' which means 'cold.' The cold regions of our planet influence our entire world's climate. The cryosphere is central to the daily lives of people, plants, and animals. Also, it has a great impact on the global cooling and distribution of cold winds. According to scientists the cryosphere, are places where water is in solid form, where very low temperature freezes water and turns it into ice. Cryosphere covers most of the top and bottom of our planet, in the polar regions. These are areas around the North Pole, the Arctic and the area around the South Pole, the Antarctic. Moreover, it covers many other locations on Earth far away from the cold poles including high elevations. For example, the snow on top of Mount Kilimanjaro found in Tanzania. Frozen soil can be found in the high mountains of the United States, as well as in the northern reaches of Canada, China, and Russia. The cryosphere expands during the cold winter months. Seasonal areas of the cryosphere include places where snow falls, and where soil, rivers, and lakes freeze.