CLIMATE | Class 11 Geography Notes

COMPOSITION AND STRUCTURE OF ATMOSPHERE

The atmosphere is a vital component of the Earth, essential for sustaining life. It is a mixture of gases, water vapour, and dust particles enveloping the Earth. Air is indispensable for life; humans can survive for days without food and water but only minutes without air. The atmosphere contains life-supporting gases such as oxygen, necessary for animals and humans, and carbon dioxide, essential for plants. Approximately 99% of the atmosphere's total mass is confined within 32 km from the Earth's surface. The air itself is colourless and odourless, perceptible only when it moves as wind. The atmosphere's composition varies with altitude; for example, oxygen becomes almost negligible at about 120 km, and carbon dioxide and water vapour are present only up to 90 km. Carbon dioxide plays a crucial meteorological role by being transparent to incoming solar radiation but opaque to outgoing terrestrial radiation, thus contributing to the greenhouse effect. Its volume has been increasing due to fossil fuel combustion, leading to global warming. Ozone, found between 10 and 50 km altitude, acts as a protective filter absorbing harmful ultraviolet rays from the sun. Water vapour is a variable gas that decreases with altitude and latitude, constituting up to 4% of air volume in warm, wet tropics but less than 1% in cold, dry regions. It absorbs solar radiation and preserves Earth's heat, acting like a blanket to moderate temperature extremes. Dust particles, including sea salts, soil, smoke, ash, pollen, and meteor debris, are concentrated mostly in the lower atmosphere but can be transported higher by convection. These particles serve as nuclei for water vapour condensation, forming clouds. The atmosphere is stratified into five layers based on temperature: troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The troposphere, extending up to 13 km on average (8 km at poles, 18 km at equator), contains dust and water vapour and is the site of all weather and climate changes. Temperature here decreases with height at about 1°C per 165 meters. The tropopause separates the troposphere from the stratosphere and maintains a nearly constant temperature. The stratosphere extends to about 50 km and contains the ozone layer, which shields life from ultraviolet radiation. Above lies the mesosphere (up to 80 km), where temperature decreases again, reaching about -100°C at the mesopause. The ionosphere, between 80 and 400 km, contains charged particles (ions) that reflect radio waves back to Earth. The outermost layer, the exosphere, merges gradually into outer space and contains extremely rarefied gases. While all layers influence Earth, geographers mainly focus on the troposphere and stratosphere, where weather and climate phenomena occur. The main elements of weather and climate include temperature, pressure, winds, humidity, clouds, and precipitation, which are elaborated in subsequent chapters.

📊 Diagram: Figure 7.1 illustrates the structure of the atmosphere showing the five layers: troposphere (lowest), stratosphere (containing ozone layer), mesosphere, thermosphere (including ionosphere), and exosphere (outermost). The diagram depicts temperature variations with altitude and the approximate heights of each layer.

🧪 Activity: No specific activity in this section.

🔗 Connection: This section lays the foundation for understanding elements of weather and climate, which are discussed in detail in subsequent chapters.