Atmosphere and Climate.

Class 9 Geography – Detailed Notes Atmosphere and Climate 1. What is the Atmosphere? The atmosphere is the thin layer of gases surrounding the Earth. It is held around the Earth by the force of gravity. The atmosphere is essential for life because it: • Provides oxygen for respiration. • Provides carbon dioxide for photosynthesis. • Protects Earth from harmful solar radiation. • Maintains a suitable temperature for life. • Helps in the formation of clouds and rainfall. • Supports the water cycle and weather processes. PART A – ATMOSPHERE 2. Composition of the Atmosphere The atmosphere consists mainly of gases, water vapour and dust particles. Major gases Gas Approximate proportion Importance Nitrogen 78% Essential for plant growth and protein formation Oxygen 21% Necessary for respiration and combustion Argon 0.93% Inert gas Carbon dioxide About 0.04% Required for photosynthesis and helps regulate temperature Other gases Very small amounts Include neon, helium, ozone, hydrogen etc. Nitrogen • It is the most abundant gas. • Plants cannot directly use atmospheric nitrogen. • Certain bacteria convert nitrogen into forms that plants can absorb. • Nitrogen is essential for the formation of proteins. Oxygen • Makes up about 21% of the atmosphere. • Essential for respiration. • Supports combustion. Carbon Dioxide • Plants use CO₂ during photosynthesis. • It is an important greenhouse gas. • An increase in CO₂ contributes to global warming. Water Vapour • The amount of water vapour varies from place to place. • It is generally higher in warm and humid regions. • It plays an important role in cloud formation, rainfall and weather. Dust Particles Dust particles are present mainly in the lower atmosphere. They act as condensation nuclei, around which water vapour condenses to form clouds. 3. Structure of the Atmosphere The atmosphere is divided into different layers according to changes in temperature.

A. Troposphere • It is the lowest layer of the atmosphere. • Its average height is about 13 km. • It is approximately 8 km thick over the poles and 18 km over the equator. • Almost all weather phenomena occur here. • It contains most of the atmospheric water vapour and dust particles. • Temperature decreases with increasing height. • The rate of decrease is approximately 6.5°C per kilometre. Important term: Normal Lapse Rate The average decrease in temperature with height in the troposphere is called the normal lapse rate. Normal lapse rate = 6.5°C per 1000 m B. Stratosphere • Located above the troposphere. • Extends roughly from 13 km to 50 km. • It is relatively free from clouds and weather disturbances. • The ozone layer is found mainly in this region. • Temperature increases with height in the upper part because ozone absorbs ultraviolet radiation. Ozone Layer Ozone is a form of oxygen containing three oxygen atoms (O₃). It absorbs much of the Sun's harmful ultraviolet (UV) radiation, thereby protecting living organisms. C. Mesosphere • Extends approximately from 50 km to 80 km. • Temperature decreases with height. • It is one of the coldest regions of the atmosphere. • Most meteors burn up in this layer because of friction with atmospheric gases. D. Thermosphere • Extends approximately from 80 km upward. • Temperature increases rapidly with height. • It contains electrically charged particles. • The ionosphere is associated with this region. • Radio communication is influenced by the ionosphere. • Auroras occur in the upper atmosphere. E. Exosphere • It is the outermost layer of the atmosphere. • Air is extremely thin here. • It gradually merges with outer space. • Very light gases such as hydrogen and helium are found here. PART B – WEATHER AND CLIMATE 4. Weather Weather refers to the day-to-day condition of the atmosphere at a particular place. Weather can change within hours. Examples: • It is raining today. • The temperature is 35°C. • Strong winds are blowing. • The sky is cloudy. 5. Climate Climate refers to the average weather conditions of a large area over a long period of time, generally **30 years or more**. Example: India has a **monsoon type of climate**, characterised by seasonal reversal of winds and a distinct rainy season. 6. Difference between Weather and Climate Weather Climate Refers to atmospheric conditions at a particular time Refers to average atmospheric conditions over a long period Changes frequently Changes comparatively slowly Short-term Long-term Can change within hours or days Studied over several decades Example: Heavy rainfall today Example: India has a monsoon climate 7. Elements of Weather and Climate The major elements are: 1. Temperature 2. Atmospheric pressure 3. Wind 4. Humidity 5. Precipitation A. Temperature Temperature refers to the degree of hotness or coldness of the air. Factors affecting temperature • Latitude • Altitude • Distance from the sea • Ocean currents • Winds • Relief features Distribution of Temperature Temperature generally: • Decreases from the equator towards the poles. • Decreases with increasing altitude. 8. Atmospheric Pressure Atmospheric pressure is the pressure exerted by the weight of air on the Earth's surface. It is measured using a barometer. Unit : Atmospheric pressure is commonly expressed in millibars (mb) or hectopascals (hPa). Relationship between temperature and pressure • Warm air → expands → becomes lighter → rises → creates low pressure • Cold air → contracts → becomes heavier → sinks → creates high pressure This difference in pressure is one of the major causes of wind movement. 9. Wind Wind is the movement of air from an area of high pressure to an area of low pressure. Types of Winds 1. Permanent/Planetary Winds These blow throughout the year in fairly regular directions. Examples: • Trade Winds • Westerlies • Polar Easterlies 2. Seasonal Winds Their direction changes according to seasons. Example: Monsoon winds. 3. Local Winds These blow over a small area for a short period. Examples: Loo– hot, dry wind in northern India during summer. Land breeze Sea breeze 10. Humidity Humidity refers to the amount of water vapour present in the atmosphere. Types Absolute Humidity: The actual amount of water vapour present in a given volume of air. Relative Humidity: The percentage of moisture present in the air compared with the maximum amount the air can hold at that temperature. Warm air can hold more water vapour than cold air. 11. Precipitation Precipitation is the process by which moisture from the atmosphere falls to the Earth's surface. It may occur as: • Rain • Snow • Sleet • Hail Rainfall Rain occurs when water droplets in clouds become sufficiently large and heavy and fall to the ground. 12. Factors Affecting the Climate of a Place The climate of a place is influenced by several factors. 1. Latitude Latitude has a major influence on temperature. • Places near the equator receive more direct sunlight. • Places towards the poles receive slanting rays. • Therefore, temperature generally decreases from the equator towards the poles. 2. Altitude As altitude increases, temperature generally decreases. Therefore: • Mountains are cooler than plains. • Hill stations are often cooler than nearby lowlands. 3. Distance from the Sea Water heats and cools more slowly than land. Coastal areas • Experience moderate temperatures. • Have smaller annual temperature ranges. Interior areas • Experience more extreme temperatures. • Have hotter summers and colder winters. 4. Pressure and Winds Pressure conditions and prevailing winds strongly influence temperature and rainfall. For example, monsoon winds are responsible for bringing seasonal rainfall to much of India. 5. Ocean Currents Warm and cold ocean currents influence the temperature of coastal areas. • Warm currents raise temperatures. • Cold currents lower temperatures. 6. Relief Features Mountains can affect temperature and rainfall. Example: The Himalayas act as a climatic barrier: • They prevent cold continental winds from Central Asia from entering India during winter. • They force moisture-laden monsoon winds to rise, contributing to rainfall. PART C – INDIAN CLIMATE 13. India's Climate India has a monsoon type of climate. The word monsoon is derived from the Arabic word 'mausim', meaning season. The most important feature of India's climate is the seasonal reversal of wind patterns. Major characteristics • Seasonal reversal of winds. • Distinct wet and dry seasons. • Uneven distribution of rainfall. • Variations in temperature. • Monsoon rainfall is highly variable. • Rainfall is concentrated in a few months. 14. Factors Influencing India's Climate India's climate is influenced by: A. Latitude The Tropic of Cancer passes through India, dividing the country into almost two equal parts. The southern part lies closer to the equator and experiences tropical conditions, while the northern part experiences greater seasonal variation. B. Altitude The Himalayas influence India's climate significantly. C. Pressure and Surface Winds Seasonal changes in pressure cause the reversal of winds. D. Distance from the Sea Coastal regions experience moderate temperatures, while inland areas have greater temperature variations. E. Relief The Himalayas and other mountain ranges influence the distribution of rainfall. 15. Seasons of India According to the **Indian Meteorological Department**, India experiences four main seasons: 1. Cold Weather Season - December to February 2. Hot Weather Season - March to May 3. Southwest Monsoon Season - June to September 4. Retreating/Post-Monsoon Season - October to November 16. Cold Weather Season Period - December to February Main features • Temperatures are generally low. • Northern India experiences cold conditions. • Days may be relatively warm and nights colder in northern inland areas. • The northeast trade winds generally prevail. Western Disturbances Western disturbances originate in the Mediterranean region and bring winter rainfall to north western India. This rainfall is important for rabi crops, especially wheat. 17. Hot Weather Season Period - March to May Main features • Temperature increases rapidly. • Northern and central India become very hot. • Low-pressure conditions develop over north western India. Important Local Winds and Storms Loo • Hot, dry and strong winds. • Blow mainly during the afternoon in northern and north western India. Kal Baisakhi • Violent thunderstorms occurring mainly in West Bengal and Assam. • They can be beneficial for tea and other crops. Mango Showers • Pre-monsoon showers in Kerala and Karnataka. • They help in the early ripening of mangoes. 18. Southwest Monsoon Season Period - June to September The southwest monsoon is the most important feature of India's climate. Arrival of Monsoon The monsoon generally reaches: • Kerala around the beginning of June. • It then advances northwards. Two branches of the Southwest Monsoon 1. Arabian Sea Branch It enters India from the southwest and brings rainfall to: • Western coast • Western Ghats • Parts of central and northern India 2. Bay of Bengal Branch It moves towards: • North eastern India • Northern plains It is responsible for heavy rainfall in many parts of north eastern India. 19. Distribution of Rainfall in India Rainfall is unevenly distributed. Very Heavy Rainfall • Western coast • North eastern India Low Rainfall • Western Rajasthan • Ladakh • Some parts of the Deccan Plateau Rain Shadow Area - The eastern side of the Western Ghats receives less rainfall because it lies in the rain-shadow region. 20. Retreating Monsoon Period - October to November After September, the southwest monsoon begins to withdraw from northern India. Main features • Skies become clearer. • Temperature remains high in some areas. • Humidity can be high. • This period is sometimes called the October Heat. Northeast Monsoon During this period, winds generally reverse direction and blow from land towards the sea. The Coromandel Coast** receives substantial rainfall from the retreating/northeast monsoon. Tamil Nadu receives a significant part of its annual rainfall during this season. 21. Distribution of Rainfall and Relief Mountains play an important role in rainfall distribution. Windward Side - The side facing moisture-bearing winds receives heavy rainfall. Leeward Side - The opposite side receives less rainfall and forms a rain-shadow region. 22. Why is India's Climate Called Monsoon Type? India's climate is called monsoon type because: 1. Winds reverse their direction seasonally. 2. Most rainfall occurs during a particular season. 3. There is a clear distinction between wet and dry seasons. 4. Monsoon winds strongly influence temperature and rainfall. 5. The timing and amount of rainfall vary considerably from year to year. 23. Important Terms • Insolation - The amount of solar energy received by the Earth's surface. • Lapse Rate - The rate at which temperature decreases with increasing altitude. • Atmospheric Pressure - The force exerted by the weight of air on the Earth's surface. • Humidity - The amount of water vapour present in the atmosphere. • Precipitation - Any form of water falling from the atmosphere to the Earth's surface. • Monsoon - Seasonal reversal of wind direction. • Rain Shadow - An area receiving relatively little rainfall on the leeward side of a mountain. • Western Disturbances - Temperate cyclones originating near the Mediterranean region that bring winter rainfall to north western India. • October Heat - Hot and humid weather experienced during the retreating monsoon period. Exam-Oriented Key Points • 78% of the atmosphere is nitrogen. • 21% is oxygen. • Weather occurs mainly in the troposphere. • The ozone layer is mainly found in the stratosphere. • Most meteors burn up in themesosphere. • The outermost atmospheric layer is the exosphere. • Air moves from high pressure to low pressure. • Climate refers to average weather conditions over a long period. • India's climate is monsoon type. • India has four major seasons according to the Indian Meteorological Department. • The Himalayas have a major influence on India's climate. • The southwest monsoon is the main rainy season. • The Coromandel Coast receives important rainfall from the retreating/northeast monsoon. • Loo is a hot, dry local wind. • Kal Baisakhi refers to violent pre-monsoon thunderstorms in parts of eastern and northeastern India. • Mango showers are pre-monsoon rains that help in the early ripening of mangoes.