Earth Atmosphere: Composition, Extent and Structure

Introduction

• Earth’s Atmosphere is the gaseous envelope surrounding the earth. It is held around the earth by gravitational force and plays a vital role in maintaining life. The atmosphere supplies essential gases, regulates temperature, protects living organisms from harmful solar radiation, and controls weather and climate.
• The study of the atmosphere is very important in Agrometeorology because atmospheric conditions directly influence crop growth, rainfall, temperature, humidity, and agricultural productivity.

Definition of Atmosphere “Atmosphere is the blanket of gases surrounding the earth and extending outward from the earth’s surface.”

Composition of Atmosphere

The atmosphere is composed of various gases, water vapour, and dust particles. The composition of dry air remains nearly constant up to a certain height.

Major Gases of Atmosphere

Importance of Atmospheric Gases

The gases present in Earth’s Atmosphere are essential for maintaining life, regulating climate, and supporting agricultural activities. Each atmospheric gas performs specific functions important for plants, animals, and environmental balance.

1. Nitrogen (N₂): Nitrogen is the most abundant gas in the atmosphere, constituting about 78% of air. Although plants cannot directly use atmospheric nitrogen, it becomes available to plants through nitrogen fixation by microorganisms and lightning. Nitrogen is an essential component of proteins, enzymes, and chlorophyll, making it vital for plant growth and crop production. It also helps maintain atmospheric stability by diluting oxygen and reducing rapid combustion.

2. Oxygen (O₂): Oxygen forms about 21% of the atmosphere and is essential for respiration in humans, animals, plants, and microorganisms. It releases energy from food required for life processes. Oxygen also supports combustion, which is important in natural and industrial activities. Adequate oxygen in soil air is necessary for healthy root growth and microbial activity.

3. Carbon Dioxide (CO₂): Carbon dioxide is present in small quantities but has great importance in agriculture and climate. It is the basic raw material for photosynthesis through which green plants prepare food. CO₂ also acts as a greenhouse gas that traps heat and helps maintain earth’s temperature suitable for life. However, excessive CO₂ contributes to global warming and climate change.

4. Ozone (O₃): Ozone is mainly concentrated in the stratosphere, forming the ozone layer. It absorbs harmful ultraviolet (UV) radiation from the sun and protects living organisms from skin diseases, genetic damage, and harmful effects on crops and ecosystems. Thus, ozone plays a vital role in protecting life on earth.

Structure of Atmosphere

Earth’s Atmosphere is divided into different layers on the basis of temperature variation with height. Each layer has distinct characteristics and importance in weather, climate, and life processes.

Layers of Atmosphere

1. Troposphere

• The troposphere is the lowest and most important layer of the atmosphere where all major weather phenomena occur. It extends up to about 8 km over the poles and nearly 18 km over the equator. This layer contains most of the atmospheric mass, water vapour, and dust particles.
• Cloud formation, rainfall, storms, winds, and other weather activities mainly occur in this layer. Temperature decreases with increase in altitude at the normal lapse rate of:
• 6.5∘C / 1000 m6.5^\circ C\,/\,1000\ m6.5∘C/1000 m

Importance

• Most important layer for agriculture and living organisms
• Contains water vapour necessary for cloud formation and rainfall
• Supports crop growth and weather processes

Upper Boundary: The upper limit of troposphere is called the Tropopause.

2. Stratosphere

• The stratosphere lies above the troposphere and extends up to about 50 km above the earth’s surface. Unlike the troposphere, temperature increases with height in this layer because ozone absorbs ultraviolet radiation from the sun.
• This layer contains the ozone layer, which protects life on earth from harmful UV rays. The stratosphere is comparatively stable with very little turbulence and weather activity.

Importance

• Ozone layer absorbs harmful ultraviolet radiation
• Protects humans, animals, and plants from UV damage
• Stable atmospheric conditions favor aircraft movement

Upper Boundary: The upper boundary of the stratosphere is known as the Stratopause.

3. Mesosphere

• The mesosphere is the third layer of Earth’s Atmosphere and extends from about 50 km to 80 km above the earth’s surface. In this layer, temperature again decreases with increase in altitude, making it the coldest layer of the atmosphere.
• The air in the mesosphere is very thin, and atmospheric pressure is extremely low.

Importance

• Most meteors entering the earth’s atmosphere burn due to friction in this layer, protecting the earth from meteor impacts.
• Helps in maintaining atmospheric balance between lower and upper layers.

Upper Boundary: The upper boundary of the mesosphere is called the Mesopause.

4. Thermosphere

• The thermosphere lies above the mesosphere and extends from about 80 km to nearly 400–500 km. In this layer, temperature increases rapidly with height because gases absorb high-energy solar radiation.
• The thermosphere contains ionized gases, forming the ionosphere, which is important for communication systems.

Importance

• The ionosphere reflects radio waves and helps long-distance radio communication.
• Natural light displays known as aurora borealis and aurora australis occur in this layer.
• Many satellites and space stations operate within this region.
• The thermosphere plays an important role in space science and atmospheric communication systems.

5. Exosphere

• The exosphere is the outermost layer of Earth’s Atmosphere and gradually merges into outer space. It extends beyond the thermosphere to several thousand kilometers above the earth’s surface. The air in this layer is extremely thin and mainly composed of light gases such as hydrogen and helium.
• Because of very low gravitational force and extremely low atmospheric density, gas molecules can easily escape into space.

Characteristics

• Outermost and least dense atmospheric layer
• Mainly contains hydrogen and helium gases
• Satellites move through this region
• No weather phenomena occur in this layer
• The exosphere forms the transition region between the earth’s atmosphere and outer space.

Importance of Atmosphere

Earth’s Atmosphere is essential for sustaining life and maintaining environmental balance on earth. It acts as a protective blanket around the earth and supports various biological, climatic, and agricultural processes.

1. Provides Essential Gases for Life: The atmosphere supplies oxygen required for respiration by humans and animals and carbon dioxide required by green plants for photosynthesis. Nitrogen present in the atmosphere is also important for plant growth after nitrogen fixation.

2. Regulates Earth’s Temperature: The atmosphere maintains suitable temperature conditions for life by absorbing, storing, and distributing heat energy. Greenhouse gases prevent excessive heat loss from the earth’s surface and maintain thermal balance.

3. Protects from Harmful Radiation: The ozone layer present in the stratosphere absorbs harmful ultraviolet (UV) rays from the sun and protects living organisms from skin diseases, genetic damage, and harmful effects on crops.

4. Responsible for Weather and Climate: All weather phenomena such as rainfall, clouds, wind, storms, humidity, and temperature changes occur in the atmosphere. It controls climate and seasonal variations essential for agriculture and natural ecosystems.

5. Helps in Water Cycle: The atmosphere plays a major role in the hydrological cycle through evaporation, condensation, cloud formation, and precipitation. This maintains water availability on earth.

6. Protects Earth from Meteors: Most meteors entering the atmosphere burn due to friction in the mesosphere before reaching the earth’s surface, protecting life and property.

7. Supports Agricultural Production: Atmospheric factors such as temperature, rainfall, humidity, solar radiation, and wind directly affect crop growth, livestock production, and agricultural productivity. Therefore, the atmosphere is highly important in Agrometeorology.

8. Medium for Sound and Communication: The atmosphere acts as a medium for sound transmission and supports radio communication through the ionosphere.