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The National Commission on Agriculture, duly constituted by Government of India, in their report Volume IV on Weather and Agriculture (1976) recommended that all the State Agricultural Universities (SAUs) should establish a separate Department of Agricultural Meteorology and start teaching program in Agricultural Meteorology at Post Graduate level to generate well trained manpower to undertake research in Agricultural Meteorology for better understanding the effects of weather and climate in planning and management of agricultural production which is very much necessary for guiding farmers in carrying out field operations based on the information continuously generated by the National Meteorological and Hydrological Services. As a consequence few of the very progressive State Agricultural Universities started separate Department of Agricultural Meteorology and introduced MSc and PhD Programs in Agricultural Meteorology from early 1980 onwards . As on now, there are more than 60 Agricultural Universities in the Country and it is unfortunate that there are only 19 SAUs have a separate Division/ Department of Agricultural Meteorology and rest of the Universities have either ignored or unaware of the recommendations of National Commission on Agriculture. The Association of Agrometeorologists in collaboration with the ICAR-Central Research Institute of Dryland Agriculture, Hyderabad organized a Brain Storming Workshop on Reorientation of Education and Research in Agricultural Meteorology in the Context of Climate Change and Agrometeorological Advisory Services during September 29-30, 2023. The workshop has recognized the importance of revising the syllabus followed for MSc and PhD programs taking into consideration the recent technological advances made in weather forecasting, creation of data base, modern methods available for analysis and interpretation of multiple data sets available with wide range spatial and temporal variability. The Workshop also identified lack of uniformity in the Syllabus prescribed by the Indian Council of Agricultural Research and the syllabus followed by the Agricultural Scientists Recruitment Board, as a result of which there was some deficiency in training of students.

Aristotle (384 to 322 BC), an ancient Greek philosopher and scientist (Fig. 1.1) developed the concept of meteorology as a study of the lower atmosphere. Over the years meteorology developed as a science of atmosphere which deals with the physics, dynamics and chemistry of the atmosphere and their effect upon the earth’s surface, oceans and thereby life in general either directly or indirectly. As the atmospheric conditions change from time to time in a given area and from place to place in a given time, climatology was developed as a science which studies the average conditions of the weather and its variability over a long period of time. All most all the human activities are greatly influenced by the weather and climate to a greater extent. Therefore, the scope of meteorology encompasses various activities which are of considerable economic significance as detailed here below; 1.1 Housing The design and construction of houses, buildings, industrial structures etc. depends upon the weather and climate of the region. Orientation of doors, windows roofs of the buildings are usually designed to provide maximum comfort and convenience to the people.

Air is one of the primary things that make life on earth possible. The earth’s atmosphere or air is made up of variety of gases and other particles. The dynamic layer surrounding the earth’s surface containing various mixture of gases, moisture and solid particles (aerosols) is called atmosphere. The atmosphere is generally defined as the gaseous envelop surrounding the earth. The formation of the earth was believed to be about five billion years ago. In the first 500 million years, a thick atmosphere emerged from the vapour and gases that were expelled from the interior of the earth. Hydrogen (H2 ), water vapour, methane and carbon dioxide were some of the gases believed to be initially formed in the atmosphere due to out gassing from the interior of the earth. About 3.5 billion years ago, the atmosphere consisted of carbon dioxide (CO2 ), Carbon monoxide (CO), water vapour (H2 O), nitrogen (N2 ) and hydrogen present in atmospheric layers like troposphere, stratosphere, mesosphere, thermosphere (Fig. 2.1). The condensation of water vapour into water led to the formation of oceans in which sedimentation occurred and the hydrosphere was created. In the initial and ancient environment, oxygen was not present. Anaerobic is technical word which literally means without air (where air is generally used to mean oxygen and aerobic indicates the presence of oxygen). Evidence of such an anaerobic reducing atmosphere was hidden in early rock formations such as iron and uranium in their reduced states.

Three fundamental processes that are responsible for inducing changes in weather are transfer of heat, energy and mass. The Sun, whose inter-surface temperature is estimated to be around 6000°C emits heat energy in the form of solar radiation which is received at the earth’s surface after it is partly depleted in the atmosphere. Therefore, it is essential to understand the processes that govern the transfer of heat in the earth-atmosphere system. 3.1 Conduction The transfer of heat from one point to another point within the material medium without the actual movement of the particles in the medium is called conduction i.e. the heat energy transfer is conducted by the particles in the medium from hot end to cold end. If a metal rod is heated at one end, the other end of rod becomes hot gradually due to transfer of heat by conduction. In the earth atmosphere system, conduction of heat is observed at the ground surface only. The amount of heat energy transferred due to conduction (Q) is

In meteorology, temperature is always referred to as the temperature of the air unless it is stated otherwise. There will always be exchange of heat in the atmosphere from a point at high temperature to a point at lower temperature. The transfer of heat through convection and radiation is always observed in the atmosphere whereas the conduction of heat takes place near the earth’s surface. 4.1 Diffusion of Heat Heat diffuses (spreads) at different rates through different media, the rate at which diffusion of heat takes place is called thermal diffusivity. The thermal diffusinty of of some selected materials is given below: One can see from the above table that iron has got higher diffusivity compared to wood. Suppose one end of iron rod is kept at hot temperature, the heat spreads faster in iron rod and the other end gets warmer quicker than in the case of a wooden stick which means that the transfer of heat is quicker in bodies with higher thermal diffusivity.

The mass of a body is usually defined as the material content of the body whereas the weight of the body is defined as the force with which the body is attracted towards the earth due to gravity. Therefore, the weight of a body is the product of its mass and acceleration due to gravity. The air in the atmosphere, which is a mixture of several gases has certain mass and although not very dense, will acquire some weight due to gravitational force of attraction when the entire column of the atmosphere from the ground level to the top of it is considered. Pressure is defined as the force for unit area. Therefore, the atmospheric pressure is defined as the force exerted by an air column of units area of cross section extending from earth’s surface to the top of atmosphere. As we go up in the atmosphere the height of the air column decreases, the air also become rarer and the atmospheric pressure decreases with increase in height.

Clouds play very important role on weather and climate. The formation of clouds takes place when water condenses in the sky. Clouds are visible accumulation of tiny water droplets and ice crystals in the Earth’s atmosphere. The moisture is always present in the atmosphere though it may not be visible. Clouds form when air becomes cooler until the water vapour present in it condenses to liquid form. The condensation over oceans occurs around solid particles like salt, dust, smoke etc. suspended in the atmosphere. The type of clouds depends upon temperature and wind. The clouds are classified based on physical appearance and the height at which clouds are formed (Fig. 7.1).

Clouds play very important role on weather and climate. The formation of clouds takes place when water condenses in the sky. Clouds are visible accumulation of tiny water droplets and ice crystals in the Earth’s atmosphere. The moisture is always present in the atmosphere though it may not be visible. Clouds form when air becomes cooler until the water vapour present in it condenses to liquid form. The condensation over oceans occurs around solid particles like salt, dust, smoke etc. suspended in the atmosphere. The type of clouds depends upon temperature and wind. The clouds are classified based on physical appearance and the height at which clouds are formed (Fig. 7.1).

The general circulation of the atmosphere explains the formation of pressure belts and wind systems in the earth’s atmosphere. It is known that the earth receives more solar radiation near the equator and gets heated more. Consequently, the air gets heated more and rises vertically upwards. As there is more convection of the air near the equator, the atmospheric pressure will be less. The warm air which moves upwards up to a height about 14 km near the equator gets cooled and move towards the poles. The upper air gets gradually cooled when it reaches a latitude of 30°N in the northern hemisphere and 30°S in the southern hemisphere. At about 30°latitude the cold air descends to the earth’s surface. Some of the cold air after descending to earth’s surface moves towards the equator. The circulation of air between the equator and 30°N or 30°S latitudes is called Hadley cells (Fig. 8.1). The winds blowing from 30° latitude towards the equator at surface level are called trade winds. These trade winds which move towards the equator in both northern hemisphere and southern hemisphere converge near the equator and the zone of the convergence of these winds is called Inter Tropical Zone of Convergence. As the convergence of air takes place, due to convection of heat as a result of high temperatures leads to vertical lifting of moist air. Therefore, the Equatorial regions receive more rainfall and there are tropical forests in this region.

The division of earth’s climates into worldwide system of contiguous regions, each one of which is defined by relative homogeneity of the climatic elements (Fig. 9.1) 9.1 Koppen’s Classification Koppen’s climatic classification describes 5 major groups of climate A, B, C, D and E which are sub-divided into a total of 14 individual climate types, along with a special category of Highland (H) climate. A - Tropical: Average temperature of coldest month is 18°C or more. B - Dry climate: Potential evapotranspiration exceeds precipitation. C - Warm temperature: The average temperature of the coldest month (of the middle latitudes) is higher than -3°C but below 18°C.

Drought is a situation when the demand for water is much greater than the availability of water. Drought is a creeping phenomenon and it is experienced only after it has set in. The scarcity of water compared to demand will have greater impact on every activity of human life including failure of crops, food shortages, decline in ground water table, drying tanks, lakes or reservoirs, acute shortage of drinking water for humans and livestock shortage for cattle etc. Drought will have very serious impact on economy of households as well as that of nation. There are five different types of droughts depending upon the nature of shortage of water 10.1 Meteorological Drought Meteorological drought occurs when the water received through rainfall during a given period is much less than long period average rainfall of the area. The meteorological drought is associated with either failure of rain or due to long gap between the successive rainfall events. In India, meteorological drought occurs due to failure of south west monsoon or long breaks in south west monsoon rainfall. In Tamil Nadu the drought occurs due to failure of north east monsoon rainfall during months of October to December.

Whenever the daily maximum temperature exceeds more than average values during summer season the heat wave will occur. Similarly, whenever the daily minimum temperature falls below the average values cold wave will occur. The India Meteorological department has developed the criteria to identify the occurrence the heat waves and cold waves. 11.1 Heat Waves Heat wave is defined as the condition when the daily maximum temperature is 45ºC or more. Based on departure from normal daily maximum temperature of 40ºC onwards the heat waves are defined as follows:

The weather parameters that may occur outside India will also play a dominant role on weather conditions in India. Such weather parameters are called as global weather parameters and were found to play very important role on weather even in most parts of the world. During normal conditions in the Pacific Ocean, trade winds blow west along the equator, taking warm water from South America towards Asia. To replace that warm water, cold water rises from the depths-a process called upwelling. These global parameters are popularly known as El Ni?o, La Ni?a, Southern Oscillation, and ENSO (combination of El Ni?o and Southern Oscillation).

India is one of the unique countries in the world with great diversity in climate. The annual rainfall varies from about 180 mm in North western part of India to almost 11,777 mm at Cherrapunji in Meghalaya. For a considerable amount of time, Cherrapunji had occupied the position of wettest place in India. However, off late, observations revealed that it is Mawsynram which receives the maximum amount of annual rainfall. Hence, Mawsynram with 11,872 mm annual rainfall is the wettest place in India. Both the places-Mawsynram and Cherrapunji-are located within 16 kilometers from each other in the East Khasi Hills district of Meghalaya. The maximum temperature at Churu in western Rajasthan is 50ºC and the lowest minimum temperature at Leh in Ladakh region is as low as -35ºC. In view of such great diversity in the distribution of rainfall and temperature in the country, it is necessary to have clear understanding on regional, seasonal and diurnal variation in different weather parameters (Fig. 13.1).

According to Thornthwaite’s method of climatic classification, six major types of climates exist in India (Fig. 14.1) as stated below: 14.1 Perhumid Regions of India (A) These regions receive rainfall of more than 2500 mm and consist of Konkan region along Western Ghats, Kerala and most parts of northeast India covering Assam, Meghalaya, Arunachal Pradesh and Tripura etc. 14.2 Humid Regions of India (B) These regions receive annual rainfall ranging from 2000-2500 mm and are found in areas adjoining perhumid climate.

From the beginning of 18th century, the human activities started increasing gradually. Industrialization, increased use of petroleum products, deforestation, mining and increased use of chemicals have contributed to the pollution of natural resource base. The proportion of gases such as carbon dioxide, nitrous oxide, methane etc started increasing in the atmosphere and more heat is getting trapped in the atmosphere leading to rise in atmospheric temperatures (Fig. 15.1). The changes in atmosphere temperatures have led to changes in weather patterns. This changes in weather pattern triggered extreme situations like heat wave, cold wave, severe cyclonic activity, strong winds and hail storms, f loods and droughts of greater intensity creating new records.

In meteorology, we often use some of the physical parameters to explain the physical processes taking place in the atmosphere. These parameters must have already appeared in the earlier chapters. However, these are given in this chapter to enable the readers to get acquainted once again. 16.1 Momentum Momentum is defined as the product of mass and its velocity. In S.I. system units mass is expressed in kilograms and velocity in meters per second. Momentum = Mass x Velocity Units for momentum (S.I. system) = kg. m/sec In C.G.S. System, mass is expressed in grams and velocity in cm/sec. Units for momentum (C.G.S System) = gm.cm/sec

Balasubramanian, T.N., Jagannathan, R. and Geethalaxmi, V. 2021. Agro-Climatology Advances and Challenges. New India Publishing Agency (NIPA). ISBN: 9788194766896. Pp 332 Bishnoi, O.P. 2007. Principles of Agricultural Meteorology. Oxford Book Company. ISBN -13: 9788189473013. Pp 420. Buchan, A. 2023. Introductory Text-Book of Meteorology. Legare Street Press.ISBN-13 : 978 1020278716. Pp 256. Byers, H.R. 1974. General Meteorology. New York (McGraw-Hill), 4th Edition. Pp. 461. Chandrasekhar, A. 2010. Basics of Atmospheric Science. PHI Learning Private Limited, New Delhi. ISBN: 13-9789391818241. Pp.464. Chang, J.H. 1968. Climate and Agriculture–An Ecological Survey. Aldine Publishing Company. ISBN: 9781351527972, 13-51527975. Pp. 320.