Atmosphere: Composition and Structure
Atmosphere of Earth
The atmosphere is an envelope of gases surrounding the earth, these gases are commonly called air. The air is one of the necessary conditions for the existence of life on the planet. The atmosphere of Earth protects life on Earth.
- The atmosphere is a mixture of gases that is odourless, colourless, tasteless, and formless, blended so thoroughly that it behaves like a single gas. It contains life-giving gases like Oxygen for humans and animals for breathing and life support and Carbon di Oxide for plants.
- Earth’s atmosphere is unique as it supports life.
- The stabilization of the atmosphere in the Cambrian Period (about 600 million years ago).
Composition of Atmosphere
The Earth’s atmosphere is made up of different gases, which are not static, as their composition changes with time and place.
- There are three major constituents of Earth’s atmosphere nitrogen 78.084 %, 20.94% oxygen, and 0.934 % argon.
- Water vapour is 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.
- Other noble gases, neon, helium, krypton, and xenon are also present in the Atmosphere.
The composition of the Atmosphere is given in the table below:
Structure of Atmosphere:
Earth’s atmosphere can be divided (called atmospheric stratification) into five main layers. Excluding the exosphere, the atmosphere has four primary layers, which are the troposphere, stratosphere, mesosphere, and thermosphere.
- Troposphere: 0 to 12 km (0 to 7 miles)
- Stratosphere: 12 to 50 km (7 to 31 miles)
- Mesosphere: 50 to 80 km (31 to 50 miles)
- Thermosphere: 80 to 700 km (50 to 440 miles)
- Exosphere: 700 to 10,000 km (440 to 6,200 miles)
The troposphere is the lowest layer, and closest to the Earth’s surface. Some important facts about the troposphere are as follows:
- It extends up to the tropopause defined by a temp. -57 °C.
- The temperature decreases at the rate of 6.4° C per 1000 meters in the troposphere, the decrease in temperature is termed as normal lapse rate.
- The height of tropopause varies with season, latitude, surface temperature, and pressure.
- the lowest part of the troposphere (i.e. Earth’s surface) is typically the warmest section of the troposphere
- It extends from Earth’s surface to an average height of about 12 km, varies at an altitude of 19km at the equator, 13km in the middle latitude, and about 8 km near the poles.
- the troposphere contains approximately 90% of the total mass of the atmosphere and the bulk of all water vapour, clouds, weather, and air pollution.
- Nearly all atmospheric water vapour or moisture is found in the troposphere, so in this layer most of Earth’s weather takes place.
- basically, it contains all the weather-associated cloud genus types generated by active wind circulation.
- In the troposphere most conventional aviation activity takes place, and it is the only layer that can be accessed by propeller-driven aircraft.
The stratosphere lies above the troposphere. Important facts about the stratosphere are discussed below:
- it is the second-lowest layer of Earth’s atmosphere
- The stratosphere is separated from the troposphere by the tropopause. This layer extends from the top of the troposphere at roughly 12 km above Earth’s surface to the stratopause at an altitude of about 50 to 55 km.
- The stratosphere defines a layer in which temperatures rise with increasing altitude, because of the absorption of UV radiation from the sun by the Ozone layer.
- The stratosphere is characterized by the presence of the Ozonosphere, also called the Ozone Layer. Ozone is made up of 3 oxygen atoms (O3).
- Ozone observes ultraviolet radiation (UV) radiation from the Sun.
- Most harmful Ultraviolet radiation filtered which are coming from solar radiation.
- By filtering Solar radiation, it protects the earth’s surface.
- The stratosphere is almost completely free of clouds and other forms of weather.
- sometimes polar stratospheric or nacreous clouds are seen in the lower part of this layer of the atmosphere where the air is coldest
- The stratosphere is the highest layer that can be accessed by jet-powered aircraft.
The mesosphere is the area from 50 to 80 km, it is the third highest layer of Earth’s atmosphere. Some important facts about the mesosphere are given below:
- Found above the stratosphere and below the thermosphere
- It extends from the stratopause at an altitude of about 50 km (160,000 ft) to the mesopause at 80–85 km (260,000–280,000 ft) above sea level.
- The mesopause is the point of minimum temperature at the boundary between the mesosphere and the Thermosphere atmospheric regions.
- Meteors burn up in this layer.
The thermosphere is the second-highest layer of Earth’s atmosphere. Important facts about the thermosphere are as follows:
- The layer extends from the mesopause (which separates it from the mesosphere) at an altitude of about 80 km (260,000 ft) up to the thermopause at an altitude range of 500–1000 km (1,600,000–3,300,000 ft).
- 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 ranges from 80 to 550 kilometres above the Earth’s surface and contains the ionosphere.
- The temperature of the thermosphere gradually increases with the increase in height
- The layer is completely cloudless and free of water vapour
- In the thermosphere, non-hydrometeorological phenomena such as the aurora borealis and aurora australis are occasionally seen.
- in this layer the satellites orbiting the earth are present.
The exosphere is the outermost layer of the Earth’s atmosphere (Upper limit of atmosphere). Some important facts about Exosphere are as follows:
- The layer 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 (33,000,000 ft) where it merges into the solar wind.
- The exosphere 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 gravitational force is very weak.
- The exosphere no longer behaves like a gas, and the particles constantly escape into space, where the earth’s atmosphere meets outer space.
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