Met-Olympiad-Study-Material-Senior PDF 2025

NATIONAL METEOROLOGICAL OLYMPIAD on Weather & Climate Science for Society 2025 Jointly organized by India Meteorological Department, South Asian Meteorological Association and Indian Meteorological Society Study Materi...

NATIONAL METEOROLOGICAL OLYMPIAD on Weather & Climate Science for Society 2025 Jointly organized by India Meteorological Department, South Asian Meteorological Association and Indian Meteorological Society Study Materials (Senior) i  Authors Dr. Ananda Kumar Das Dr. M. T. Bushair Dr. Poulomi Chakravarty Dr. S.N. Mishra Editors Dr Prashant Kodgire Dr. Someshwar Das AVM (Retd) Dr Ajit Tyagi Dr. M. Mohapatra Support Team: Dr Swagata Payra, Mr. Nimish Pandey, Mr. Ankit Yadav, Ms. Laxmi Patak, Dr TV Lakshmi Kumar Partners India Meteorological Department (IMD) South Asian Meteorological Association (SAMA) India Meteorological Society (IMS) Vidyarthi Vigyan Manthan (VVM) ii  Content P. No. Chapter-1, Introduction to Weather & Climate System………………….…………………………… 1-9 1.1 Introduction to Weather and Climate………….…………………………………….……...…………….. 1 1.2 The Earth in the Solar System………………………………………………………..…………………… 5 1.3 Origin and Evolution of Our Atmosphere and Ocean…………………………………………………...... 6 1.4 Composition and Structure of Atmosphere and Ocean……………………………………………...……. 7 1.5. Role of Atmosphere and Ocean…………………………………………….……………....……………... 8 1.6 Land, Air, Water, and Ecosystem……………………..……………………….………...………………... 9 Chapter 2: Weather and Climate Processes………...……………….……………………….………… 10-27 2.1 Principles of Weather and Climate Systems……………..……………………………………………….. 10 2.2 Weather and Climate forecasting process………………………………………………………………… 13 2.3 Dynamics if atmosphere and ocean …………………………………………………………………….. 17 2.4 Seasons ……………….………………………………………………..………………………………... 22 2.5 Rainfall and Monsoons...………………………………………….……………………………………… 23 2.5 Basic Atmospheric Parameters and Their Measurements……………….………………………………... 27 Chapter 3: Natural Hazards and Disasters……………………………………..…………...……...... 28-44 3.1. Introduction to Natural Hazards……………...…………………….……...……………………………… 28 3.2 Hydro-meteorological hazards over India………………………….……………………………………... 29 3.3. Cyclones…………………………………………………...……………………………………………… 29 3.4. Thunderstorm and Lightning…………...…………………….…………….…………………………....... 32 3.5. Heat Waves and Cold Waves……………………………………………………………………………... 36 3.6. Floods and Droughts……………………………………………………………………………………… 39 3.7 Fog……………………………………………………………………….………...……………………… 42 Chapter 4: Weather and Climate, Hazard Prediction, Warning Systems, and Disaster Management………………………………………………………………………………..................... 45-57 4.1. Prediction of Natural Hazards………………………………...………………………….………………... 45 4.2. Disaster Preparedness and Management……………………………………..…………….…...………..... 54 iii 4.3. IMD’s Website: A Vital Resource for Weather Forecasting and Disaster Management………………….. 55 4.4 Involving Communities in Hazard Preparedness…………………………...……………….…...……....... 57 Chapter 5: Climate Change: Causes, Impacts, Mitigation and Adaptation………………………...... 58-67 5.1. Climate Change…………………………………………………………………...…………….…………. 58 5.2. Global Warming and Climate Change…………………………………………………………………….. 60 5.3 Climate Change: Proof for Climate Change……………………………………………………………..... 60 5.4 Impacts of Climate Change………………………………………………………………………………... 61 5.5 International and National Initiatives……………………….…………………...………………………… 63 5.6 Mitigation to Climate Change……………………………………………….…………………………….. 65 5.7 Adaptation to Climate Change….………………………………………………………………….…….. 67 Chapter 6: Climate Services to Socio-economic sectors…..………………………………………..... 68-73 6.1 Introduction………………………………………………………………………………………………... 68 6.2 Global Framework for Climate Services (GFCS) ……………………………………………….………... 68 6.3 National Framework for Climate Services (NFCS) ……………………………………..………………... 69 6.4 Climate Services by IMD and IITM…………………………………………………...…………………... 70 6.5 Applications of Weather and Climate Information in Socio-Economic Sectors……………...…………… 72 List of Figures Figure 1.1: The Solar System …………………….…………………………..…………..…………………….…... 5 iv Figure 1.2: The Earth System …………………………………………………………………………………….… 6 Figure 1.4: The Atmosphere ………………………………………..…………..………………………………...… 8 Figure 2.1: Type of clouds ……. ……………………………………………………………………...…………... 11 Figure 2.2: IMD’s Weather Forescasting System……………………………………………………...…………... 13 Figure 2.3: Integrated Observing System……………………………………………………………………………. 14 Figure 2.4: IMD Surface Observatory and Radiosonde……………………………………………………………. 15 Figure 2.5: Satellite Image of Cyclone…………………………………………………………………………….. 15 Figure 2.6: A Squall line observed by Doppler radar at Agartala on 11 May 2011…………………………………. 16 Figure 2.7: Source Centre for climate and energy solution ……………………..…………………………………. 18 Figure 2.8: Atmospheric Circulation………………………………………………………………………………… 20 Figure 2.9: Weather System and Extreme Weather in India…………………………………………………………... 20 Figure 2.10: The map of Indian climatic zones ……………………………………………………………………… 22 Figure 2.11. : All India District wise annual rainfall normals using the data during 1971-2020……………………….. 23 Figure 2.12. : Map of Onset of Southwest Monsoon in India ……………………..………………………………... 24 Figure 2.13 : Map of All India Districtwise Rainfall Normals using the data during 1971-2020 for the southwest Monsoon season (June to September) …………………………………………………………………….…………... 25 Figure 2.14. : District-wise Post Monsoon Season Rainfall…………………….…………………………………….. 27 Figure 3.1. Vertical structure of tropical cyclone & Image of cyclone captured from satellite.………………………. 30 Figure 3.2 Cyclone hazard prone districts of India based on frequency of total cyclones, total severe cyclones, actual/estimated maximum wind, probable maximum storm surge associated with the cyclones and probable maximum precipitation for all districts………………………..………………………………………...………….. 32 Figure 3.3 Simplified model depicting the life cycle of an ordinary thunderstorm that is nearly stationary. (Arrows show vertical air currents. Dashed line represents freezing level, 0°C isotherm.)……………………….. 33 Figure 3.4: Annual thunderstorm climatology (average number of thunderstorm days) over India based on the data of the period 1981-2019…………………………………………………………………………..………….. 34 Figure 3.5: Total number of lightening days in annual over India based on the data of the period 1969-2019.......... 35 Figure 3.6 Total heat wave days (Total Number of Disasterous Heat Wave Days) using the data during the Period from 1969 to 2019……………………………………………………………………………..…………. 37 Figure 3.7 Total cold wave days (Total Number of Disasterous Heat Wave Days) using the data during the Period from 1969 to 2019…………………………………………………………………………………………. 38 Figure 3.8: A Comprehensive Guide for Do's and Don'ts During Flood………………………………………......... 40 Figure 3.9: Total Number of Flood Events during the Period from 1969 to 2019 & Drought Normalized Vulnerability Index over India based on Standardized Precipitation Index ………. …………………………… 41 Figure 3.10: An image depicting parts of Delhi on a foggy day……………………………………………… 44 Figure 3.11: 13 Fog blanket extensively covering a large part of the Indo-Gangetic plains, as detected by the INSAT-3D satellite at 08:30 hours on January 5, 2018…………………………………………………………… 44 Figure 4.1: The matrix Decision of Colour Code for Warning……………………………………………………. 47 Figure 4.2 Operational forecast by the IMD for the track of Cyclone ‘DANA,’ ……………………………………... 49 Figure 4.3 IMD’s multi-hazard warning map ………………………………………………………………………... 53 Figure 4.4: Screenshot of IMD’s website…………………………………………………………………………. 56 Figure 5.1: H2 Concentration Measured at Mauna Loa Lab, Source: Global Climate Change (NASA)…….…... 59 Figure 6.1: Five foundational pillars of GFCS…………………………………………………………………….. 69 v  Chapter 1 Introduction to Weather & Climate System 1.1. Introduction to Weather and Climate Weather and climate are fundamental components of Earth's environmental system, influencing all living organisms, human activities, agriculture, and water resources. Understanding the difference between weather and climate is essential for comprehending their impact on society and the natural world. Weather refers to the short-term atmospheric conditions in a specific place at a particular time, including temperature, humidity, precipitation, wind speed, and visibility. These conditions can change frequently, often within minutes, hours, or days. For example, a sunny morning can quickly turn into a rainy afternoon due to shifts in atmospheric pressure, temperature, and moisture levels. Climate, on the other hand, refers to the long-term patterns and averages of weather conditions in a region, typically measured over 30 years or more. It encompasses trends in temperature, humidity, wind, and precipitation over extended periods. For instance, a tropical climate is characterized by hot, humid conditions with significant rainfall throughout the year, while a polar climate is defined by persistently cold temperatures. 1.1.1. Importance of Weather and Climate Weather and climate have far-reaching impacts on ecosystems, agriculture, infrastructure, human health, and economies. They control the distribution of water through rain, snow, and other forms of precipitation, which directly affects agriculture and freshwater availability. Extreme weather events such as droughts, floods, hurricanes, and heatwaves can lead to food shortages, water scarcity, property damage, and even loss of life. A notable example is the 1999 Odisha Super Cyclone in India, which resulted in nearly ten thousand deaths and displaced over a million people. Understanding and monitoring weather patterns are essential for disaster preparedness, while studying climate trends is critical for predicting long-term changes that influence natural resources, human health, energy supplies, infrastructure, and transportation systems. Changes in climate can alter agricultural productivity, biodiversity, and water availability, presenting significant long-term challenges for human societies and the environment. 1.1.2. Climate Impact on Society Climate has a significant impact on our daily lives, natural resources, and ecosystems. Weather patterns influence the distribution of water on Earth, which is essential for the survival of all living organisms. The availability of fresh water for drinking and agriculture depends on weather patterns and seasonal changes. For example, droughts can devastate agricultural production, leading to food shortages and the loss of livelihoods. Historical records reveal that severe droughts have caused millions of deaths worldwide. Extreme weather events, such as hurricanes, tornadoes, floods, and heatwaves, profoundly affect human civilization. For instance, the Odisha Super Cyclone in 1999 resulted in nearly 10,000 deaths and displaced over a million people. These catastrophic events highlight the importance of understanding weather and climate to minimize their societal impact. Climate Change poses new challenges to society by impacting health, infrastructure, food and water security, and ecosystems. The 1 frequency and intensity of natural disasters are increasing, emphasizing the need for society to adapt to changing weather patterns and mitigate its adverse effects. 1.1.3. Weather and Climate Services To monitor and predict weather and climate patterns, scientists use advanced observational techniques and technologies. Data on atmospheric conditions such as temperature, humidity, pressure, wind speed, and precipitation are gathered globally through surface weather stations, satellites, weather radars, and upper-air measurements. These observations form the foundation for weather forecasts, climate research, and disaster preparedness. Surface observations are conducted at weather stations worldwide, which use instruments like thermometers (to measure temperature), barometers (for pressure), hygrometers (to measure humidity), anemometers (for wind speed), and rain gauges (to track precipitation). Upper air data is collected using pilot balloons and radiosondes, providing crucial information on temperature, humidity, and pressure at various altitudes, which is key to predicting weather patterns. Weather radars detect precipitation, storm intensity, and movement, offering real-time monitoring of hazardous weather phenomena such as heavy rainfall, thunderstorms, and cyclones, thereby facilitating timely warnings. Satellites provide a global perspective by monitoring land, oceans, and the atmosphere, collecting data on cloud formation, sea surface temperatures, rainfall, and upper atmospheric conditions. For instance, the INSAT series of satellites in India contributes significantly to weather forecasting and climate research by providing essential meteorological data. 1.1.4. Ancient Meteorology The beginnings of meteorology in India can be traced to ancient times. Early philosophical writings of the 3000 B.C. era, such as the Upanishadas, contain serious discussion about the processes of cloud formation and rain and the seasonal cycles caused by the movement of earth round the sun. Varahamihira's classical work, the Brihatsamhita, written around 500 A.D. provides clear evidence that a deep knowledge of atmospheric processes existed even in those times. It was understood that rains come from the sun (Adityat Jayate Vrishti) and that good rainfall in the rainy season was the key to bountiful agriculture and food for the people. Kautilya's Arthashastra contains records of scientific measurements of rainfall and its application to the country's revenue and relief work. Kalidasa in his epic, 'Meghdoot', written around the seventh century, even mentions the date of onset of the monsoon over central India and traces the path of the monsoon clouds. Meteorologica written by Aristotle around 350 BCE is one of earliest treatise on meteorology. Long before the development of modern scientific instruments, cultures around the world attempted to explain weather patterns through a combination of observation, mythology and rudimentary techniques. Many folklores developed in earlier periods hold good even today. Meteorology, as we perceive it now, may be said to have had its firm scientific foundation in the 17th century after the invention of the thermometer and the barometer and the formulation of laws governing the behaviour of atmospheric gases. It was in 1636 that Halley, a British scientist, published his treatise on the Indian summer monsoon, which he attributed to a seasonal reversal of winds due to the differential heating of the Asian land mass and the Indian Ocean. India is fortunate to have some of the oldest meteorological observatories of the world. The British East India Company established several such stations, for example, those at Calcutta in 1785 and Madras (now Chennai) in 1796 for studying the weather and climate of India. The Asiatic Society of Bengal 2 founded in 1784 at Calcutta. A disastrous tropical cyclone struck Calcutta in 1864 and this was followed by failures of the monsoon rains in 1866 and 1871. In the year 1875, the Government of India established the India Meteorological Department, bringing all meteorological work in the country under a central authority. Mr. H. F. Blanford was appointed Meteorological Reporter to the Government of India. The first Director of Bombay (now Mumbai), promoted scientific studies in meteorology in India. Captain Harry Piddington at Calcutta published 40 papers during 1835-1855 in the Journal of the Asiatic Society dealing with tropical storms and coined the word "cyclone", meaning the coil of a snake. In 1842 he published his monumental work on the "Laws of the Storms". In the first half of the 19th century, several observatories began functioning in India under the provincial governments. 1.1.5. India Meteorological Department (IMD) A disastrous tropical cyclone struck Calcutta in 1864 and this was followed by failures of the monsoon rains in 1866 and 1871. In the year 1875, the Government of India (under British rule) established the India Meteorological Department, bringing all meteorological work in the country under a central authority, with its headquarters at Calcutta (now Kolkata). Mr. H. F. Blanford was appointed Meteorological Reporter to the Government of India. The IMD, is India’s primary agency for meteorological observations, and weather forecasting. Current national Headquarters of the IMD is located in New Delhi. It plays a critical role in monitoring weather patterns and issuing forecasts that aid agriculture, aviation, shipping, disaster management, and other sectors vital to the economy and public safety. IMD’s responsibilities include cyclone warnings, which help coastal regions prepare for storms, and monsoon forecasts, essential for agriculture and water management. Equipped with a network of weather stations, radars, and satellites, IMD continuously monitors atmospheric conditions, providing real-time updates to the public while supporting climate research. IMD provides its services up to village level through six Regions Meteorological Centres located at Delhi, Mumbai, Kolkata, Chennai, Guwahati, Nagpur and State Met Centres located at the state capitals. IMD has also established itself as a global leader in weather science. It was among the first meteorological institutions to adopt telegraphy for weather communication, setting up observatories that contributed to international data sharing. Its historical focus on monsoon and cyclone forecasting has made IMD a key contributor to the World Meteorological Organization (WMO) and a pioneer in global weather science. Monsoon forecasting is one of IMD’s most significant contributions. The Indian monsoon is crucial for the nation’s agriculture and water resources, and its impact extends globally, affecting the climate of the Indian Ocean region and Southeast Asia. IMD’s expertise in understanding atmospheric patterns, such as the El Niño-Southern Oscillation (ENSO), contributes to international efforts in climate prediction. The department also collaborates with initiatives like the Asian-Australian Monsoon Project, advancing global monsoon research. Cyclone forecasting is another area where IMD excels. Using advanced numerical weather prediction (NWP) models and satellite observations, IMD accurately predicts the intensity and path of cyclones in the Bay of Bengal and the Arabian Sea. Its Regional Specialized Meteorological Centre (RSMC) provides cyclone advisories to neighbouring countries, helping to minimize the loss of life and property. This service is part of the global early warning system under the WMO framework. 3 IMD’s technological advancements have further enhanced global weather forecasting. The department utilizes India’s INSAT and Oceansat series of satellites, providing real-time weather data shared with international organizations. Its network of Doppler Weather Radars (DWRs) has improved short-term forecasts and monitoring of severe weather events. IMD also uses sophisticated NWP models like the Global Forecast System (GFS) and the Weather Research and Forecasting (WRF) model, contributing to global weather predictions. In addition to its technological contributions, IMD collaborates with global organizations like the WMO and UNFCCC, providing technical assistance and training to regional meteorologists. It leads initiatives such as the South Asian Climate Outlook Forum (SASCOF), offering seasonal climate forecasts that support resilience in South Asia. IMD’s research contributions, especially in climate change studies, have been invaluable. Its long-term climate records are critical for analyzing trends in extreme weather events, and its work in developing climate adaptation strategies benefits global climate resilience efforts. IMD also offers essential services to marine and aviation sectors, providing weather forecasts for maritime operations and real-time weather information for air travel safety. Its oceanographic data contributes to global programs like the Global Ocean Observing System (GOOS), while its aviation services are integral to both domestic and international flights. Looking ahead, IMD continues to expand its role in global weather science, enhancing its forecasting capabilities through initiatives like the Monsoon Mission and exploring the use of artificial intelligence and machine learning. Its ongoing leadership in weather forecasting, climate research, and disaster management ensures that IMD will remain a key player in global meteorology for years to come. Further details on evolution of India Meteorological department are described in Appendix-B 1.1.6. World Meteorological Organization (WMO) The World Meteorological Organization (WMO), established in 1950, with its headquarters at Geneva (Switzerland) is a specialized agency of the United Nations that coordinates global efforts in weather, climate, and water resource monitoring. Its mission is to promote international cooperation in sharing meteorological data and information, ensuring standardized methods for observation and forecasting. WMO’s global network of observational systems forms the foundation of weather forecasting, climate research, and disaster risk reduction. This coordination helps improve weather predictions and supports international climate science efforts. One of WMO's primary functions is to support climate change research by compiling and analyzing weather and climate data from around the world. This data is crucial for informing policymakers and assisting them in developing effective mitigation and adaptation strategies to address the impacts of climate change. The organization’s Global Framework for Climate Services (GFCS) ensures that climate-sensitive sectors, such as agriculture and disaster management, have access to reliable climate information. WMO also plays a significant role in disaster risk reduction by enhancing early warning systems for extreme weather events like cyclones, floods, and droughts. Through initiatives such as the Multi- Hazard Early Warning Systems (MHEWS) and Regional Specialized Meteorological Centers (RSMCs), WMO provides timely warnings to vulnerable regions, helping to mitigate the impacts of these hazards. 4 In addition, WMO is involved in capacity building, particularly in developing countries. India has been an active member of the WMO since its inception. It provides training, resources, and technical support to strengthen national meteorological and hydrological services. This support helps improve local forecasting abilities and enhances preparedness for weather-related disasters. By fostering international collaboration and providing critical weather and climate services, WMO plays a vital role in addressing the global challenges posed by climate variability and extreme weather events. 1.2. The Earth in the Solar System The Earth is one of the eight planets in our solar system, part of the vast Milky Way galaxy. Our solar system consists of the Sun, the eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), their moons, and other celestial objects like asteroids and comets (Figure 1.1). Earth is the third planet from the Sun and is unique because it supports life. This ability is due to several factors: its perfect distance from the Sun, the presence of liquid water, and an atmosphere that contains essential gases like oxygen. These features allow life to thrive in various forms, from microscopic bacteria to large mammals like elephants and whales. The Earth is like a giant system made up of four main parts that work together to support life. The atmosphere is the layer of air that surrounds our planet, providing oxygen for us to breathe and protecting us from the Sun’s harmful rays. It also helps regulate the Earth's temperature. A conceptual diagram of the Earth system is shown in the figure 1.2. The biosphere includes all living things, like plants, animals, and humans, and it depends on the other parts of the Earth system to survive. The lithosphere is the solid, outer layer of the Earth made up of rocks and soil. It forms mountains, valleys, and the land we live on. Lastly, the hydrosphere covers all the water on Earth, including oceans, rivers, lakes, and even the water in the air. These parts are connected, and any change in one can affect the others. Together, they create the perfect balance that makes life on Earth possible. Figure 1.1: The Solar System (Image Credit: Pixabay) 5 Figure 1.2: The Earth System (Image Credit: ID 210304128 © VectorMine| Dreamstime.com) 1.2.1. The Sun: The Sun is a giant star at the center of our solar system. It is composed mainly of Hydrogen and Helium gases. The Sun's immense energy powers weather systems and climate patterns on Earth. The sunlight we receive is crucial for processes like photosynthesis, which allows plants to produce food and release oxygen—both vital for life. 1.3. Origin and Evolution of our Atmosphere and Ocean 1.3.1. Origin of the Atmosphere: The Earth's atmosphere was formed about 4.5 billion years ago during the planet's early development. Initially, the atmosphere was composed mainly of hydrogen and helium, captured from a cloud of gas and dust called the solar nebula. This primary atmosphere was lost because the Earth's gravity was too weak to hold onto these light gases, and the strong solar wind blew them away. A new atmosphere, known as the secondary atmosphere, formed from gases released by volcanic eruptions, a process called volcanic outgassing. Volcanic eruptions released water vapor, carbon dioxide, ammonia, and methane into the atmosphere. These gases were crucial in creating an environment that could support life. 1.3.2. Evolution of the Atmosphere: As the Earth cooled, water vapor in the atmosphere condensed to form liquid water, leading to the creation of oceans. This was a pivotal moment in Earth's history, as it allowed for the development of life. Early life forms, such as cyanobacteria, began using sunlight to make food through photosynthesis, a process that produces oxygen. Over millions of years, oxygen accumulated in the atmosphere, forming the ozone layer. The ozone layer is important because it protects life on Earth from the Sun's harmful ultraviolet (UV) rays, allowing more complex life forms to develop. 6 1.3.3. Formation of the Oceans: The formation of oceans is closely linked to the atmosphere. Water vapor released by volcanic activity and possibly from icy comets that collided with Earth contributed to the early oceans. These bodies of water played a vital role in regulating the planet's climate and providing a habitat for early life. The oceans cover about 71% of the Earth's surface and play a crucial role in regulating the planet's climate. They absorb carbon dioxide from the atmosphere, helping to stabilize global temperatures. Oceans also support diverse ecosystems, ranging from tiny plankton to large marine mammals. 1.4. Composition and Structure of Atmosphere and Ocean 1.4.1. Composition of the Atmosphere: The Earth's atmosphere is composed of several gases: nitrogen (78%), oxygen (21%), and trace amounts of other gases such as argon, carbon dioxide (0.04%), and neon. The presence of water vapor and greenhouse gases like carbon dioxide and methane is vital for maintaining the Earth's temperature. Structure of the Atmosphere: The Earth's atmosphere consists of several layers, each with distinct characteristics. A conceptual diagram of the layers of atmosphere is shown in the figure 1.3. The troposphere is the lowest layer, extending up to about 12 kilometers (7.5 miles) above the Earth's surface. It is where all weather occurs and contains about 75% of the atmosphere's mass. In the troposphere, temperatures decrease as you go higher. The stratosphere is located above the troposphere and extends from 12 to 50 kilometers (7.5 to 31 miles) high. This layer contains the ozone layer, which absorbs and scatters UV radiation from the Sun. In the stratosphere, temperatures increase with altitude. The mesosphere extends from 50 to 85 kilometers (31 to 53 miles) above the Earth. In the mesosphere, temperatures decrease with altitude, and it is where most meteorites burn up upon entering the Earth's atmosphere. The thermosphere ranges from 85 to 600 kilometers (53 to 373 miles) above the Earth. It is characterized by very high temperatures due to the absorption of high-energy radiation from the Sun. The thermosphere is also where the auroras (northern and southern lights) occur. The exosphere is the outermost layer of the atmosphere, extending from the thermosphere into space. It is composed of very sparse particles that gradually escape into space. 7  Figure 1.3: The Atmosphere (Image credit: Freepik images) 1.4.2. Composition of the Ocean: The Earth's oceans cover about 71% of the planet's surface and contain 97% of the Earth's water. Ocean water is composed of 96.5% water and 3.5% dissolved salts, mainly sodium chloride. 1.4.3. Structure of the Ocean: The Earth's oceans are vast and deep, and they play a critical role in regulating the planet's climate and supporting life: The surface layer is the uppermost part of the ocean, extending to about 200 meters (656 feet). It is well-mixed by wind and waves, allowing sunlight to penetrate and support photosynthetic life, such as algae. The thermocline is a layer in the ocean where temperatures decrease rapidly with depth. The thermocline acts as a barrier between the warm surface waters and the cold deep ocean. The deep ocean extends from the thermocline to the ocean floor. It is characterized by cold temperatures, high pressure, and limited light. Despite the harsh conditions, it is home to a variety of life forms adapted to these environments. 1.5. Role of Atmosphere and Ocean 1.5.1. Role of the Atmosphere: The atmosphere regulates temperature by trapping heat through the greenhouse effect, which keeps the Earth's surface warm enough to support life. Without this effect, the planet would be too cold for most life forms. The atmosphere also distributes heat around the planet through wind and weather systems. This redistribution helps maintain a relatively stable climate across different regions. Finally, the atmosphere protects life from harmful solar radiation, particularly UV rays, through the ozone layer. 8 1.5.2. Role of the Oceans: The oceans are equally vital in maintaining the Earth's climate and supporting life. The oceans act as a heat reservoir by absorbing and storing solar energy. This stored heat helps moderate global temperatures and influence weather patterns. Ocean currents regulate climate by distributing heat around the globe, affecting regional climates. For example, the Gulf Stream carries warm water from the tropics to the North Atlantic, moderating the climate of Western Europe. The oceans support marine ecosystems by providing habitats and resources for a diverse range of marine life. They are also a source of food and oxygen, playing a crucial role in the Earth's biosphere. 1.6. Land, Air, Water, and Ecosystem Land: The land includes various landscapes such as mountains, plains, and valleys, providing habitats and resources for terrestrial life. Landforms can affect weather patterns, such as mountains influencing wind and precipitation. Air: The air is essential for respiration and photosynthesis, maintaining the balance of gases necessary for life. The air's composition includes nitrogen (78%), oxygen (21%), and trace gases. Water: Water covers most of the Earth's surface and exists in various forms: liquid (oceans, rivers, lakes), solid (ice, snow), and gas (water vapor). Water is vital for life, weather, and climate systems. Ecosystem: An ecosystem consists of living organisms interacting with their physical environment. Ecosystems include biomes such as forests, deserts, tundras, and aquatic systems, each supporting different life forms. 9  Chapter 2 Weather and Climate Processes 2.1 Principles of Weather and Climate Systems Understanding the principles of weather and climate systems is vital for comprehending how these natural processes impact our daily lives and the planet as a whole. Weather and climate shape ecosystems, agriculture, water resources, and even human activities. By studying these processes, we can better prepare for weather changes and understand long-term climate trends, which is increasingly important in the context of climate change. 2.1.1 Weather: Short-Term Atmospheric Conditions Weather refers to the short-term state of the atmosphere at a specific place and time. It encompasses various elements, including temperature, humidity, precipitation, wind, and visibility. Weather can change rapidly, often within minutes or hours, and is influenced by several factors such as atmospheric pressure, temperature differences, moisture levels, and the movement of air masses. Each of these elements plays a critical role in defining the weather of a particular area: Temperature: This measures how hot or cold the atmosphere is at a given place and time. Temperature is typically measured in degrees Celsius (°C) or Fahrenheit (°F). The Sun is the primary source of heat energy, warming the Earth's surface. When the Sun's rays hit the Earth, they warm the land, air, and water, resulting in varying temperatures across different regions. During the day, temperatures are generally higher because of direct sunlight, while at night, the absence of sunlight causes the temperature to drop. Humidity: Humidity refers to the amount of water vapor present in the air. It is usually expressed as a percentage, indicating how saturated the air is with moisture. For example, if the humidity is 100%, the air is fully saturated and cannot hold more water vapor, which often leads to the formation of clouds and precipitation. High humidity levels can make warm temperatures feel even hotter because the body's ability to cool itself through sweat evaporation is reduced. Humidity plays a crucial role in weather patterns, especially in tropical climates, where high humidity levels can lead to frequent rainfall. Precipitation: Precipitation includes all forms of water—liquid or solid—that fall from clouds to the ground. This includes rain, snow, sleet, and hail. Precipitation occurs when clouds, which are composed of tiny water droplets or ice crystals, become too heavy to remain suspended in the air. When this happens, gravity pulls the water down to Earth. Precipitation is essential for maintaining freshwater supplies and supporting plant and animal life. Wind: Wind is the movement of air from areas of high pressure to areas of low pressure. It is caused by the uneven heating of the Earth's surface by the Sun. Wind speed and direction are influenced by factors such as the Earth's rotation, pressure systems, and geographic features like mountains and oceans. Winds play a vital role in transporting heat, moisture, and even pollutants around the globe, influencing weather patterns. For example, sea breezes occur when cooler air from the sea moves toward the land, creating a refreshing breeze during the day. 10  Visibility: Visibility is a measure of the distance one can clearly see. It is affected by weather conditions such as fog, rain, snow, or smoke. Reduced visibility can impact daily activities, such as driving and air travel, making it important for weather forecasts to include visibility reports. Clouds: Formation, Categories, and Role in Rainfall: Clouds play a crucial role in weather forecasting, as their types and patterns can provide early signs of changing weather conditions. Clouds form when warm, moist air rises and cools, causing the water vapor within it to condense into tiny droplets. These droplets cluster together to create clouds. Clouds are classified based on their appearance, height, and the weather they bring. There are three main types: cirrus, cumulus, and stratus. Cirrus clouds are high, thin, and wispy, often seen in fair weather. They are often seen ahead of a change in the weather, such as an approaching storm. Cumulus clouds are fluffy, white clouds with a flat base, typically indicating fair weather. However, when they grow larger, they can develop into cumulonimbus clouds, which are towering and dark, bringing thunderstorms, heavy rain, and sometimes even hail. Stratus clouds form low, gray layers that cover the sky, bringing overcast weather or light rain/drizzle. A graphical representation of different types of clouds based on its height is Figure 2.1. Figure 2.1: Cloud types (Image credit (Science Facts) Weather phenomena: Weather phenomena refer to various atmospheric events and conditions that occur naturally in the Earth's atmosphere and affect different regions around the globe. These phenomena are driven by complex interactions between temperature, humidity, air pressure, and wind patterns. Common weather phenomena include precipitation events such as 11  rain, snow, sleet, and hail, as well as storms like thunderstorms, cyclones, tornadoes, and hurricanes. Lightning, thunder, and windstorms are also categorized under weather phenomena. Fog and mist, which reduce visibility, are other examples. Severe weather phenomena, like heatwaves, cold snaps, and droughts, can significantly impact human activities, agriculture, and the environment. Weather phenomena vary widely depending on geographic location and the season, and their study helps meteorologists forecast weather patterns and develop strategies for mitigating their effects on communities. Understanding these phenomena is crucial for safety, disaster preparedness, and efficient management of natural resources. 2.2 Weather and Climate forecasting process Weather and climate forecasting involves using scientific principles and tools to predict atmospheric conditions over various time scales. Understanding the dynamics of the atmosphere and ocean is crucial for accurate forecasting. The forecasting system encompasses everything from seasonal predictions to real-time nowcasts, providing timely and accurate weather information for various sectors. Long Range Weather Prediction (LRF) focuses on seasonal patterns, such as the monsoon, months in advance by integrating oceanic, atmospheric, and land surface data to predict large-scale climate phenomena. Extended Range Weather Prediction (ERF) models provide forecasts ranging from 10 to 30 days, assisting in predicting trends like heatwaves and rainfall patterns. For medium-range forecasting (up to 10 days) and short-range forecasts (up to 3 days), Numerical Weather Prediction (NWP) models are utilized, enhancing accuracy through observed weather data from different sources. Nowcasting, which offers immediate forecasts (for the next 3 hours), employs high-resolution models and integrates radar, satellite, surface, and upper air observations to track weather events. A schematic diagram of IMD’s operational forecasting system is provided in the Figure 2.2. Figure 2.2: IMD’s Weather Forecasting System 12 2.2.1 Data Observations Weather forecasting begins with the collection of meteorological data, and the IMD has established a comprehensive network to gather this data from across the country, as well as global data through various collaborations. A map of integrated observing system is provided in the Figure 2.3. The data collection methods include: Surface Observatories: IMD operates around 206 meteorological observatories (synoptic stations) across India, which collect essential weather parameters like temperature, humidity, pressure, wind speed/direction, clouds, and rainfall, etc. These observatories form the backbone of IMD’s data network, ensuring continuous monitoring of weather patterns. In addition, IMD has also deployed around 735 automatic weather stations (AWS) (Figure 2.4a) and 1350 automatic rain gauge (ARG) stations, which are equipped with sensors to measure various weather parameters without the need for human intervention. These stations provide real-time data, particularly in remote areas where manual observations might be limited. Upper Air Observation: IMD conducts upper air observations using radiosondes (Figure 2.4b) and pilot balloons to measure atmospheric conditions at various altitudes. Currently, the India Meteorological Department (IMD) conducts radiosonde launches from 56 locations and pilot balloon launches from 62 locations across India. These upper air data include critical parameters like temperature, humidity, wind, and pressure at different heights, which are essential for accurate weather forecasting, particularly in tracking large-scale weather systems and cyclonic developments. Satellites: Satellite data plays an indispensable role in modern meteorological predictions. IMD utilizes the Indian Space Research Organization (ISRO)’s INSAT-3D, INSAT-3DR, INSAT-3DS, and Oceansat-3 satellites, which provide a continuous feed of meteorological data, including cloud cover, atmospheric temperature, humidity, sea surface temperatures, rainfall, wind speed & direction, clouds, cloud temperature, ocean surface wind, cyclone position, cyclone tracking, etc. Satellite image of cyclone “Mocha” captured from INSAT-3D satellite is shown in Figure 2.5. These satellites deliver real-time data crucial for weather forecasting and disaster warnings. IMD also utilizes weather satellites of foreign nations 13  Figure 2.3: Integrated Observing System (a) IMD Surface observatory (b) Radiosonde Figure 2.4: IMD Surface Observatory and Radiosonde 14  Figure 2.5: Satellite Image of Cyclone “Mocha” Figure 2.6: A Squall line observed by Doppler radar at Agartala on 11 May 2011. Doppler Weather Radars (DWR): The Doppler Weather Radar network is a crucial component of IMD’s forecasting infrastructure. These radars provide real-time information on rainfall intensity, storm structure, wind patterns, and the movement of cyclonic storms. IMD’s Doppler radars are particularly vital in predicting short-term weather phenomena like thunderstorms and severe storms. 15  A Squall line (narrow band of thunderstorms that can produce strong winds, heavy rain, and severe weather) observed by Doppler radar at Agartala on 11 May 2011 is shown in Figure 2.6. Global Data Networks: In addition to domestic data, IMD is part of global data-sharing networks facilitated by the World Meteorological Organization (WMO). This ensures that IMD has access to global weather data, which is essential for long-term forecasting and understanding broader weather patterns. 2.2.2 Climate: Long-Term Atmospheric Patterns Weather is a short-term phenomenon, often varying from day to day. For instance, it might be sunny in the morning and rainy by afternoon, or warm today and cold tomorrow. Understanding weather elements and how they interact helps meteorologists predict short-term weather patterns, allowing people to prepare for immediate changes. Climate, on the other hand, refers to the long-term average of weather patterns in a specific region, typically measured over 30 years or more. While weather is what happens in the atmosphere on a day-to-day basis, climate is the overall trend and pattern of these conditions over an extended period. Climate is influenced by several key factors: Latitude: This is the distance of a place from the equator, measured in degrees. Latitude plays a crucial role in climate because it affects the amount of solar energy a region receives. Areas near the equator receive direct sunlight year-round, resulting in warm, tropical climates. In contrast, regions near the poles receive less direct sunlight, leading to colder, polar climates. For example, equatorial regions like the Amazon rainforest experience hot and humid conditions, while the Arctic has a frigid polar climate. Altitude: Altitude is the height of a place above sea level. Higher altitudes tend to have cooler temperatures because the atmosphere becomes thinner, and air pressure decreases as you go higher. This is why mountainous regions, even those near the equator, can have colder climates than surrounding lowlands. For instance, the Himalayan mountains in Asia have snow-covered peaks year-round despite being in a tropical region. Ocean Currents: Ocean currents are large-scale movements of seawater that play a significant role in regulating climate. Warm currents, such as the Gulf Stream, carry heat from the tropics toward higher latitudes, influencing the climate of nearby coastal areas. For example, the Gulf Stream warms the climate of Western Europe, making it milder than other regions at similar latitudes. Conversely, cold currents can lower temperatures along coastlines, as seen in the cool waters off the west coast of South America. Human Activities: Activities such as deforestation, urbanization, and the burning of fossil fuels release greenhouse gases into the atmosphere, contributing to climate change. These human-induced changes can alter climate patterns by increasing the Earth's overall temperature, causing shifts in weather, sea level rise, and more frequent extreme weather events like hurricanes and droughts. Understanding climate is essential because it helps predict long-term changes and their potential impacts on the environment and human societies. For example, knowing that a region has a tropical climate with a rainy season helps farmers plan their planting and harvesting schedules. 16 2.2.3 Solar Radiation and Heat Balance: The Earth receives energy from the Sun in the form of solar radiation. This energy is absorbed, reflected, and emitted by the Earth's surface and atmosphere. The balance between incoming and outgoing radiation determines the Earth's temperature and climate. Factors such as greenhouse gases, albedo (reflectivity), and cloud cover influence this heat balance. Greenhouse Gases: These gases, such as carbon dioxide, methane, and water vapor, trap heat in the atmosphere and keep the Earth's surface warm. Albedo: This is the reflectivity of the Earth's surface. Surfaces like ice and snow have high albedo and reflect most of the sunlight, while darker surfaces like oceans and forests have low albedo and absorb more heat. ⚫ Cloud Cover: Clouds can both reflect sunlight back into space and trap heat the atmosphere influencing the Earth’s’ heat balance. Greenhouse Effect and Greenhouse Gases (GHGs) The greenhouse effect is a natural process that warms the Earth's surface. When the Sun's energy reaches Earth, some of it is reflected back into space, while the rest is absorbed and re-radiated by greenhouse gases. This effect is crucial for life on Earth, as it keeps the planet warm enough to sustain most forms of life. The natural greenhouse effect maintains the Earth's average surface temperature at around 15°C (59°F). Without it, the average temperature would be about -18°C (0°F), making the planet too cold for most life forms. This process is vital for sustaining life on Earth by maintaining a temperature suitable for habitation. Figure 2.7: Source Centre for climate and energy solution 17 Key greenhouse Gases Greenhouse gases trap heat in the Earth’s atmosphere and contribute to global warming (Fig 2.7). The most significant GHGs are: 1. Carbon Dioxide (CO₂): Released by burning fossil fuels (coal, oil, natural gas) and deforestation. 2. Methane (CH₄): Emitted during the production of coal, oil, and natural gas; also released by livestock and other agricultural practices. 3. Nitrous Oxide (N₂O): Mainly from agricultural activities, such as the use of fertilizers. 4. Water Vapor (H₂O): Naturally occurring but amplifies the warming effect. 5. Ozone (O₃): Exists naturally in the stratosphere, but human-made ozone at ground level is a pollutant. The Greenhouse Effect: The greenhouse effect is a natural process where greenhouse gases trap some of the sun’s heat in the form of terrestrial radiations, keeping Earth warm enough to support life. However, human activities have intensified this effect, leading to global warming. 2.2.4 Weather and Climate: Differences and Similarities Both weather and climate are influenced by similar processes, such as atmospheric pressure, solar energy, and the Earth's rotation, but they operate on different timescales. Weather is concerned with short-term changes in the atmosphere, while climate focuses on the long-term trends and averages of these weather patterns. Studying both is crucial for understanding the Earth's atmospheric behaviour and how it affects ecosystems, agriculture, and human activities. 2.3 Dynamics of Atmosphere and Ocean The dynamics of the atmosphere and ocean play a crucial role in shaping weather patterns and influencing climate variability. Atmospheric dynamics focuses on the movement of air, which is influenced by forces such as pressure gradients, the Coriolis effect, friction, and gravity. Solar radiation and Earth's energy balance drive atmospheric circulation patterns, including Hadley, Ferrel, and Rossby cells, which in turn create wind patterns like trade winds and the jet stream. These circulations impact air masses and fronts, leading to cloud formation and precipitation processes. Ocean dynamics involves the movement of water driven by ocean currents, wind stress, temperature and salinity gradients, and Earth's rotation (Coriolis effect). The interaction between the atmosphere and the ocean is key to understanding larger-scale weather systems, such as El Niño and La Niña, which have global climate impacts. Together, the interplay of these atmospheric and oceanic forces shapes the Earth's weather and climate. Atmospheric Dynamics: This refers to the study of the motion of air in the atmosphere, influenced by various forces, such as: Pressure gradients, Coriolis effect, Friction, Gravity. Ocean Dynamics: The movement of ocean water is affected by: Ocean currents, Wind stress, Temperature and salinity gradients, Earth's rotation (Coriolis effect). 18 The interaction between the atmosphere and ocean is vital in understanding weather patterns and climate variability. 2.3.1 Atmospheric Circulation One of the main drivers of both weather and climate is atmospheric circulation. The Earth's rotation and the uneven heating of its surface cause air to move, creating wind patterns that transport heat and moisture around the globe. This circulation is responsible for the movement of air masses and the formation of weather systems. A diagram of which represents atmospheric circulation is provided in the Figure 2.8. Key features of atmospheric circulation include: Trade Winds: Steady winds that flow from east to west between 30 degrees latitude and the equator in both hemispheres. These winds are crucial for moving warm ocean waters and influencing weather patterns in tropical regions. For example, they play a key role in bringing moisture-laden air to coastal areas, leading to rainfall. Westerlies: Winds that blow from west to east between 30 and 60 degrees latitude in both hemispheres. The westerlies influence weather patterns in the mid-latitudes, where many major cities are located. For instance, they bring warm air from the tropics to Europe, moderating its climate. Polar Easterlies: Winds that blow from east to west near the poles. They contribute to the formation of polar weather systems, which are characterized by extremely cold temperatures. These winds play a part in shaping weather patterns in regions like Antarctica and the Arctic. Figure 2.8: Atmospheric Circulation Atmospheric circulation not only dictates daily weather but also contributes to the long-term climate of a region. For example, the steady flow of trade winds helps create tropical rainforests near the equator, while the westerlies contribute to the temperate climates found in parts of North America and Europe. 19 2.3.2 Weather Systems: Weather systems, such as cyclones, anticyclones, and frontal systems, are formed by the interaction of air masses with different temperatures and humidity levels. These systems bring changes in weather conditions, including precipitation, wind, and temperature variations. Major weather affecting India is shown in Figure 2.9. Figure 2.9: Weather System and Extreme Weather in India Cyclones: Low-pressure systems characterized by inward spiraling winds and often associated with stormy weather. In the Northern Hemisphere, they rotate counterclockwise, while in the Southern Hemisphere, they rotate clockwise (Details in Chapter 3). Anticyclones: High-pressure systems characterized by outward spiraling winds and typically associated with calm and clear weather. In the Northern Hemisphere, they rotate clockwise, while in the Southern Hemisphere, they rotate counterclockwise. Frontal Systems: Boundaries between two air masses of different temperatures and humidity levels. They can cause dramatic weather changes, including storms and heavy precipitation. 2.3.3 Climate Systems: Climate systems are influenced by long-term factors such as ocean currents, volcanic activity, solar radiation variations, and human activities. These factors interact in complex ways to shape regional and global climate patterns. Understanding these interactions is crucial for predicting future climate changes and their potential impacts. Ocean Currents: Large-scale movements of seawater that distribute heat around the globe. For example, the Gulf Stream carries warm water from the tropics to the North Atlantic, moderating the climate of Western Europe. 20  Volcanic Activity: Volcanic eruptions can release large amounts of ash and gases into the atmosphere, affecting climate. For example, the eruption of Mount Pinatubo in 1991 caused global temperatures to drop temporarily. Solar Radiation Variations: Changes in the amount of solar energy reaching the Earth can influence climate. For example, periods of reduced solar activity, such as the Maunder Minimum, have been associated with cooler global temperatures. Human Activities: The burning of fossil fuels, deforestation, and industrial processes release greenhouse gases into the atmosphere, contributing to global warming and climate change. 2.3.4 Climate Classification Climate classification is a system used to categorize the world’s different climate types based on various factors like temperature, rainfall, and seasonal patterns. It helps scientists, farmers, and policymakers understand and predict the climate conditions of a particular region. By grouping areas with similar weather patterns, climate classification makes it easier to study global climate systems and how they affect agriculture, ecosystems, and human activities. For example, the Köppen Climate Classification divides climates into groups like tropical, dry, temperate, continental, and polar, based on temperature and precipitation. Similarly, the Thornthwaite Classification focuses on moisture availability and how water is used and stored in an area. The map of Indian climatic zones is depicted in Figure 2.10. Overall, climate classification helps us make informed decisions about water management, farming, and preparing for weather changes. Figure 2.10: The map of Indian climatic zones. 21 2.4 Seasons Winter: Winter occurs when a hemisphere is tilted away from the Sun, resulting in shorter days and cooler temperatures. In India, winter typically lasts from December to February. The weather is generally dry and cold, with occasional rain brought by Western Disturbances in northern regions. Snowfall occurs in mountainous areas. The Indo-Gangetic Plains (IGP) of northern India frequently experience fog and poor air quality, especially during the winter months. Fog occurs when cooler air traps moisture near the surface, leading to reduced visibility. This phenomenon is particularly common from November to January, when cooler temperatures and high humidity contribute to dense fog formation. The situation is exacerbated by human activities such as vehicular emissions, industrial pollution, and agricultural burning, particularly the practice of stubble burning in states like Punjab and Haryana. These pollutants, combined with the stagnant air, contribute to severe air pollution, forming a toxic mixture of fog and smog, often called "smog episodes." The lack of wind and atmospheric circulation traps pollutants close to the surface, leading to hazardous air quality. Poor air quality over the IGP not only disrupts transportation and daily activities due to reduced visibility but also poses significant health risks, including respiratory problems and other chronic conditions, affecting millions of people across densely populated areas. Pre-Monsoon (Summer): The pre-monsoon season, also known as summer, spans from March to May. During this time, temperatures rise significantly, especially in inland regions. Hot winds, known as "loo" in northern India, blow across the plains. This period also marks the buildup to the monsoon, with increased humidity and occasional thunderstorms. Southwest Monsoon (Rainy Season): The southwest monsoon season extends from June to September. Moist winds from the Indian Ocean and the Arabian Sea blow towards the land, bringing heavy rainfall across much of India. This rain is crucial for agriculture, replenishing water sources and supporting crop growth. However, excessive monsoon rains can lead to floods and landslides, especially in coastal and hilly regions. Post-Monsoon (Autumn): The post-monsoon season, or autumn, occurs from October to November. During this period, the southwest monsoon withdraws, and the weather becomes more stable. Temperatures begin to cool, and the skies are generally clear. This season is also known as the retreating monsoon, as the winds shift direction, leading to sporadic rainfall in certain regions like the southeastern coast of India. 2.5 Rainfall and Monsoons The monsoon is a seasonal wind system that plays a crucial role in shaping the climate and weather patterns of India and many other regions in South Asia. In India, the monsoon is divided into two primary phases: the Southwest Monsoon and the Northeast Monsoon, each with distinct characteristics and effects on the subcontinent. The climate and rainfall of India and its neighbourhood to a large extent is determined by the geographic location. India is an extension of the great Asiatic continent with the vast expanse of the Indian ocean to the south and the loftiest Himalayas to the north. The rainfall over India and neighbourhood is dominated by two monsoons - Southwest monsoon and Northeast monsoon. 22  Figure 2.11.: All India District wise annual rainfall normal using the data during 1971-2020. All India normal rainfall is 118 cm of which 75% (86.8 cm) rainfall takes place in the monsoon season. Rainfed agriculture in India covers 68% of the net sown area. Rainfall monitoring and forecast for rainfed agriculture areas is important as it accounts for almost 40% of the country's food production. Figure 2.11 shows the district-wise annual normal rainfall map of India, generated using data from 1971 to 2020. 2.5.1 Southwest Monsoon (June to September) The Southwest Monsoon is the most significant monsoon season in India, responsible for around 70- 90% of the country’s annual rainfall. It occurs from June to September and is crucial for the country's agriculture, economy, and water resources. 1. Mechanism: The Southwest Monsoon is driven by the differential heating of land and water. During summer, the Indian landmass heats up more quickly than the surrounding oceans, creating a low-pressure zone over the northern plains. At the same time, the Indian Ocean remains cooler, generating a high-pressure zone. This difference in pressure causes moist winds from the southwest, originating over the Arabian Sea and the Bay of Bengal, to move towards the Indian subcontinent. 2. Onset: The monsoon generally makes landfall over the Kerala coast around June 1st. It progresses northwards, reaching the central and northern parts of India by mid-July. The winds carry moisture from the ocean, which condenses as they move over land, leading to widespread 23 rainfall. The normal dates for onset of southwest monsoon over different part of the country is illustrated in the Figure 2.12. Figure 2.12.: Map of Onset of Southwest Monsoon in India (Image Credit: IMD) Rainfall Distribution: The Western Ghats and northeastern states receive the heaviest rainfall due to the orographic effect, where mountains force the moisture-laden air to rise, cool, and release rain. States like Kerala, Karnataka, Maharashtra, and the northeastern regions experience significant downpours. Central and northern India also benefit from the monsoon rains, which are vital for crops like rice, cotton, and sugarcane. Figure 2.13 shows the district-wise normal rainfall map of India during southwest monsoon season, generated using data from 1971 to 2020. 24 Figure 2.13: Map of All India District-wise Rainfall Normals using the data during 1971-2020 for the southwest Monsoon season (June to September) (Image Credit: IMD) The 4 zones of rainfall throughout India are categorized below. a). Insufficient Rainfall Zone (less than 50cm of rainfall): This rainfall zone is found in Andhra Pradesh, some regions of Karnataka as well as regions of Maharashtra, Ladakh, and a vast area of Rajasthan. Jaisalmer is an area in Rajasthan that counts for receiving the least rainfall in India. b). Low Rainfall Zone (50cm – 100cm of rainfall): This rainfall zone is found in Maharashtra, some areas in Gujarat, some places in Karnataka, Tamil Nadu, Andhra Pradesh, Madhya Pradesh, Punjab, Haryana, and sparsely in Western Uttar Pradesh. c). Medium Rainfall Zone (100cm – 200cm of rainfall): The various zones of abundant rainfall in India are geographically separated from each other. First, the Western Ghats have a thin strip that runs North-to-South across the Ghats’ entire length. The frequency of rainy seasons rises as one travels South. For example, the North has 4 rainy periods between June and September whereas the Midlands have 5 rainy months ranging from June to October. 25  d). High Rainfall Zone (200cm – 300cm of rainfall) : The most notable rainfall occurs on the West side, in the Western Ghats, including the sub-Himalayan areas of the upper East along Meghalaya’s slopes. The North-Eastern region and also the windward portion of the Central Highlands receive an average of 400cm of rain every year. Rain in the Brahmaputra Valley and its surrounding hills experience less than 200cm of rainfall. This zone includes locations that receive 200cm-300cm of rain each year. This zone is primarily found in Eastern India. 3. Impact: The Southwest Monsoon is essential for the Kharif crop season, and the livelihood of millions of farmers depends on its timely arrival and adequate distribution. However, the monsoon can also cause challenges such as flooding, landslides, and waterlogging, especially in regions prone to heavy rainfall like Assam, Bihar, and parts of Maharashtra. 2.5.2 Northeast Monsoon (October to December) The Northeast Monsoon, also known as the retreating monsoon, occurs from October to December and mainly affects the southeastern part of India, particularly Tamil Nadu, Andhra Pradesh, and parts of Karnataka and Kerala. 1. Mechanism: The Northeast Monsoon is caused by the reversal of wind patterns as the Indian subcontinent begins to cool after the intense heat of summer. By October, the low-pressure zone over the northern plains weakens, and a high-pressure zone forms over the landmass as temperatures drop. Cold, dry winds from the northeast blow toward the ocean, but as they pass over the Bay of Bengal, they pick up moisture and bring rainfall to southeastern India. 2. Onset: The Northeast Monsoon typically begins in October and continues until December. Unlike the Southwest Monsoon, which covers most of India, the Northeast Monsoon affects a smaller region, primarily the southeastern coastal areas. 3. Rainfall Distribution: Tamil Nadu receives about 50-60% of its annual rainfall from the Northeast Monsoon. Coastal areas like Chennai, Puducherry, and other parts of Tamil Nadu and southern Andhra Pradesh benefit from this monsoon. The rainfall is generally less intense compared to the Southwest Monsoon but is vital for the Rabi crops grown in these regions. Figure 2.14 shows the district-wise normal rainfall map of India during northeast monsoon season, generated using data from 1971 to 2020. 4. Impact: The Northeast Monsoon is important for replenishing water reservoirs and supporting agriculture in the southeastern states. However, it can also cause localized flooding, especially in low-lying areas and coastal cities like Chennai. 26  Figure 2.14.: District-wise Post Monsoon Season Rainfall (Image Credit: IMD) 2.5.3 Importance of Monsoons in India Monsoons play a critical role in India’s agrarian economy, determining water availability for agriculture, drinking water, and hydropower generation. The timely arrival and adequate spread of the monsoons are vital for the country’s food security. However, the monsoons also present challenges, as both excessive and deficient rainfall can lead to floods, droughts, and economic losses. Understanding and predicting monsoon patterns is therefore crucial for disaster preparedness, agricultural planning, and water resource management in India. 27  Chapter 3 Natural Hazards and Disasters 3.1. Introduction to Natural Hazards 3.1.1. Overview of Natural Hazards on Earth Natural hazards are naturally occurring events that can pose significant threats to human life, property, and the environment. These hazards are caused by different Earth processes, including atmospheric, geological, and hydrological phenomena. They vary in type, scale, and impact, ranging from localized events like landslides to widespread disasters like hurricanes or earthquakes. While natural hazards themselves are inevitable, their effects can be mitigated through preparedness, early warning systems, and effective disaster management strategies. 3.1.2 Types of Natural Hazards Natural hazards can be broadly categorized into Hydrometeorological hazards and Geological hazards: (i) Hydrometeorological Hazards These are hazards associated with weather and climate processes. They often stem from atmospheric conditions and include the following: Cyclones: Cyclones are large-scale storm systems characterized by low pressure at the center and strong winds rotating around it. These storms can cause extensive flooding, damage to infrastructure, and loss of life. Thunderstorms: A localized storm often accompanied by lightning, thunder, and heavy rainfall. Thunderstorms can cause flash floods, landslides, and damage due to strong winds and lightning strikes. Heavy Rainfall: Excessive rain over a short period of time can lead to urban flooding, river overflows, landslides, and soil erosion, severely impacting communities and agriculture. Heat Waves: Prolonged periods of excessively high temperatures can lead to health crises, particularly affecting vulnerable populations like the elderly and those with pre-existing conditions. Cold Waves: Periods of unusually low temperatures can cause severe cold stress on humans and animals, damage crops, and disrupt energy systems. Fog: Dense fog can reduce visibility, leading to transportation accidents, particularly in aviation and road traffic. It also poses health risks, especially for respiratory conditions. 3.1.3 Geological Hazards Geological hazards arise from Earth’s internal processes and include the following: Landslides: Landslides refer to the downward movement of soil, rock, or debris due to gravity, often triggered by factors such as heavy rainfall, earthquakes, or human activities like deforestation. They can block roads, damage buildings, and cause fatalities. 28 Earthquakes: Sudden shaking or movement of the Earth’s crust due to the release of energy along fault lines. Earthquakes can lead to severe structural damage, ground rupture, and secondary hazards like tsunamis and landslides. Volcanic Eruptions: When magma from beneath the Earth’s surface is ejected through a volcano, causing pyroclastic flows, ash clouds, lava flows, and volcanic gases. Volcanic eruptions can devastate communities, ecosystems, and infrastructure. 3.2 Hydro-meteorological hazards over India India is prone to a variety of hydro-meteorological hazards that vary by season and region, significantly impacting the lives and livelihoods of its population. During the monsoon season, which typically lasts from June to September, heavy rainfall leads to flooding in many states, often resulting in loss of life and property. In contrast, the pre-monsoon months of April and May witness severe heatwaves affecting northern and central India, with states like Rajasthan, Gujarat, Uttar Pradesh, Haryana, Delhi and Madhya Pradesh experiencing soaring temperatures that can lead to heat-related illnesses and agricultural losses. The winter months, particularly from December to February, bring dense fog to northern plains, disrupting transportation and posing risks to travellers. Additionally, the coastal regions of India, including Tamil Nadu, Odisha, West Bengal, Gujarat and Andhra Pradesh, are frequently impacted by cyclones, particularly during April to June and September to December, leading to storm surges, heavy rainfall, and widespread destruction. Furthermore, regions like Uttarakhand and Himachal Pradesh are vulnerable to landslides triggered by heavy rains, especially during the monsoon. Overall, the diverse climatic conditions across India create a complex landscape of hydro-meteorological hazards that necessitate comprehensive disaster management strategies to mitigate their impacts. 3.3 Cyclones 3.3.1. Formation and Structure of cyclones Cyclones typically form over warm tropical waters where sea surface temperatures exceed 26.5°C, as warm water provides the energy needed to fuel the cyclone. In addition to warm water, other favorable conditions include high humidity in the atmosphere, low vertical wind shear (where wind speeds and directions do not change drastically with height), and a pre-existing disturbance, such as a low-pressure area. A cyclone is a large-scale air mass that rotates around a strong center of low atmospheric pressure, characterized by inward spiraling winds. Cyclones are classified based on their location and intensity, with tropical cyclones (hurricanes or typhoons) forming over warm ocean waters and featuring strong winds, heavy rainfall, and storm surges. Cyclones form due to the Coriolis effect, which causes the wind to rotate in a counterclockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere. When all the favorable conditions combine, they create the perfect environment for cyclones to develop and grow in strength. However, these powerful storms can lead to significant damage and pose serious threats to life and property. In the Indian Ocean, particularly the Bay of Bengal and the 29  Arabian Sea, cyclones are common during pre-monsoon (April-June) and post-monsoon (October- December) seasons.The shape of a tropical cyclone is typically circular or spiral, characterized by a well-defined central core called the eye (Figure 3.1). Surrounding the eye is the eye wall, a ring of towering thunderstorms where the most intense winds and heavy rainfall occur. The cyclone's outer structure consists of spiral rainbands, which are bands of clouds and precipitation that spiral outward from the center. These rainbands contribute to the storm's overall spiral shape, resembling a massive, rotating system when viewed from above. The symmetrical structure is maintained by the Coriolis force, which causes the cyclone to rotate, and the storm's energy is driven by warm ocean waters. Figure 3.1. (Left) Vertical structure of tropical cyclone; (Right) Image of the cyclone Mocha captured from satellite with eye of the cyclone visible. Cyclones are intense low pressure areas - from the center of which pressure increases outwards. The amount of the pressure drop in the center and the rate at which it increases outwards gives the intensity of the cyclones and the strength of winds. The criteria followed by the India Meteorological Department (IMD) to classify the low pressure systems in the Bay of Bengal and in the Arabian Sea as adopted by the World Meteorological Organisation (W.M.O.) are given in Table 3.1. Table 3.1. Criteria for classification of cyclonic disturbances over the North Indian Ocean Type of disturbance Associated maximum sustained wind 1. Low Pressure Area Not exceeding 17 knots (