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Preface Weather is a basic input for agriculture. It affects the agricultural planning in a big way starting from land preparation to harvesting, processing and transportation. Thus, climate and weather may be considered as the most critical factor determining the sustainability of agricultural systems. However, among all the resources that are essential for agricultural production, climate and weather received the least attention in agriculture literature till few decades ago. Agrometeorology is now recognised as a full-fledged discipline and 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, based on the syllabus of the course ‘Introductory Agrometeorology & Climate Change’ meant for undergraduate students of agriculture, horticulture and forestry. The interdisciplinary nature of agrometeorology and its applications in agriculture and allied sectors like horticulture, animal husbandry, fishery, forestry, etc. is well explained in this book. The book has been divided into fifteen chapters covering all aspects of agrometeorology. Concepts, definition, importance and scope of agrometeorology are described in chapter 1. Chapter 2 details the basic information of atmosphere. Chapter 3 to chapter 10 include the weather parameters like radiation, temperature, humidity, pressure, wind, clouds, monsoon and precipitation and their importance in agriculture. Applied aspects of meteorology like weather hazards, climatic normals for crops and livestock production, weather forecasting are discussed in chapter 11 to chapter 13. Climate change and its importance in agriculture is described in chapter 14. Chapter 15 gives an overall idea about the features of an agrometeorological observatory. Each chapter contains objective type of questions which will help the students to prepare for various examinations. The glossary and relevant bibliography including the weblinks will also help for easy access to the literature on the subject.

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. 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.

Atmosphere can be defined as the gaseous envelope surrounding the earth. It is retained by earth’s gravity. The atmosphere has a mass of about 5.15 × 1018 kg, three quarters of which is within about 11 km of the surface. 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. 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.

Air in motion is called wind. Winds move moisture and heat around the world and also produce much of our weather. Winds are part of a global air circulation system that acts to balance temperature and pressure around the world. Any difference in temperature will always cause a difference in air pressure, and therefore, winds will blow. There are two types of movement in the atmosphere, horizontal movement and vertical movement. The term wind generally refers to the horizontal movement of air relative to the earth’s surface. Vertically moving air columns are called as currents. Upward and downward air currents are referred to as updrafts and downdrafts, respectively. Vertical motion in the atmosphere is of significance for the formation of clouds, precipitation and various types of storms. The horizontal differences in air pressure are far more variable than vertical changes and give rise to a wide range of air motions. Uneven distribution of pressure over the globe is balanced through the movement of winds. Winds also act as a means of transporting heat, moisture and other properties from one part of the earth to another.

The sun 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. 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. The sun makes up about 99.86% of the mass of the entire solar system. The visible disk or photosphere has a radius of 6.599 × 105 km, and the solar mass is 1.989 × 1030 kg. The sun is a completely gaseous body. The chemical composition (by mass) of the outer layers of sun is 71% hydrogen, 26.5% helium, and 2.5% heavier elements like oxygen, carbon, neon, and iron among others. Since the sun is not a solid body, different parts of the sun rotate at different rates. At the equator, the sun spins once about every 25 earth days, but at its poles the sun rotates once on its axis every 36 days.

Temperature is the physical property of an object that quantitatively expresses the hotness or coldness. Most of the weather parameters are dependent on temperature, 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 equalize the energy content of the different regions of the earth. Thus, atmospheric temperature is one of the basic elements of weather and climate. The temperature at a given location and time is affected by energy transfer processes including conduction, convection, advection and radiation. The controls of temperature include those factors that bring about spatial variations in temperature. The important factors that determine the temperature of a particular place on the earth’s surface are latitude, altitude, distribution of land and water, topography, ocean currents, prevailing wind, cloudiness, precipitation, nature of the surface, and turbulence.

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. Water vapour is constantly being cycled between the atmosphere and the earth's surface. The input rate (the rate at which water vapour enters the atmosphere from the ground through evaporation) and output rate (the rate at which water moves from the atmosphere to ground through precipitation) are equal. The average length of time that individual molecules of water remain in the atmosphere is called the residence time. It can be found by dividing the mass of the substance in the atmosphere (in kilograms) by the rate (in kg per year) at which the substance enters to and exists in the atmosphere. The total volume of atmospheric water vapour at any instance over the globe is about 14000 km3, which is 0.001% of the hydrosphere. Although water vapour is lighter than air, the combination of higher temperatures and the ocean source tends to restrict it to the lower atmosphere. The decrease of water vapour with elevation is quite important in many meteorological processes. Infact, most of the water vapour is contained within the lower 5 km of the atmosphere (> 90%) and more than half is actually contained within the lowest 1.5 to 2 km. Less than 5 - 6% of the water is above 5 km, and less than 1% is in the stratosphere.

Precipitation is the deposition of atmospheric moisture on earth. All the water sources on the earth depend on precipitation only. Precipitation involves condensation, growth of hygroscopic nuclei, cooling of nuclei at different stages and their fall on to the earth's surface. If the cloud particles are too small and remain suspended in the sky, they may not fall on the earth. There may be several clouds but no precipitation. 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 longer remain floating due to buoyancy of air, do they fall as precipitation. FORMS OF PRECIPITATION Precipitation form is the state that the moisture is in, i.e., liquid, freezing, or frozen (solid). 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.

Clouds are the most important form of suspended water droplets caused by condensation. 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. 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 million. 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 complete the following functions.

In the Indian sub-continent, monsoon is one of the oldest weather observations, an economically important weather pattern over June through September every year. Due to its effects on agriculture, flora and fauna, and other economic, social, and environmental effects, a monsoon is one of the most anticipated, followed and studied weather phenomena of the Indian subcontinent. The word 'monsoon' is derived from the Arabic word mawsim meaning season. The term 'monsoon' 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. 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 south-west during one half of the year and from the north-east 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 south-west winds, and the winter monsoon period characterised by dry and cool north-east 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.

There are various weather hazards that affect agricultural productions. These hazards affect crops and agricultural productivity negatively if they are not checked. Weather hazards include cyclones, droughts, floods, hailstorms, heatwaves 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. 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. DROUGHT Drought is a relative term used with reference to deficiency of rainfall when compared to normal rainfall of a given location. However, drought cannot be defined in terms of amount of rainfall or number of days without rain. Since ancient times droughts have had far-reaching effects on humankind by causing the failure of crops, decreasing natural vegetation, and depleting water supplies. Drought is different from other hazards; in that it develops slowly. It is a hazard that requires many months to emerge and that may persist for many months or years thereafter. This type of hazard is known as a ‘creeping’ hazard. Generally, this occurs when a region receives consistently below average precipitation.

The crops 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. Weather and climate have also great influence on farm animal production. The environment in which an animal lives is referred to as its habitat. A habitat includes both biotic (living) and abiotic (non-living) components of the animals’ environment. Abiotic components of an animal’s environment include temperature, humidity, oxygen, wind, soil composition, day length and elevation. These potential environmental stressors can directly and adversely affect animal farming. The indirect consequences of weather include their impact on feed quality and availability.

Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. 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. Farming in many parts of the world, especially in the arid and semiarid regions is risky, because climate is highly variable. 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. Under rainfed conditions, even a less reliable, but earlier forecast may be more valuable than an accurate but late forecast. In about 340 B.C., Aristotle described weather patterns in Meteorologica. Later, Theophrastus compiled a book on weather forecasting, called the Book of Signs. Ancient weather forecasting methods usually relied on observed patterns of events. This experience was accumulated over the generations to produce weather knowledge. However, not all of these predictions prove reliable, and many of them have since been found not to stand up to rigorous statistical testing. Two officers from of British Royal Navy, Francis Beaufort and Robert FitzRoy formed the basis for all of today’s weather forecasting knowledge by middle of 19th century. A terrible storm in 1859 inspired FitzRoy to develop charts to allow predictions to be made, which he called ‘forecasting the weather’, thus coining the term ‘weather forecast’. The ‘Weather Book’ which FitzRoy published in 1863 was far in advance of the scientific opinion of the time. By the early 20th century, scientists had begun to believe that weather forecasting could be predicted based on mathematical methods. Computers were first used to create surface analyses in 1992 and by 1998 digital imagery were used for satellite, radar and surface observations, encompassing nearly every factor of weather interpretation. Modern technology, particularly computers and weather satellites, and the availability of data provided by coordinated meteorological observing networks, has resulted in enormous improvements in the accuracy of weather forecasting.

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. 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.

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. Table 15.1. Instruments used for measuring weather parameters in observatories and laboratories

Answer Keys 1. Agrometeorology and Its Scope Fill in the blanks with suitable word / figure.