eChapters

Aim

To study the different materials used for chain survey

Theory

Chain surveying is the method of land surveying in which only linear measurements are made. This method of surveying quit simple and it requires only a chain, a tape, few ranging rods and few arrows. It is used for securing data for exact description, marking the boundaries of land and preparing the maps of the area to show various details. The term details mean a natural or man-made feature at the ground surface.

Aim

To design of greenhouse cooling system

To determine the rate of air exchange in an active summer and winter cooling system.

Theory

Elevation factor (F_elev)

Elevation factor is directly proportional to the elevation. The values of elevation factors, used to correct the rate of air removal for a particular elevation. The rate of air removal from the greenhouse increase as the elevation of the greenhouse site increases.

Theory

Heat required for heating greenhouses is based on the quantity of heat lost through different structural components of greenhouses per unit time. Shapes of greenhouse (A-frame and quonset) and structural components (roof, wall, gable, curtain wall, curved covering and end) are mainly influencing the calculation of heat requirement. The amount of heat lost from a greenhouse depends on glazing materials, exposed surface area of the house, inside desired temperature, outside existing temperature, wind speed, and condition of house (old, new, tight or loose). The outside temperature should be based on the lowest average expected temperature during the heating season.

Greenhouse selection

Selection of greenhouse to be constructed is important as each type has its own merits and demerits. There are two common types of commercial greenhouses namely; (i) Freestanding (single span) and (ii) Ridge and furrow (gutter connected or multi-span). Multi-span structure permits economy in construction, heating and cooling but the single span gives flexibility as far as microclimate inside the greenhouse in concerned. The freestanding Quonset type is usually accommodates many growing situations but there height restrictions near the side walls. Another freestanding type is single gable greenhouse and its many variations. The ridge and furrow or gutter connected greenhouse are joined at the eave by a common gutter.

Aim

To determine cost of operation of processing machineries with available data.

Total operation cost

For calculation of cost of operation of processing machineries, two types of cost viz., fixed cost and variable cost to be estimated.

Description

Bucket elevator is one of the equipments used in the material handling to convey the material vertically. The lab model bucket elevator (Fig.34.1) consists of number of buckets/cups arranged to an endless flat belt. The belt with cups run vertically and cups are placed at a particular interval in the belt. The belt is wrapped over two pulleys, one at the top is called head pulley which receives power from an electrical motor 0.5 hp through suitable power concentration and rotates at required rpm. The belts, cups and pulleys are mounted on a steel frame. The grain from the hopper is reaches to the bottom pulley. The bucket is filled with grains during upward movement and is discharged at the top due to the centrifugal force. An outlet is provided at the top to collect the discharge grain. The capacity depends on the bucket size and seed of operation. For proper discharge of the material, the weight of the material in each cup should be equal to the centrifugal force created due to rotation.

Description

Screw conveyor is one of the equipment used in material handling to convey the material horizontally. The screw conveyor consists of screw blade, screw shaft, coupling, tubular trough, cover, inlet and outlet gates, bearings and drive mechanism. A shaft with spiral screw revolves in the tubular trough. The screw shaft is supported by end and hanger bearings. The rotation of screw pushes the grain along the trough. The pitch of a stranded screw which is the distance from the center of one thread to the center of the next thread is equal to its diameter

Aim

To design a belt conveying system

Theory

The design of belt conveying system is based on available space, horizontal conveying length, conveying lift, characteristics of the materials to be conveyed and capacity requirement.

On the basis of overall requirement and information, the belt width, belt speed, required horsepower, maximum belt tension and breaking strength of the belt, diameter of the pulleys and idlers and quantity of belt (thickness) are determined to design a belt conveyor (Fig.32.1).

Aim

To design a bucket elevator

Theory

The bucket elevator’s capacity mainly depends on bucket size, conveying speed, bucket design and spacing, the way of loading and unloading, the bucket and the characteristic of bulk material.

Bucket elevators with a belt carrier can be used at fairly speeds of 2.5 to 4 m/s. The speed of the belt depends on the head pulley.

If the belt speed is too low, the discharge of the grains become more difficult, with too high speed the buckets are not fed well.

Aim

To design a screw conveyor

Theory

The screw conveyor consists of screw blade, screw shaft, coupling, tubular trough, cover, inlet and outlet gates, bearings and drive mechanism.

A shaft with spiral screw revolves in the tubular trough. The screw shaft is supported by end and hanger bearings. The rotation of screw pushes the grain along the trough.

The screw blade or flight is a continuous one-piece helix, shaped from a flat strip of steel welded on to the shaft.

The screw shaft is usually a joint less tube with thick sides and a high tensile strength to reduce the weight.

Aim

To design shallow and deep bins

Theory

The structure behaves as a deep or shallow bin depending upon its relative dimensions such as depth and diameter or width. It is decided from the concept of the plane of rupture.

Plane of rupture: The plane of rupture is that surface down which a wedge of material bounded by one wall face, the free surface, and the plane of rupture would start sliding if bounding wall were to move.

Specifications

These structures are generally used for the storing grain in the bags. The length of the structure is about twice the width or greater than that. Special care should be taken to make the side walls, floor and roof in constructing the structure. The walls are made of bricks or stones laid either in lime mortar (1:2) or cement mortar (1:6). The thickness is restricted to a minimum of 37.5 cm or maximum of 45 cm. The height of the walls on which the trusses are placed is kept about 5.5 m. Each of the structures is provided with two large size doors of 2.4 × 2.4 m and top ventilators.

Rubber-roll Sheller consists of two rubber rolls rotating in opposite direction at different speeds. One of the roll is fixed while the other is adjustable to obtain desired clearance between them. The rolls are driven by mechanically. The adjustable roll normally runs about 25% slower than the fixed one. A feeder feeds paddy uniformly to the machine (Fig.27.1). Paddy is fed in thin layer through the clearance between two rubber rolls, rotating in opposite directions at different speed. Difference in speeds of the rolls develops a shearing force on grain surface results in the splitting, breaking and separating the husk from the grains. One part of the husk is subjected to shearing forces whereas the other part in contact with the slower roll is under compression and is thus subjected to breaking forces. Husking is done by the action of these forces. The clearance between them should be smaller than the mean thickness of paddy and may be adjusted subsequently by judging the shelling efficiency. If the gap between rolls is properly adjusted, it can shell up to 95% of paddy fed to it. At decreased gap excess pressure results more breakage of grain and can also cause coloring of shelled rice. During shelling the faster roll should have a peripheral speed of 10 -13 m/ s, while the slower roll 8-10 m/s.

Theory

Paddy parboiling is a hydrothermal process where the paddy is partially boiled to gelatinize the starch. During the gelatinization process the void of the rice kernel are filled and become hard on drying resulting minimum breakage and maximum head rice recovery. The nutrients are also absorbed into the inner mass and loss of nutrient is avoided. In general, the steps are involved in parboiling process are (i) Soaking (ii) Steaming (iii) Drying.

Theory

Removal of outer sheath is called husking and removal of kernel from maize from maize cob is called shelling. The performance of the sheller is expressed in terms of capacity, percentage of unshelled kernels (corn) and percentage of breakage. The above terms are defined as:

Aim

To determine the efficiency of groundnut decorticator

Apparatus required

Manually operated groundnut decorticator

Weighing balance

Groundnut pods

Aim

To determine the effectiveness of the spiral separator.

Materials required

Spiral separator

Weighing balance

Sample divider

Stopwatch

Grain mixture (round and others)

Aim

To evaluate the performance of the gravity separator.

Materials required

Specific gravity separator

Weighing balance

Sample divider

Stopwatch

Grains

Aim

To determine the effectiveness of inclined belt separator

To determine the capacity of inclined belt separator

Materials required

Inclined belt separator

Weighing balance

Feed material

Sample divider

Aim

To study the working principle and cleaning efficiency of air-screen seed cleaner cum grader

Apparatus required

Lab model air-screen seed cleaner cum grader

Seed sample

Weighing balance

Theory

Fluidized bed drying is a method of drying by keeping the material in fluidized/ floating condition inside a chamber by hot air. In this drying process higher rate of moisture migration takes place.

Aim

To draw the drying characteristic curve for given grain by thin layer drying.

To determine the drying constant, heat utilization factor and coefficient of performance.

Aim

To study the determination of drying rate of agricultural products

Problem

In a drying experiment on paddy at an air temperature of 60ºC, the following data were obtained. Prepare a drying rate curve for the experiment. Initial weight of the sample is 1250 gm and initial moisture content is 35% (d.b).

Aim

To study the numerical problems on grain moisture content and equilibrium moisture content.

Problem 1

Five hundred kilograms of paddy at 22% moisture content (wb) is dried to 14% moisture content (wb) for milling. Calculate the amount of moisture removed in drying.

Theory

This type of moisture measurement is called brown-duvel fractional distillation method. The method is recognized as an official method for determination of moisture content. In this method, directly measures the volume of moisture in cubic centimeter. This apparatus consists of flask, burner, thermometer, condenser, graduated cylinder. This apparatus consists of flask in which grain sample is boiled with mineral oil.

Theory

Indirect methods are based on the measurement of electrical properties of the grain that depends upon moisture content. There are two indirect methods namely (i) electrical resistance and (ii) dielectric methods are used. The electrical conductivity or resistance of a product measures its moisture content at a given compaction and temperature. The electrical resistance is depends on its moisture content, degree of compaction, grain temperature and impurities present in the sample. The universal digital moisture meter measures the electrical resistance of the grain at a given compaction. Universal moisture meter gives fairly accurate readings of moisture content on wet basis. In dielectric method, grain moisture content can be measured with the help of dielectric properties of the grains.

Theory

The amount of moisture in a product is given on the basis of the weight of water present in the product and is usually expressed in percent. Moisture content is designated by (i) wet basis (wb) and (ii) Dry basis (db). The moisture content on dry basis is more simple to use in calculation as the quantity of moisture present at any time is directly proportional to the moisture content on dry basis.

Theory

Texture testing is a well-established technique for evaluating the mechanical properties of raw materials, food structure, pre- and post- harvest quality control checks. The texture testing has many applications across wide range of foods including cereals, fruits, vegetables, meat, poultry, fish, baked foods, confectionaries, snacks, dairy, gelatins, pasta, etc. Food texture analysis of foods can highlight the quality improvement throughout the supply chain. At research and development stage, new or alternative ingredients can be compared with existing ingredients.

Texture profile parameters

The texture profile parameters including hardness, firmness, crispiness, rupture strength springiness, chewiness, cohesiveness, gumminess, gel strength, adhesiveness, consistency, crunchiness, elasticity, extensibility, fracturability, stiffness, stringiness, toughness, work to cut, penetrate and shear can be measured by the mechanical food texture analyzer.

Theory

The energy available in food is expressed in unit is called joules or calories. One kilocalorie is equal to 4.184 kilojoules. The kilocalorie is defined as the amount of heat required to raise the temperature of 1 kg of water by 1°C. The calorific/ energy value of foods is usually determined using the instrument called bomb calorimeter. It is working based on the principles of calorimetry. Basically, this device burns a food sample and transfers the heat into a known mass of water. It measures the heat produced when the food sample is ignited by an electric spark in the presence of oxygen and platinum which acts as a catalyst. The quantity of food sample to be tested is placed inner part and known volume of water is taken in outer part of the bomb calorimeter. When the food sample is electrically ignited, the surrounding water absorbs the heat produced. The energy value of the food is calculated by measuring the rise in temperature of water.

Theory

Specific heat is defined as the amount of heat needed to raise the temperature of one kg of the material by one degree at constant pressure without a change in state. With food materials, specific heat is a function of different components of a food, moisture content, temperature, and pressure. Specific heat is an important parameter in thermal analysis of food processing and equipment used in heating or cooling of foods. The numerical values of specific heat of the food materials are used in designing food processes and processing equipment. It can be used to calculate the heat load imposed on the refrigeration equipment by the freezing of foods. An apparent specific heat is used in freezing or thawing and in most food processing applications.

The specific heat of a food increases as the product moisture content increases. In unfrozen foods, specific heat capacity is relatively constant with respect to temperature. However, for frozen foods, there is a large decrease in specific heat capacity as the temperature decreases. The specific heat of a material can be determined experimentally by static (adiabatic) calorimetry or differential scanning calorimetry. It can be also predicted quite accurately with the help of a number of empirical equations. The equations are empirical expressions, obtained by fitting experimental data into mathematical models. The mathematical models are based on one or more constituents of the food. Since water is a major component of many foods, a number of models are expressed as a function of water content.

Aim

To determine the thermal conductivity of the agricultural materials probe method.

Materials required

Thermal conductivity probe

Stainless-steel sample container

Grain or powder sample

Graph sheet

Power source

Aim

To determine the angle of repose of the given biological materials.

Materials required

Angle of repose apparatus
Grains
Scale

Aim

To determine the water solubility and absorption index of given food powder.

Materials required

Digital centrifuge machine

Graduated measuring cylinder

Beaker and distilled water

Theory

The volume of food material play an important role in design of storage bins, conveying equipments, separation equipments, etc. Volume of larger products such as fruits and root crops can be determined using the platform scale or water displacement method.

Theory

The specific gravity is defined as the unit weight of the product mass divided by the unit weight of distilled water at 4°C. The unit density is obtained by measuring the volume and weight of a unit product. Sometimes it is required to compare the density of the product to the density of water. This comparison is in the form of ratio and is termed as the specific gravity.

Specific gravity also known as relative density is the ratio of density of a substance to the density of a standard substance at the same temperature and pressure. For solids and liquids the standard substance is water while for gasses it is air. The specific gravity of food material play an important role in design of storage bins, conveying equipments, separation equipments, etc. The specific gravity is frequently used to solve the various phase relationships, including void ratio, porosity and degree of saturation together with the moisture content and unit weight. It is also used in calculations associated with the grain size analysis, consolidation and compaction. Pycnometer method, which is a standardized vessel for determining specific gravity. It involves a standard container of accurately defined volume used to determine the specific gravity of solids and liquids.

Theory

Porosity is a physical property characterizing the texture and the quality of dry and intermediate moisture foods. Porosity means inter-granular space. It is the measure of void space between the materials. Porosity may be defined as the percentage of volume of inter-grain space to the total volume of grain bulk. Porosity depends on size, shape, dimension, and roughness of grain surface.

The porosity of an unconsolidated agricultural material can either be determined experimentally using the porosity tank method or theoretically from bulk and true densities of the material. Porosity can be determined from the difference between bulk volume of a porous material and its volume after destruction of all voids by compression. Porosity determination requires the knowledge of the total or apparent volume, and the void volume included in the material matrix; their difference is also known as the particle volume.

Aim

To determine the tapped bulk density and flowability of the food powder.

Materials required

Raw material (powder)

Measuring cylinder

Electronic weighing balance

Tapped bulk density of the powder

Tapped bulk density is measured for powdery samples. The volume is measured after tapping the powdery material so that the product is adjusted to inter-granular spaces, but it cannot eliminate all the inter-granular spaces. The tapped density is an increased bulk density attained after mechanically tapping a container containing the powder sample.

Theory

Bulk density of the food material play an important role in many applications such as drying, storing, separation of undesirable material, determining the purity of seeds, grinding, maturity evaluations, design of storage structures, etc. It is an important parameter in packaging requirement and is often 2-20% of true density. Bulk density is the density of material when stacked or packed in bulk. Bulk density of powder can be determined under loose conditions. Loose packing is defined as the state obtained by pouring a powder sample into a vessel without any consolidation.

Bulk density

Bulk density of the food material can be defined as the ratio of weight of an untapped material to its volume including the contribution of the inter-particulate void volume. There are two kinds of voids, voids between particles and void within particles. Weight of the material can be determined by weighing balance. Volume can be determined by measured dimensions or using liquid or gas or solid displacement methods.

Determination of size of the food materials

Size is an important physical attribute of foods used in screening solids to separate foreign materials, grading of fruits and vegetables, and evaluating the quality of food materials. Various types of cleaning, grading and separation equipment are designed on the basis of size. The size and shape of the grains are used to determine the size of screen openings, angle of inclination, vibration amplitude and frequency of screen in designing an air screen grain cleaner and conveying characteristics of the solid materials.

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

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.

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 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.

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.

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.

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.

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.

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.

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.

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.