Comprehensive Guide to Earth’s Atmospheric Composition – Geography Notes

Atmosphere of Earth

Atmosphere

• The atmosphere is a thin layer of gasses that surrounds the Earth and is held in place by the force of gravity. 
• It is an integral part of the Earth’s system, playing a crucial role in regulating the climate and supporting life.
• Atmospheric pressure is the force per unit area exerted by the weight of the atmosphere above that point.
•  It is caused by the weight of the air molecules pressing down on the Earth’s surface.
• Atmospheric pressure decreases with altitude, as there is less air above to exert pressure.
• At sea level, the standard atmospheric pressure is around 1013 millibars, which is equivalent to 1.01325 kilograms per square centimeter, or 14.7 pounds per square inch. The pressure can vary due to changes in temperature, humidity, and weather conditions.
• Understanding atmospheric pressure is important for many applications, such as aviation, weather forecasting, and scuba diving. For example, changes in atmospheric pressure can affect airplane performance, and knowing the pressure gradient can help pilots adjust their flight path. 
• In weather forecasting, changes in atmospheric pressure can indicate the movement of high and low pressure systems, which can affect the weather.
• Atmospheric pressure plays a crucial role in regulating the Earth’s climate, as it affects the movement of air and the distribution of heat.

Role of Earth’s Atmosphere

• The atmosphere is composed of a mixture of gasses, including oxygen, nitrogen, carbon dioxide, and other trace gasses. 
• These gasses are essential for the survival of living organisms, as they provide the necessary elements for respiration and photosynthesis.
• The atmosphere also plays a critical role in regulating the Earth’s temperature and protecting it from harmful solar radiation. 
• The greenhouse gasses in the atmosphere, such as carbon dioxide and water vapor, trap heat and keep the planet warm enough for life to exist. Without the atmosphere, the temperature on Earth would fluctuate wildly between day and night, making life impossible.
• The atmosphere also protects the Earth from harmful ultraviolet radiation from the sun. The ozone layer in the stratosphere absorbs much of the ultraviolet radiation, preventing it from reaching the Earth’s surface and causing damage to living organisms.
• The atmosphere is also responsible for creating weather patterns, which have a significant impact on many natural and man-made processes. Understanding weather patterns is critical for agriculture, transportation, and many other fields.
• The atmosphere is a critical component of the Earth’s system, regulating the climate, protecting against harmful radiation, and supporting life on our planet.

Composition of Atmosphere

• The atmosphere is not only composed of gasses, but also contains solid and liquid particles called aerosols. 
• The composition of the atmosphere can vary depending on location and time, but there are some permanent components, such as nitrogen and oxygen, which make up the vast majority of the atmosphere.
• The proportion of gasses changes with altitude, with oxygen becoming increasingly negligible at higher altitudes. Carbon dioxide and water vapor are found only up to certain altitudes as well.
• The remaining gasses in the atmosphere are mostly inert, meaning they do not react readily with other substances. These gasses include argon, neon, helium, krypton, and xenon. Although they are present in relatively small quantities, they still play important roles in atmospheric processes.
• Water vapor is also an important component of the atmosphere. It is responsible for the formation of clouds and precipitation, which are critical for sustaining life on Earth. Dust particles and other aerosols can also affect weather patterns and climate.
• The composition of the atmosphere is complex and plays a crucial role in regulating the Earth’s climate, supporting life, and protecting against harmful radiation.

Different constituents of the atmosphere, with their individual characteristics, are discussed below:

1. Oxygen: Constitutes 21% of the total volume of the atmosphere. It is essential for respiration and combustion.
2. Nitrogen: Accounts for 78% of the total atmospheric volume. It is relatively inert and an important constituent of all organic compounds. It helps control combustion and indirectly helps in oxidation.
3. Carbon dioxide: Constitutes only about 0.03% of the dry air and is a product of combustion. It is absorbed by green plants through photosynthesis and plays a crucial role in the heat energy budget. Increased burning of fossil fuels has led to an alarming increase in the percentage of carbon dioxide in the atmosphere, which could induce drastic climatic changes.
4. Ozone: Formed at higher altitudes and transported downwards, it plays a crucial role in blocking the harmful ultraviolet radiation from the sun.
5. Water vapor: One of the most variable gaseous substances present in the atmosphere, constituting between 0.2% and 4% of the total volume. It plays a significant role in the insulating action of the atmosphere, absorbing long-wave terrestrial radiation and a part of the incoming solar radiation. It is also the source of precipitation and clouds.
6. Solid particles: Consist of sand particles, pollen grains, small organisms, soot, ocean salts, and fragments of meteors. They perform the function of absorbing, reflecting, and scattering the radiation and are responsible for the orange and red colors at sunset and sunrise, the blue color of the sky, and the formation of clouds, fog, and hailstones.
7. Greenhouse gasses: Carbon dioxide, ozone, and water vapor are the major greenhouse gasses that absorb terrestrial radiation and reflect back some of it towards the earth’s surface, thus largely responsible for the greenhouse effect. Methane, produced from the decomposition of animal wastes and biological matter, is another important greenhouse gas.

Structure of Atmosphere

There are five layers in the structure of the atmosphere depending upon temperature. These layers are:

1. Troposphere
2. Stratosphere
3. Ozonosphere
4. Mesosphere
5. Thermosphere
6. Exosphere

Troposphere

• It is the atmospheric layer between the earth’s surface and an altitude of 8 km at the poles and 18 km at the equator.
• The main reason for higher height at the equator is due to the presence of hot convection currents that push the gasses upward.
• The troposphere is thickest at the equator because warm air rises due to convection and creates a higher layer of atmosphere. The tropopause is the upper boundary of the troposphere and marks the transition zone to the stratosphere. Above the tropopause, the temperature remains constant or increases with height.
• The temperature in this layer, as one goes upwards, falls at the rate of 5°C per kilometer, and reaches -45°C at the poles and -80°C over the equator at Tropopause (greater fall in temperature above the equator is because of the greater thickness of troposphere – 18 km).
• The troposphere plays a crucial role in the Earth’s weather and climate systems and is affected by a number of factors, including the changing seasons and the position and strength of jet streams. 
• The jet streams, for example, are fast-flowing, narrow air currents in the upper troposphere that can influence weather patterns by steering storms and other weather systems. The position and strength of these jet streams can change seasonally and from year to year, affecting weather patterns and regional climates.
• Temperature decreases with increasing height of the atmosphere at the rate of 1 degree Celsius for every 165 m of height, This is called the Normal lapse rate.

Tropopause

1. The topmost layer of the troposphere.
2. It acts as a boundary between the troposphere and the stratosphere 
3. This layer is marked by constant temperatures.

Stratosphere

• The stratosphere is the layer of the Earth’s atmosphere that lies beyond the troposphere, extending up to an altitude of about 50 km. 
• Unlike the troposphere, the temperature in the stratosphere initially remains constant but then rises to reach a level of 0°C at an altitude of 50 km. 
• This rise in temperature is due to the presence of a layer of ozone that absorbs harmful ultraviolet radiation from the sun.
• Because the stratosphere is almost free from clouds and associated weather phenomena, it is considered an ideal layer for flying airplanes. 
• Airplanes can fly at higher altitudes in the lower stratosphere or sometimes in the upper troposphere, where weather conditions are calm.
• However, sometimes cirrus clouds can be present at lower levels in the stratosphere.

Ozonosphere

• It lies at an altitude between 30 km and 60 km from the earth’s surface and spans the stratosphere and lower mesosphere.
• Because of the presence of ozone molecules, this layer reflects the harmful ultraviolet radiation.
• The ozonosphere is also called chemosphere because a lot of chemical activity goes on here.
• The temperature rises at a rate of 5°C per kilometer through the ozone layer.

Mesosphere

• The mesosphere is indeed the layer above the stratosphere and is the coldest of all the atmospheric layers.
• It is characterized by a temperature drop with increasing altitude and can reach temperatures as low as -100 degrees Celsius at its upper limit, the mesopause. 
• The mesosphere extends from about 50 km above the Earth’s surface to 80 km. 
• Meteors that enter the Earth’s atmosphere burn up in this layer, producing the streaks of light that we see as shooting stars.
• The temperature gradually falls to -100°C at 80 km altitude.

Thermosphere

• The thermosphere is the layer above the mesosphere and is characterized by a rapid increase in temperature with height due to absorption of high-energy solar radiation. 
• The ionosphere, which is a part of the thermosphere, extends from 80-400 km and is responsible for the reflection and propagation of radio waves. 
• The extremely low pressure in this layer means that a person or object would not feel warm despite the high temperature. 
• The International Space Station and satellites orbit in this layer due to its low air resistance.
• Aurora’s or the Northern and Southern lights are also observed in the lower parts of the thermosphere due to interaction between the Earth’s magnetic field and solar wind particles.

Ionosphere

• The lower thermosphere is also known as the ionosphere because it is the layer of the Earth’s atmosphere where the concentration of ions is high due to the ionization of gas molecules by cosmic and solar radiation. 
• The ionosphere extends from about 80 km to 400 km above the Earth’s surface and is an important region for radio communications and satellite navigation because of its ability to reflect and refract radio waves. 
• Additionally, the ionosphere plays an important role in the Earth’s climate system and is affected by changes in solar activity and space weather.
• Temperature again starts increasing with height because of radiation from the sun.

Exosphere

• The Exosphere, which is the outermost layer of the Earth’s atmosphere. 
• It begins at the top of the thermosphere, around 500 km above the Earth’s surface, and gradually fades into the vacuum of space. 
• The exosphere is characterized by an extremely low density of gas molecules and a very high temperature due to the absorption of intense solar radiation. 
• The temperature in the exosphere can reach up to thousands of degrees Celsius, despite the lack of molecular collisions that would normally cause a gas to heat up. 
• As a result of its low density, gasses such as helium and hydrogen can escape into space from the exosphere. The exosphere is considered to be part of outer space and marks the boundary between the Earth’s atmosphere and interplanetary space.

Speed of sound follows the temperature profile

• This is because speed of sound is directly proportional to temperature as we move away from earth.

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