Preface Agrometeorology is primarily concerned with the interactions among meteorological, hydrological and pedological factors that influence production systems in agriculture and allied sectors like horticulture, animal husbandry, fishery, forestry, etc. Since agrometeorology is multidisciplinary in nature it has a well defined approach in theory and applications. Thus, a good understanding of the interrelationships that exist among all the concerned disciplines is required. An agrometeorologist defines all these interactions and correlates physical environments to biological responses and applies the acquired and relevant meteorological skills to help farmers for exploiting weather conditions to improve agricultural production both in quality and quantity. In this backdrop, agrometeorology has been recommended as a core subject in the curriculum of the agricultural universities. Keeping this in view, an effort has been made to write a textbook on agrometeorology which would be useful to the students as well as to the teachers. This book is primarily based on the syllabus of the course ‘Introductory Agrometeorology & Climate Change’ meant for under graduate students of agriculture, horticulture and forestry. In writing this book, the literature on agrometeorology developed by various organisations, agencies and institutes like WMO, FAO, NOAA, BOM, MAFF, IPCC, NASA, IMD, NDMA, CRIDA, ICRISAT, ICAR, AICRPAM, etc. is freely used. I am thankful to the organisations permitting me to quote or cite their literature in the text. I extend my sincere thanks to the authors and editors of various books, journals and periodicals which have been used as reference material in this book. Every care has been taken to cite the bibliographic references. However, any omissions, misrepresentations, incorrect citations or other mistakes that may have occurred are regretted. I am grateful to my colleagues in the All India Coordinated Research Project on Integrated Farming Systems, Odisha University of Agriculture & Technology, Bhubaneswar for their help in various ways during the preparation of this textbook. I express my gratitude to the New India Publishing Agency, New Delhi for bringing out the book timely and nicely. I am thankful to my wife Jharashree and daughter Prachurya for their support and encouragement. I also express my indebtedness and gratitude to my beloved parents who are a constant source of inspiration to me throughout my academic journey.

Meteorology is derived from the Greek word meteoro which means above the earth’s surface; i.e., the atmosphere. The state of the atmosphere experienced at a given time, in any location on the surface of the earth is known as weather. Thus meteorology is the scientific study of the atmosphere or the weather. It is a branch of physics dealing with atmosphere and it is often quoted as the ‘physics of the lower atmosphere’. The main focus of meteorology is to understand the weather processes and forecast the weather. Solar radiation, the air mass and moisture are the three key ingredients that make up weather. The exchange of heat, moisture and momentum between the earth’s surface and the atmosphere determines the characteristic of weather. The observable weather events are known as meteorological phenomena and are defined by a set of variables called weather elements. Atmospheric temperature, atmospheric pressure, humidity, precipitation, cloudiness, wind and visibility, etc. are examples of such weather elements. The weather elements are not separate entities. They are closely related with each other and are highly variable in nature. Meteorology explains and analyses the changes of these weather elements. METEOROLOGY The major subdivisions of meteorology are dynamic meteorology (or atmospheric dynamics) and physical meteorology (or atmospheric physics). The dynamic meteorology is concerned with the analysis and interpretation of the three-dimensional, time-varying, macro scale motions and energy transformations in the atmosphere. The science of dynamic meteorology is meant to develop models based on the hydrodynamic and thermodynamic equations. The physical meteorology is concerned with the processes that alter the physical properties and the chemical composition of air parcels as they move through the atmosphere.

The dynamic layer surrounding the earth above its surface containing various gases, moisture, aerosols, etc. is called atmosphere. Atmosphere can be defined as the gaseous envelope surrounding the earth. It is retained by earth’s gravity. The atmosphere protects life on earth by absorbing ultraviolet solar radiation, warming the surface through heat retention and reducing temperature extremes between day and night. The common name given to the atmospheric gases used in breathing and photosynthesis is air. The atmosphere has a mass of about 5.15 × 1018 kg, three quarters of which is within about 11 km of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The imaginary ‘Karman line’ lies at an altitude of 100 km above the earth’s sea level, and commonly represents the boundary between the earth’s atmosphere and outer space. The study of earth’s atmosphere and its processes is called atmospheric science or ‘aerology’.

Atmospheric pressure is the force per unit area exerted on a surface by the weight of air above that surface in the atmosphere of earth. The gas molecules of the atmosphere are pulled towards the earth by the force of gravity and hence exert a force on earth surface. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Atmospheric pressure inf luences other weather elements in a significant way. Small changes in air pressure may not be felt, but they are important for bringing a change in weather. Atmospheric pressure can be calculated as the summed weight of all gas molecules between a horizontal plane and top of the atmosphere and divided by the area of the plane. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location. FACTORS AFFECTING ATMOSPHERIC PRESSURE Air pressure never remains constant. Air pressure is greatly influenced by air temperature, altitude, water vapour presence in the air, rotation of earth, etc. However, these factors are not independent of each other in affecting the atmospheric pressure. Temperature: Hot air expands and exerts low pressure. Cold air contracts and exerts high pressure. Thus, increase in temperature decreases the air pressure. Places nearer to equator experience low pressure due to prevalence of high temperature. The air being cold and heavy throughout the year a high pressure belt is created in both polar regions. Altitude: Pressure varies smoothly from the earth’s surface to the top of the mesosphere. The pressure depends on the density or mass of the air. The density of air depends on its temperature. The density of air decreases with increase in height so the pressure also decreases with increase in height. At low altitudes above the sea level, the pressure decreases by about 1.2 kPa for every 100 metres. Water vapour: Water vapour content of air is low in cold areas than that in dry areas. Hence, moist air of high temperature in dry areas exerts less pressure compared to cold air.

A big layer of air called the atmosphere surrounds the earth. The air within this layer moves from place to place when it warms up or cools down. This moving air is known as wind. In other words, air in motion is called wind. Winds move moisture and heat around the world and also produce much of our weather. The strength of the wind can vary enormously. Sometimes air moves slowly and the wind is barely noticeable. When the weather is clear there may be a gentle breeze. When the wind is still very light, but it can be felt and there may be rustlings by leaves. At other times, the air can move very quickly and become a gale or hurricane, blowing down trees and damaging electric poles and buildings. Winds are part of a global air circulation system that acts to balance temperature and pressure around the world. Different parts of the world receive different amounts of heat from the sun. This differential heating in turn results in differences in temperature and air pressure around the world, which drives the world’s winds. As equatorial areas are heated most, the air above them warms and rises as it becomes lighter than the surrounding air, causing an area of low pressure. In cooler areas, the air sinks because it is heavier and results in an area of high pressure. Winds will blow as air is squashed out by the sinking cold air and drawn in under the rising warm air. Any difference in temperature like this will always cause a difference in air pressure, and therefore winds will blow. These movements result in a global wind pattern, with air moving between different areas around the world and also at different heights in the atmosphere. Colder air from the poles tends to sink and move towards the equator closer to the surface of the earth. In contrast, warm air from the equator rises and moves towards the poles high in the atmosphere because it is lighter.

Sun is the prime source of energy injected into the earth’s atmosphere. The sun is the nearest star to the earth. It is a hot ball of glowing gases at the heart of the solar system. Its radiant energy is practically the only energy source to the earth. Very small and insignificant quantities of energy are available from other sources such as the interior of the earth, the moon, and other stars. Without the sun’s intense energy and heat, there would be no life on earth. NATURE AND PROPERTIES OF SOLAR RADIATION The mean distance of the sun from the earth, also known as one astronomical unit (1 AU), is approximately 1.496 × 108 km, though the distance varies as the earth revolves round the sun in an elliptical orbit. At this average distance, light travels from the sun to earth in about 8 minutes and 19 seconds. The energy of the sunlight supports almost all life on earth by photosynthesis, and drives earth’s climate and weather. The minimum sun-earth distance is about 0.983 AU and the maximum is approximately 1.017 AU. The earth is at its closest point to the sun (perihelion) on around January 3 and at its farthest point (aphelion) on around July 4.

Temperature is the physical property of an object that quantitatively expresses the hotness or coldness. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot. When a heat transfer path between them is open, heat spontaneously flows from bodies of a higher temperature to bodies of lower temperature. If no net heat flow occurs between two objects, the objects have the same temperature which are then said to be in ‘thermal equilibrium’. Temperature plays an important role in all fields of natural and applied sciences. Most of the weather parameters are dependent on it, directly or indirectly. It has important role to play in the development and growth of all living organisms in all stages of their growth. Atmosphere receives the heat energy from the sun and its temperature increases. Due to different amount of heat energy receipt at different places, the air temperatures at different places also vary. The variation in air temperature basically results into air motion, so as to equalise the energy content of the different regions of the earth. Thus, atmospheric temperature is one of the basic elements of weather and climate.

Water is the only substance that exists in the atmosphere in all the three phases, i.e., solid, liquid and gas. The moisture on the earth’s surface is in a constant phase change. The moisture content of the atmosphere is about 0.035% of all fresh water. The atmosphere receives its supply of moisture from the earth’s surface through evaporation from oceans, lakes, rivers, damp soil, etc. and through transpiration from vegetation. Water vapour is the gaseous state of water and is invisible. The amount of water vapour present in the atmosphere is called atmospheric moisture or humidity. It is a measure of the amount of water vapour dissolved in the air, not including any liquid water or ice falling through the air. While humidity itself is a climate variable, it also interacts strongly with other climate variables. The humidity is affected by winds and by rainfall. Moisture content is determined by the local rate of evaporation, air temperature and the horizontal transport of moisture. Humidity indicates the likelihood of precipitation, dew or fog. It also plays a major role in the heat budget and the weather phenomenon of the atmosphere.

Clouds are visible mass of little drops of water or frozen crystals suspended in the atmosphere above the surface of the earth. The World Meteorological Organisation defined cloud as a visible aggregate of minute particles of water or ice or both, in the free air. This aggregate may include larger particles of water or ice and particles such as those present in fumes, smoke or dust. Clouds are the most important form of suspended water droplets caused by condensation. The typical size of these water droplets and ice crystals range from less than 1 µm to 50 µm. When surrounded by billions of other droplets or crystals, they become visible as clouds. In the atmosphere sufficient moisture is required to allow condensation to take place. Then, moisture needs a suitable surface upon which it can condense. In free air, condensation begins on hygroscopic nuclei such as aerosols. These are microscopic particles and include dust, smoke, salts and chemical compounds. Nuclei range in size from 0.001 µm radius to over 10 µm. On an average, oceanic air contains one million condensation nuclei per litre and land air holds some 5 or 6 millions. On hygroscopic particles such as sea salts, condensation can begin before the air is saturated. Supersaturation in clouds rarely exceeds one per cent. On average, clouds contain only four per cent of the total water in the atmosphere at any one time. Almost all clouds exist in the troposphere, although the tops of some severe thunderstorms occasionally extend to the stratosphere. Clouds move along with the wind and remain stationary in calm winds. They grow vertically and horizontally, depending on the temperature of the cloud and its ambient temperature. They may dissipate or precipitate. Under favourable atmospheric conditions, they burst and fall as rain. Clouds are essential to the earth-atmosphere system. They complete the following functions.

Precipitation is water in liquid or solid forms, falling on to the earth. In other words, precipitation is the deposition of atmospheric moisture on earth. Formation of clouds does not mean precipitation. Precipitation involves condensation, growth of hygroscopic nuclei, cooling of nuclei at different stages and their fall on to the earth’s surface. There may be several clouds but no precipitation. If the cloud particles are too small and remain suspended in the sky, they may not fall on the earth. Under certain conditions, water particles formed tend to fall but do not reach the surface since they get evaporated on the way. Only when the cloud droplets or ice crystals grow to certain size and can no more remain floating due to buoyancy of air, do they fall as precipitation. Precipitation is the major source of water to all the living organisms on the earth. All the water sources on the earth depend on precipitation only. FORMS OF PRECIPITATION Precipitation form is the state that the moisture is in, i.e., liquid, freezing, or frozen (solid). Different forms of precipitation may occur together, such as mixed rain and snow; but such an occurrence is simply a mixture of forms, not a separate form of precipitation. Because atmospheric conditions vary greatly both geographically and seasonally, several different forms of precipitation are possible. All forms of precipitation are collectively termed hydrometeors. Hydrometeors have been classified into 50 specific types. Out of these precipitation forms, only rain (liquid form) and snow (solid form) make significant contribution to total precipitation. In many parts of the world, the term rainfall is used interchangeably with precipitation.

The word ‘monsoon’ is derived from the Arabic word mawsim meaning season. The Arabs during their travel towards India recognised that by June strong southwesterly winds commence over the Arabian Sea and bring a lot of rains to the subcontinent. The term was first used in English in British India and neighbouring countries to refer to the big seasonal winds blowing from the Bay of Bengal and Arabian Sea in the southwest bringing heavy rainfall to the area. In the Indian subcontinent, monsoon is one of the oldest weather observations, an economically important weather pattern over June through September every year, and the most anticipated weather event and unique weather phenomenon. Due to its effects on agriculture, flora and fauna, and other economic, social, and environmental effects, a monsoon is keenly followed and studied. It has significant impact on the overall well-being of subcontinent residents. The term ‘monsoon’ is customarily used in India to refer to the rainy season during the northern hemisphere summer from June to September and is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea. Now the term monsoon has been broadened to include almost all of the phenomena associated with the annual weather cycle within the tropical and subtropical land regions of the earth. Intergovernmental Panel on Climate Change (IPCC) describes monsoon as a tropical and subtropical seasonal reversal in both the surface winds and associated precipitation, caused by differential heating between a continental-scale land mass and the adjacent ocean. India Meteorological Department defines it as the seasonal reversals of the wind direction along the shores of the Indian Ocean, especially in the Arabian Sea, that blow from the southwest during one half of the year and from the northeast during the other half. The definitions include major wind systems that change direction seasonally. The cyclic change of wind and rain can be separated into the summer monsoon period characterised by warm and humid southwest winds, and the winter monsoon period characterised by dry and cool northeast winds. In fact, monsoon systems are a major feature of the general circulation of the atmosphere in subtropical latitudes of most regions of the world.

A weather hazard is an extreme weather event that threatens people or property. There are various weather hazards that affect agricultural productions. These hazards vary from one climatic zone to another and occur at different times, although some regions are more vulnerable to certain hazards than other. These weather hazards depend on the latitude of the place on the world map. These hazards affect crops and agricultural productivity negatively if they are not checked. Every agricultural zone has one particular hazard that is associated with it and in most cases affects the crop productions and reduces the annual crop yield. Weather hazards include cyclones, droughts, f loods, hailstorms, heat-waves and cold-waves. The hazard is distinguished from an extreme event and a disaster. An extreme event is simply an unusual event; it does not necessarily cause harm. A disaster is an event that does cause harm in significant amounts. Weather hazards are often called as natural hazards. The natural hazards become natural disasters when people’s lives and livelihoods are destroyed. Hydrologic hazards are the result of either excess or a severe lack of water, i.e., flood or drought. Hydrometeorological hazard is a process or phenomenon of atmospheric, hydrological or oceanographic nature that may cause loss of life, injury or other health impacts, property damage, loss of livelihoods and services, social and economic disruption, or environmental damage. Human and material losses caused by natural disasters are a major obstacle to sustainable development. By issuing accurate forecasts and warnings in a form that is readily understood and by educating people about how to prepare against such hazards, before they become disasters, lives and property can be protected.

Different crops are grown over a wide range of agroecosystems. Though the crops are raised in varied climates, ranging from very hot to very cold, and very wet to very dry, across temperate, tropical and semiarid zones, they are very sensitive to climate. The crop production is dependent on climatic parameters like rainfall, light, temperature, relative humidity and carbon dioxide concentration. Each kind of cultivated crop plants has its own optimum growth requirements. Breeding and selection of new cultivars have allowed for a greater adaptability to less favourable growing conditions than was possible in the past, but the inherent climatic requirements of a specific kind of crop have not changed materially. CLIMATIC NORMALS FOR CROPS Crop management is very much about managing climate risk so as to have financially viable and sustainable agricultural systems. Professor Henry Nix of the Australian National University posed three questions that need to be addressed when considering what crop can be grown where (Nix, 1981). For any given crop which areas offer the greatest biophysical advantages? For any given area which crops offer the greatest advantages? For any given crop and area how much productivity be raised and sustained?

Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. Weather forecasts are made by collecting quantitative data about the past and current state of the atmosphere on a given place and using scientific understanding of atmospheric processes to project how the atmosphere will evolve on that place. Weather warnings are important forecasts because they are used to protect life and property. Forecasts based on temperature and precipitation is important to agriculture. Agricultural production is highly dependent on weather, climate and water availability, and is adversely affected by weather and climate related disasters. In many developing countries where rainfed agriculture is the norm, a good rainy season means good crop production, enhanced food security and a healthy economy. Failure of rains and occurrence of natural disasters such as floods and droughts can lead to crop failures, food insecurity, famine, loss of property and life, mass migration, and negative national economic growth (Sivakumar, 2006). Farming in many parts of the world, especially in the arid and semiarid regions is risky, because climate is highly variable. In some developing countries as much as 80% of the variability in agricultural production is due to the variability in weather conditions. This production variability in agriculture has immediate and important macroeconomic impacts. A timely seasonal forecast of favourable conditions could permit farmers to adjust cropping patterns and input use in order to benefit fully from these conditions. Also, a timely forecast could give the marketing system and downstream users, such as the food processing industry, time to prepare for a bountiful harvest (Arndt et al., 2000). It is important to remember that under rainfed conditions, even a less reliable, but earlier forecast may be more valuable than an accurate but late forecast (Mjelde et al., 1998).

Many aspects of the global climate are changing rapidly. Evidence for changes in the climate system abounds, from the top of the atmosphere to the depths of the oceans. The climate system is the highly complex system consisting of five major components; the atmosphere, the hydrosphere, the cryosphere, the land surface and the biosphere, and the interactions among them. The climate system evolves in time under the influence of its own internal dynamics and because of external forcings such as volcanic eruptions, solar variations and human-induced forcings such as the changing composition of the atmosphere and land-use. The term ‘climate variability’ is often used to denote deviations of climatic statistics over a given period of time (e.g., a month, season or year) from the long-term statistics relating to the corresponding calendar period. In this sense, climate variability is measured by those deviations, which are usually termed anomalies. ‘Climate change’ refers to a statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period (typically decades or longer). Climate change may be due to natural internal processes or external forcings, or due to persistent anthropogenic changes in the composition of the atmosphere or in land use. The United Nation’s Framework Convention on Climate Change (UNFCCC) defines ‘climate change’ as ‘a change of climate behaviour which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods’. The UNFCCC thus makes a distinction between ‘climate change’ attributable to human activities altering the atmospheric composition, and ‘climate variability’ attributable to natural causes. Climate change is a long-term shift in weather conditions measured by changes in temperature, precipitation, wind, snow cover, and other indicators. It can involve both changes in average conditions and changes in variability, including, for example, changes in extreme conditions. A key difference between climate variability and change is in persistence of ‘anomalous’ conditions. In other words, events that are used to be rare occur more frequently, or vice versa. Occasionally, an event or sequence of events occurs that has never been recorded before, such as the exceptional tsunami in the Indian Ocean in 2004 or hurricane season in the Atlantic in 2005. Yet even that could be a part of natural climate variability. If such a season does not recur within the next 30 years, by looking back it could be called as an exceptional year, but not a ‘climate change’. Only a persistent series of unusual events taken in the context of regional climate parameters can suggest that a potential change in climate has occurred.

The science of agrometeorology has great practical utility in protection against, or avoidance of adverse climate risks of crop production. Agrometeorological information can be used in efficient land use planning, determining suitable crops for a region, risk analysis of climate hazards and profit calculations in farming, production and harvest forecasts, and in adoption of farming methods and choice of farm machinery. Meteorological instruments are the equipments used to sample the state of the atmosphere at a given time. Meteorology does not use much laboratory equipments but relies more on field-mode in situ observation and remote sensing equipments. Rain was one of the first weather parameter to be measured historically. Two other accurately measured weather-related variables are wind and humidity. An agrometeorological observatory or station is a place where all the necessary instruments and structures are installed and maintained to observe and record the weather parameters at stipulated time interval.

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