Atmosphere

The atmosphere is the envelope of gases that surrounds the Earth. It forms a protective boundary between outer space and the biosphere, ensuring the conditions necessary for life to flourish. Though invisible and formless, the atmosphere is a dynamic and vital system. It is odourless, colourless, tasteless, and consists of a complex mix of gases that behave like a single unit due to thorough mixing.

The atmosphere is not a leftover from the Earth’s original formation. Instead, it has evolved over billions of years through natural processes like volcanic eruptions, hot springs, decomposition of rocks, and interactions with the biosphere. The gases that make up our modern atmosphere are a product of continuous change and life processes.

It contains life-sustaining gases such as oxygen for humans and animals, and carbon dioxide for plants. The atmosphere protects the Earth from harmful solar radiation, particularly ultraviolet rays, and plays a critical role in moderating temperature through the greenhouse effect—allowing short-wave solar radiation in, while trapping outgoing long-wave terrestrial radiation.

Without the atmosphere, temperatures on Earth would vary wildly between day and night. The atmosphere also incinerates incoming meteors through friction, shielding the surface from potential damage. Moreover, it selectively filters solar radiation, letting in only those frequencies vital for biophysical and biochemical processes of living organisms.

Composition of the Atmosphere

The atmosphere consists of gases, water vapour, and aerosols (tiny solid and liquid particles suspended in the air).

Major Gases (by volume in dry air):

• Nitrogen (78%): An inert gas that dilutes oxygen and prevents rapid combustion. It plays an essential role in plant life through the nitrogen cycle.
• Oxygen (21%): Essential for respiration and combustion. Also forms oxides and supports all aerobic organisms.
• Argon (0.93%): An inert noble gas with no significant role in weather or life, but used in industry.
• Carbon Dioxide (0.038%): Vital for photosynthesis. Also a potent greenhouse gas that traps heat.
• Ozone (O₃): A form of oxygen found mainly between 20–25 km altitude. Absorbs UV radiation. Unevenly distributed but critical for life.

Water Vapour:

• Varies from 0% to 5% by volume.
• Originates from evaporation from oceans, rivers, soil, and plants.
• Decreases with altitude and from equator to poles.
• More than 90% is concentrated below 5 km altitude.
• Responsible for humidity, cloud formation, and precipitation (rain, snow, hail, etc.).

Particulate Matter (Aerosols):

• Includes dust, salt, smoke, soot, volcanic ash, pollen, microorganisms, and even meteoric debris.
• These particles reflect, absorb, and scatter solar radiation.
• Responsible for colour effects in the sky like red sunsets and blue skies.
• Salt particles act as hygroscopic nuclei, aiding condensation and precipitation.

Structure of the Atmosphere

The atmosphere is divided into five main layers based on temperature variations:

1. Troposphere

• Lowest layer, where all weather occurs.
• Thickness: ~13 km on average (8 km at poles, 18 km at equator).
• Contains dust particles, water vapour, and all atmospheric turbulence.
• Temperature decreases at an average rate of 6.5°C per 1000 metres.
• Bounded above by the tropopause, a boundary layer where temperature stops decreasing (~ -80°C at equator, ~ -45°C at poles).

2. Stratosphere

• Extends from ~13 km to 50 km altitude.
• Contains the ozone layer, which absorbs UV radiation and causes temperature to increase with altitude.
• Stable and ideal for jet aircraft travel.
• Almost cloudless, thus free from weather disturbances.

3. Mesosphere

• Ranges from 50 km to 80 km altitude.
• Coldest layer; temperature decreases with height.
• Meteors burn up in this layer due to friction.
• Bounded above by the mesopause.

4. Thermosphere / Ionosphere

• Ranges from ~80 km to 480 km.
• Contains electrically charged particles or ions that reflect radio waves, aiding long-distance communication.
• Temperature increases with altitude due to absorption of solar radiation.
• Important for auroral phenomena and satellite operations.

5. Exosphere

• Outermost layer; gradually merges into space.
• Extremely thin and mainly composed of hydrogen and helium.
• Satellites orbit in this region.
• Not well-defined and studied due to its distance and rarefied gases.

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Subject: Geography

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