Climate - Class 9th CBSE

Cheatsheet Content

### Introduction to Climate - **Climate:** Refers to the sum total of weather conditions and variations over a large area for a long period of time (more than thirty years). - **Weather:** Refers to the state of the atmosphere over an area at any point in time. - **Elements of Weather & Climate:** Temperature, atmospheric pressure, wind, humidity, and precipitation. - **Monsoon:** The word 'monsoon' is derived from the Arabic word 'mausim' which literally means season. It refers to the seasonal reversal in the wind direction during a year. ### Climatic Controls There are six major controls of the climate of any place: 1. **Latitude:** Due to the curvature of the Earth, the amount of solar energy received varies according to latitude. As one goes from the equator to the poles, the temperature decreases. 2. **Altitude:** As one goes higher from the surface of the Earth, the atmosphere becomes less dense and temperature decreases. Hills are therefore cooler during summers. 3. **Pressure and Wind System:** This depends on the latitude and altitude of the place. It influences temperature and rainfall patterns. 4. **Distance from the Sea (Continentality):** The sea exerts a moderating influence on climate. Places closer to the sea experience moderate temperatures, while those far from the sea have extreme weather conditions (very hot summers, very cold winters). 5. **Ocean Currents:** Along with onshore winds, ocean currents affect the climate of coastal areas. Warm or cold currents can make a coastal area warmer or colder. 6. **Relief Features:** High mountains act as barriers for cold or hot winds. They may also cause precipitation if they are high enough and lie in the path of rain-bearing winds. ### Factors Affecting India's Climate #### Latitude - The Tropic of Cancer (23.5° N) passes through the middle of the country, dividing it into tropical and sub-tropical zones. - The tropical zone experiences high temperatures throughout the year. #### Altitude - India has mountains to the north (Himalayas) with an average height of about 6,000 metres. - India also has a vast coastal area where the maximum elevation is about 30 metres. - The Himalayas prevent cold winds from Central Asia from entering the subcontinent, giving it milder winters compared to other places at the same latitude. #### Pressure and Winds - **Upper Air Circulation:** Jet streams are narrow belts of high-altitude (above 12,000 m) westerly winds in the troposphere. - In India, these jet streams are located south of the Himalayas in winter. - The western cyclonic disturbances (winter rains) are brought by these westerly jet streams. - **Monsoon Winds:** - **South-West Monsoon:** Occurs during summer (June to September). These winds blow from the Arabian Sea and Bay of Bengal towards the land, bringing widespread rainfall. - **North-East Monsoon (Retreating Monsoon):** Occurs during autumn/winter (October to November). These winds blow from land to sea, causing rainfall on the Coromandel Coast (Tamil Nadu). ### The Indian Monsoon #### Mechanism of Monsoon The monsoon is influenced by several factors: 1. **Differential Heating and Cooling:** Land heats up faster than water, creating a low-pressure area over the Indian subcontinent during summer, attracting moisture-laden winds from the high-pressure area over the ocean. 2. **Shift of ITCZ:** The Inter Tropical Convergence Zone (ITCZ) is a broad trough of low pressure in equatorial latitudes. In summer, it shifts northwards over the Ganga plain (monsoon trough). 3. **High Pressure over Madagascar:** The presence of the high-pressure area, east of Madagascar, approximately at 20°S over the Indian Ocean, affects the Indian Monsoon. 4. **Tibetan Plateau:** Intense heating of the Tibetan Plateau during summer creates strong vertical air currents and a high-pressure area. 5. **Jet Stream Movement:** The westerly jet stream moves north of the Himalayas. An easterly jet stream develops over the Indian Peninsula during summer. 6. **Southern Oscillation (SO) and El Niño:** - **Southern Oscillation (SO):** A periodic change in pressure conditions between the Pacific Ocean and the Indian Ocean. - **El Niño:** A warm ocean current that appears off the coast of Peru in December, replacing the cold Peruvian current. It is associated with delayed monsoons or drought conditions in India. ### The Seasons #### 1. The Cold Weather Season (Winter) - **Duration:** Mid-November to February. - **Temperature:** Decreases from south to north. Average temperature in Chennai is 24°-25°C, while in the Northern Plains it is 10°-15°C. - **Pressure & Winds:** High-pressure conditions prevail over northern India. Winds blow from north to south, out from the land. - **Rainfall:** - Light rainfall in the Northern Plains (western disturbances). - Heavy rainfall on the Coromandel Coast due to North-East Monsoon. #### 2. The Hot Weather Season (Summer) - **Duration:** March to May. - **Temperature:** Rises from south to north. High temperatures (45°C) in Northern Plains. - **Pressure & Winds:** Low-pressure trough develops over the Northern Plains. - **Local Winds:** - **Loo:** Strong, gusty, hot, dry winds blowing during the day over the North and Northwestern India. - **Kal Baisakhi:** Localised thunderstorms associated with violent winds, torrential downpours, often with hail, in West Bengal and Assam. - **Mango Showers:** Pre-monsoon showers in Kerala and Karnataka that help in the ripening of mangoes. #### 3. Advancing Monsoon (Rainy Season) - **Duration:** June to September. - **Onset:** Early June, progresses rapidly. - **Rainfall:** Widespread, heavy rainfall over most parts of India. - **Monsoon Break:** Interruption in the monsoon rainfall. These breaks are related to the movement of the monsoon trough. - **Distribution:** Western Ghats receive very heavy rainfall. Interior Deccan Plateau and parts of Rajasthan receive scanty rainfall. Mawsynram (Meghalaya) receives the highest rainfall in the world. #### 4. Retreating/Post Monsoon (Autumn) - **Duration:** October to November. - **Pressure & Winds:** Monsoon trough weakens, low pressure shifts to the south. North-East monsoon winds blow from land to sea. - **Weather:** Clear skies and rise in temperature. The land is still moist, leading to oppressive weather known as 'October Heat'. - **Rainfall:** Cyclonic depressions originating over the Andaman Sea cause heavy rainfall on the eastern coast, especially Tamil Nadu. ### Distribution of Rainfall - **High Rainfall:** Western Coast, Northeastern India (over 400 cm). - **Moderate Rainfall:** Deccan Plateau, Ganga plains (100-200 cm). - **Low Rainfall:** Parts of Gujarat, Rajasthan, Ladakh (less than 60 cm). - **Snowfall:** Restricted to the Himalayan region. - **Variability:** Rainfall is highly variable year to year. ### Monsoon as a Unifying Bond - The Indian landscape, its animal and plant life, its entire agricultural calendar, and the life of the people including their festivals, revolve around the monsoon. - The monsoon provides water for agriculture, which is the backbone of India's economy. - The arrival of the monsoon is awaited with great enthusiasm, strengthening a sense of unity across the country.