Composition and Structure of Atmosphere 11 Geography Chapter 7 CBSE Notes 2025-26

The composition and structure of the atmosphere are crucial topics in geography, as the atmosphere supports life on Earth and drives weather and climate patterns. The Earth’s atmosphere consists of a mixture of different gases that surround our planet, forming a protective envelope that is essential for survival. Air is invisible, odourless, and can only be felt when it moves as wind. Almost 99% of the atmospheric mass is confined within 32 km from the Earth’s surface. The atmosphere contains life-supporting gases like oxygen, required by humans and animals, and carbon dioxide, important for plants.

The atmosphere is made up of three main components: gases, water vapour, and dust particles. The relative percentage of gases remains mostly constant in the lower parts but changes as altitude increases. Oxygen becomes almost absent at heights above 120 km, while carbon dioxide and water vapour are found mainly up to 90 km from the surface.

• Gases: Nitrogen makes up the majority of the atmosphere, followed by oxygen. Nitrogen is chemically inactive, while oxygen supports all forms of life and combustion. Argon, carbon dioxide, helium, ozone, and hydrogen are also present in smaller quantities.
• Carbon dioxide is especially important for climate as it is transparent to incoming solar radiation but blocks some outgoing terrestrial radiation, trapping heat near the Earth's surface and contributing to the greenhouse effect. Its quantity is increasing due to burning of fossil fuels, leading to a rise in Earth's temperature.
• Ozone, found mainly between 10 and 50 km altitude, forms a protective layer that absorbs harmful ultraviolet rays from the sun, shielding life on Earth from dangerous radiation.

Water vapour is a variable gas in the atmosphere, with its content decreasing with altitude and towards the poles. It may account for up to 4% of the air by volume in warm and humid tropical regions, but less than 1% in dry, cold areas like deserts and polar places. Water vapour is important as it absorbs some of the sun’s radiation and helps retain heat emitted from the Earth’s surface, acting much like a blanket. It also plays a key role in atmospheric stability and is vital for the formation of clouds and precipitation.

Dust particles in the atmosphere originate from sources such as sea salt, soil, smoke, ash, pollen, and even disintegrated meteor fragments. These tiny solids are mostly found in the lower layers, but strong air currents can carry them to higher altitudes. Subtropical and temperate zones, where dry winds are common, generally have a greater concentration of dust than equatorial or polar areas. Dust and salt particles serve as nuclei around which water vapour can condense, leading to cloud formation.

The atmosphere is divided into five main layers based on temperature characteristics and composition. These are the troposphere, stratosphere, mesosphere, thermosphere (which includes the ionosphere), and exosphere. Each layer has distinct features and varying densities.

• Troposphere: The lowermost and most important layer for all forms of life. Its average height is about 13 km, but it extends from 8 km near the poles up to about 18 km at the equator. This thickness difference is due to greater heat and strong convection currents at the equator. The troposphere contains almost all the water vapour and dust particles, and is where all weather phenomena (clouds, rain, winds) occur. The temperature in this layer decreases steadily with altitude, by roughly 1°C for every 165 m.
• Tropopause: The boundary zone between the troposphere and stratosphere. Here, temperature stabilizes and is nearly constant. The air temperature at the tropopause is about -80°C over the equator and about -45°C over the poles.
• Stratosphere: Extends from the tropopause up to about 50 km altitude. This layer is notable for the presence of the ozone layer, which shields life on Earth from damaging ultraviolet radiation. Temperature increases with altitude within the stratosphere.
• Mesosphere: Above the stratosphere, extending up to about 80 km. In this layer, temperature starts falling again with height and can reach as low as -100°C at its upper limit, called the mesopause.
• Thermosphere (Ionosphere): Situated between 80 km and 400 km above the Earth, this layer contains ions or electrically charged particles. The ionosphere plays an important role in reflecting radio waves back to Earth and is therefore vital for communication. Here, temperatures rise with altitude due to the absorption of solar energy by ions.
• Exosphere: The outermost layer, with extremely rarefied content, which gradually merges with outer space. Very little is known about this layer.

The main elements of the atmosphere that constantly change and affect human life include temperature, atmospheric pressure, winds, humidity, clouds, and precipitation. All these elements work together to create different weather and climate conditions at any location on Earth.

• Temperature: Determines how hot or cold the atmosphere is at a given place or time and affects almost every other climate element.
• Pressure: The weight of the air above a certain point; pressure belts around the planet influence wind patterns and weather systems.
• Winds: Movements of air from areas of high pressure to low pressure. They can be planetary, seasonal, or local. Examples include the trade winds, monsoons, and sea breezes.
• Humidity: The amount of moisture (water vapour) in the air, which influences precipitation and how we feel temperature.
• Clouds and Precipitation: Water vapour condenses to form clouds, and precipitation includes all forms of moisture falling from clouds—like rain, snow, mist, fog, and dew.

The troposphere is the most important atmospheric layer as it contains about 75% of the total air mass and almost all of the Earth's water vapour and dust. All weather changes, including rainfall, storm formation, and cloud development, occur here. It supports all living organisms and plant life, making it essential for human survival.

The ozone layer, found within the stratosphere, acts as a protective shield that absorbs the majority of the sun’s harmful ultraviolet rays. Without the ozone, these rays would reach the Earth’s surface and cause severe harm to living organisms, including skin diseases and damage to plant life.

Dust particles, aerosols, and fine solids are vital for cloud formation: they act as condensation nuclei where water vapour condenses to form droplets. These nuclei help in the development of fog, mist, dew, and clouds, which are all forms of condensation that play an essential role in maintaining Earth’s water cycle.

The chapter also provides key review questions, such as identifying the major gases in the atmosphere, defining the importance of each layer, and short and long-answer questions to reinforce understanding of these fundamental concepts.

• The Earth’s atmosphere is a complex structure that makes life possible and influences weather and climate across the globe.
• Major atmospheric components include nitrogen, oxygen, carbon dioxide, water vapour, dust, and ozone.
• Understanding atmospheric layers, their features, and the functions of different components is essential for grasping further concepts in geography and environmental science.

Class 11 Geography Chapter 7 Notes – Climate: Composition and Structure of Atmosphere for Quick Revision

These Class 11 Geography Chapter 7 notes on the Composition and Structure of Atmosphere offer a clear summary of all important concepts. Students will find detailed coverage of atmospheric layers, gases, water vapour, and their roles in climate and weather. Using these revision notes ensures you remember key points and answer CBSE exam questions confidently.

With easy-to-understand explanations and concise bullet points on climate, these notes make last-minute revision much easier. Referring to them helps you grasp tricky topics like the greenhouse effect, the importance of the ozone layer, and the classification of different atmospheric layers for CBSE Geography exams.