Chapter Notes

Composition and Structure of Atmosphere

The Atmosphere is the mixture of different gases that surrounds the Earth like a blanket. It is absolutely essential for life. While we can survive for some time without food or water, we cannot live for more than a few minutes without breathing the air from the atmosphere. It provides life-giving gases like oxygen for humans and animals, and carbon dioxide for plants.

The atmosphere is an integral part of the Earth's mass. About 99 percent of its total mass is found within 32 km of the Earth's surface. The air itself is colourless and odourless; we can only feel its presence when it moves as wind.

Composition of the Atmosphere

The atmosphere is made up of three main components: gases, water vapour, and dust particles. The mix of these components changes as you move higher up from the Earth's surface.

Gases

The atmosphere is a mixture of many gases. However, the proportion of these gases is not uniform at all heights.

• Oxygen: Becomes almost negligible at a height of 120 km.
• Carbon dioxide and Water Vapour: Are found mainly in the lower parts of the atmosphere, up to about 90 km from the surface.

Two of the most significant gases from a meteorological standpoint are Carbon dioxide and Ozone.

• Carbon dioxide (CO2): This gas is crucial for the green house effect. It is transparent to the sun's incoming radiation, allowing sunlight to pass through and warm the Earth. However, it is opaque to the outgoing heat (terrestrial radiation) from the Earth's surface. It absorbs this heat and reflects some of it back, which keeps the planet warm.

  • In recent decades, the amount of carbon dioxide has been increasing, mainly due to the burning of fossil fuels. This increase has led to a rise in the temperature of the air.

• Ozone (O3): Found between 10 and 50 km above the Earth's surface, ozone acts as a protective filter. It absorbs harmful ultra-violet (UV) rays from the sun, preventing them from reaching the surface and harming life.

Water Vapour

Water vapour is a variable gas, meaning its amount changes depending on location and altitude.

• Distribution: It decreases with altitude. In warm, wet tropical regions, it can make up to four percent of the air's volume. In contrast, in cold, dry desert and polar regions, it may be less than one percent. It also decreases as you move from the equator towards the poles.
• Function: Water vapour acts like a blanket for the Earth. It absorbs some of the incoming solar energy (insolation) and also traps the Earth's radiated heat. This prevents the Earth from getting too hot during the day or too cold at night. It also plays a key role in the stability and instability of the air.

Dust Particles

The atmosphere contains various small solid particles from different sources. These include:

• Sea salts
• Fine soil
• Smoke-soot
• Ash
• Pollen
• Disintegrated particles of meteors

Dust particles are mostly concentrated in the lower layers of the atmosphere, but strong air currents can carry them to great heights. Their concentration is higher in subtropical and temperate regions due to dry winds, compared to the equatorial and polar regions.

Structure of the Atmosphere

The atmosphere is not a single, uniform layer. It is divided into five different layers based on temperature and density. The density of the atmosphere is highest near the surface and decreases as you go higher.

Troposphere

• This is the lowermost and most important layer for all biological activity.
• Height: Its average height is 13 km. It is thicker at the equator (about 18 km) and thinner near the poles (about 8 km). This is because strong convectional currents at the equator push heat and air up to greater heights.
• Characteristics: This layer contains almost all the atmosphere's dust particles and water vapour. All changes in weather and climate, like clouds, rain, and storms, happen here.
• Temperature: The temperature in the troposphere decreases with increasing height at a rate of 1°C for every 165 meters.

Stratosphere

• The tropopause is the zone that separates the troposphere from the stratosphere. Here, the temperature is nearly constant.
• Height: The stratosphere is found above the tropopause and extends up to a height of 50 km.
• Key Feature: The most important feature of the stratosphere is the ozone layer. This layer absorbs harmful UV radiation from the sun, shielding life on Earth.

Mesosphere

• Lying above the stratosphere, the mesosphere extends up to a height of 80 km.
• Temperature: In this layer, the temperature starts decreasing again with altitude, reaching as low as -100°C at 80 km.
• The upper limit of this layer is called the mesopause.

Ionosphere

• The ionosphere is located between 80 km and 400 km above the mesopause.
• Characteristics: It contains electrically charged particles known as ions. These ions are what give the layer its name.
• Function: This layer is very important for communication. Radio waves transmitted from the Earth are reflected back to Earth by the ionosphere.
• Temperature: In this layer, the temperature starts increasing with height.

Exosphere

• This is the uppermost layer of the atmosphere.
• Characteristics: Very little is known about the exosphere. The contents are extremely rarefied (thin), and the layer gradually merges with outer space.

Elements of Weather and Climate

The main components of the atmosphere that change and influence human life are known as the elements of weather and climate. These are:

• Temperature
• Pressure
• Winds
• Humidity
• Clouds
• Precipitation

Composition and Structure of Atmosphere

The atmosphere is the mixture of different gases that surrounds the Earth like an envelope. It is essential for all life, providing oxygen for animals and humans, and carbon dioxide for plants. While we can survive for some time without food or water, we cannot live for more than a few minutes without breathing air. This shows how crucial the atmosphere is.

The air that makes up our atmosphere is colourless and odourless, and we can only feel it when it moves as wind. About 99 percent of the total mass of the atmosphere is found within 32 km of the Earth's surface.

Composition of the Atmosphere

The atmosphere is made up of three main components: gases, water vapour, and dust particles. The mix of these components changes as you go higher. For example, oxygen becomes almost negligible at a height of 120 km, while carbon dioxide and water vapour are found only up to 90 km from the Earth's surface.

Gases

Gases are the primary component of the atmosphere. While the volume of most gases is constant, some, like carbon dioxide, are changing.

• Carbon dioxide (CO2): This gas is extremely important for our planet's climate. It is transparent to the sunlight coming towards Earth but is opaque to the heat radiating back from the Earth's surface. This means it traps some of the outgoing heat, reflecting it back towards the surface. This process is largely responsible for the greenhouse effect, which keeps our planet warm. In recent decades, the amount of carbon dioxide has been rising, mainly due to the burning of fossil fuels, which has also caused an increase in air temperature.
• Ozone (O3): Found between 10 and 50 km above the Earth's surface, ozone is another critical gas. It acts as a filter, absorbing harmful ultra-violet (UV) rays from the sun and preventing them from reaching us.

Water Vapour

Water vapour is a variable gas, meaning its amount changes depending on location and altitude.

• Distribution: It decreases with altitude. In warm, wet tropical regions, it can make up to four percent of the air, but in cold, dry polar or desert regions, it can be less than one percent. It also decreases as you move from the equator towards the poles.
• Function: Like carbon dioxide, water vapour absorbs some of the incoming solar energy and traps the Earth's radiated heat. It acts like a blanket, preventing the Earth from getting too hot or too cold. It also plays a key role in the stability and instability of the air, which affects weather patterns.

Dust Particles

The atmosphere contains many small solid particles from various sources. These include:

• Sea salts
• Fine soil
• Smoke-soot and ash
• Pollen and dust
• Disintegrated particles from meteors

These dust particles are usually concentrated in the lower layers of the atmosphere but can be carried to great heights by air currents. Their concentration is higher in subtropical and temperate regions due to dry winds.

Structure of the Atmosphere

The atmosphere is not uniform; it is divided into five different layers based on temperature and density. The density of the atmosphere is highest near the Earth's surface and decreases as altitude increases.

Troposphere

The troposphere is the lowermost layer of the atmosphere and is the most important for all biological activity.

• Height: Its average height is 13 km. It is thicker at the equator (about 18 km) and thinner near the poles (about 8 km). This is because strong convectional currents at the equator push heat and air to greater heights.
• Characteristics: This layer contains almost all the atmosphere's dust particles and water vapour. As a result, all changes in climate and weather, such as clouds, rain, and storms, take place here.
• Temperature: In the troposphere, the temperature decreases as you go higher, at a rate of 1°C for every 165 metres of height.

The zone that separates the troposphere from the next layer is called the tropopause. Here, the temperature is nearly constant, reaching about -80°C over the equator and -45°C over the poles.

Stratosphere

Found directly above the tropopause, the stratosphere extends up to a height of 50 km.

• Key Feature: Its most important feature is the presence of the ozone layer. This layer is crucial for life as it absorbs harmful ultraviolet radiation from the sun, shielding the planet.

Mesosphere

The mesosphere lies above the stratosphere and extends up to a height of 80 km.

• Temperature: In this layer, the temperature once again starts to decrease with increasing altitude. It can reach as low as -100°C at 80 km.
• Boundary: The upper limit of the mesosphere is known as the mesopause.

Thermosphere

The thermosphere is the layer above the mesosphere. Within this layer is a region known as the ionosphere.

• Ionosphere: Located between 80 and 400 km, the ionosphere contains electrically charged particles called ions.
• Function: This layer is very useful for communication. Radio waves transmitted from Earth are reflected back to the planet by the ionosphere.
• Temperature: In the thermosphere, the temperature starts to increase with height.

Exosphere

The exosphere is the uppermost layer of the atmosphere.

• Characteristics: Very little is known about this layer. The contents are extremely rarefied (spread out), and it gradually merges with outer space.

Elements of Weather and Climate

The main elements of the atmosphere that are constantly changing and influence human life are:

• Temperature
• Pressure
• Winds
• Humidity
• Clouds
• Precipitation

These elements combine to create the weather we experience daily and the long-term climate of a region.