Preface Success or failure of a bio-system depends upon the weather conditions. The rate of growth and development of a bio-system is largely depending upon its ambient temperature and moisture. Agriculture is a field of the human activity which is especially sensitive to weather and climate. Each and every plant developmental phase is decided by meteorological parameters. Agricultural meteorology when properly applied can achieve the sustainability of agricultural production system through efficient management of agro-climatic resources and crop microclimate modification. The management and execution of agricultural farm practices, dairy farming, poultry, bee-keeping etc. are also weather dependent. The maximum efficiency of farm practices can be achieved with knowledge of prevailing and anticipated weather. Therefore, the knowledge of meteorological parameters and their influence on crop growth and yield is essential. Thus observations to be recorded extensively following standard methods and using well-calibrated instruments including the computation and the measurement techniques is absolutely essential. In agrometeorological, teaching and research, meteorological data play very important role and applications. Though, practical manuals of agrometeorology are published by many organisations but they are lacking in materials, lucid presentation and good photographs of instrumets. Agriculture students are not finding their course materials in one book. Hence, a practical book on agrometeorology is very much needed, after fourth Dean Committee reports, for undergraduate agricultural students. The present book of Practical Agrometerology is prepared, which provides materials on all aspect of practical agrometeorology of data acquisition techniques, instruments and method of observation with precautions.

Agriculture encounters a complex dynamic system of natural conditions. The meteorological factors among these natural conditions play a pivotal role so far as agriculture is concerned. The rate of growth and development of a bio-system is largely depending upon its ambient temperature and moisture. The plant productivity per unit area, total production, quality of the produce, operational efficiency as well as econometrics are solely governed by the meteorological parameters. Therefore, their through understanding including the computation and the measurement techniques is absolutely essential. It is no less important to have sufficient workable information on weather phenomena as well as climatic resources to determine realizable potential and an appropriate solution for a number of problems faced in agriculture.

Since meteorology and climatology are primarily observational sciences, adequate care has to be taken for getting most representative and accurate observations of weather parameters for their efficient application. This depends on site of observatory, maintenance of observatory, accuracy of instruments, timely measurements of weather parameters, knowledge and responsibility of the observer. Monitoring of weather conditions, in a country like India, which is climatologically so diverse because of its unique geographical situation is a challenging task. It is done through a large network of observatories of different types. India Meteorological Department (IMD), Pune, is entrusted with the collection and keeping the record of weather data collected from different stations and also to train the man power in this specialized area. Weather stations installed in different parts of the country are of different types.

Rainfall is precipitation (falling of any type of condensed moisture to the ground) in the form of liquid drops larger than 0.5 mm in diameter falling on the earth surface. Ordinary rain drop size varies from 0.5-4.0 mm in diameter. Rainfall is measured in term of depth of rain water (millimeters) on a solid surface provided that rain water is neither allowed to percolate nor runoff or loss in any form.

Snow is precipitation of solid water, mainly in the form of branched hexagonal crystals or stars. It is “precipitation of white opaque grains of ice”. Measurement of snow cover is related to runoff estimation and also important for hilly agriculture. 4.1. Snowfall It is the amount of fresh snow deposited over a specified period (generally 24 hours). The measurements relate to depth and water content of the snowfall.

Temperature influences most plant processes, including photosynthesis, transpiration, respiration, germination, flowering, adaptation and distribution of crops. As temperature increases (up to a point), photosynthesis, transpiration, and respiration increase. When combined with day-length, temperature also affects the change from vegetative (leafy) to reproductive (flowering) growth. Depending on the situation and the specific plant, the effect of temperature can either speed up or slow down this transition. Temperature is the measure of mean kinetic energy per molecule of the molecules in an object, while the heat is the measure of total kinetic energy of all the molecules of that object. A large object may  have a much lower temperature than a small object and may still have greater heat content by virtue of larger number of molecules in it. Thus heat and temperature are not the same.

The surface of the earth gets heated up during the day and gets cooled during the night causing diurnal changes in the top layers of the soil. Soil temperature is an important part of micro-environment, where plant roots grow and capture the water and nutrients and thus, it affects the plant growth and yield. During crop germination also soil temperatures play vital role. Since the heat regimes of these layers are governed by the soil temperature, the measurement of soil temperature becomes extremely important.

It is the actual minimum temperature experienced by the plants near ground surface. The grass minimum temperature is lower than the air temperature at 4 ft level. It is the temperature recorded in open air ground on short turf, with the bulb of the thermometer just in contact with the tips of the blades of grass. It is also described as the temperature at 5.0 cm (2 inches) above ground. It is used during October to March period only when frost occurrence is expected. The readings of the grass minimum thermometer indicate the possibility of ground frosts, which is important information for farmers, gardeners and growers.

The presence of moisture in atmosphere is termed as humidity and plays an important role in formation of weather system and plant as well as insect-pest growth and development. An abundance of moisture results in a rich natural flora and makes possible a wide choice of crops. Deficiency of moisture, on the other hand, permits only narrow range of crops. Humidity also affects the water requirement and the rate of transpiration. Reduction in transpiration reduces the translocation of food material and uptake of nutrients. High humidity with high temperature favours the outbreak of pest and disease. Moisture stress condition reduces the cell size and shape and ultimately plant growth and yield is reduced. High soil moisture and humidity condition cause failure in establishing stand of legumes and pulses. Excess of soil moisture in earlier growing season may result in more vegetative growth and delayed flowering.

Wind influences the plant life both physiologically and mechanically. Wind affects plant directly by increasing the transpiration and intake of CO2. Thus raising the supply of CO2 to the plants and thereby increase in photosynthesis. However, the increase in photosynthesis is again upto a certain wind speed beyond which its rate become constant. The warm and dry winds increase evaporative loss and cause moisture stress. Lodging is one of severe injury caused by strong winds which causes great yield loss depending upon the stage of crop. This injury is common in paddy, maize, wheat, sorghum and sugarcane. Strong winds damage fruit trees extensively by breaking leaves and twigs and causing fruit drops.

Sunshine means incoming solar radiation, which consists of a bundle of rays of different wavelengths. At the top of the atmosphere, the amount of radiant energy received is found to be constant (1.94 cal/cm2/min). The receipt of the radiant energy, which is received on the earth and on crop surface, is influence by the cloudy and sunny or clear sky conditions. The brightness hours of the sunshine are an important parameter, which not only influence microclimate of a place but also the photosynthesis of the crop. The radiant energy of intensity of 0.17 cal/cm2/min or 120 W/m2 and above is known to specify the bright sunshine.

Cloud is a visible aggregate of minute water droplets and/or ice particles, or both in the air usually above the general ground level. Clouds are the most important form of suspended water droplets caused by condensation. There is no precipitation without clouds. 11.1. Measuring Instrument The cloudy hours are measured with the help of eye or sensory parts of the body. No particular instrument is required for cloud amount or cloud cover measurement in the sky.

The solar energy is the main source of energy for the processes occurring at the earth surface and in its atmosphere. Crop production, is in fact an exploitation of solar radiation and plants are the converters of solar energy into a useful form of energy which is biomass and sustain life providing food, fodder, fiber, fuel, fruits etc. Sun energy fulfills two essential needs of the plants i.e.  (i) light for photosynthesis including many other photo processes and functions of the plants, (ii) thermal conditions required for the normal physiological functions of the plant. Thus, the crop production and agriculture is basically dependent on quality and quantity of radiation.

The change of state of water form solid or liquid to gaseous form is known as evaporation. It may be defined as the water loss in vapour form from a soil/water surface. The energy used in this process is called as latent heat. The energy of 540 cal/g of water is used in this process. This process plays a major role in the redistribution of thermal energy between the earth and the atmosphere. Water vapour diffuses into the atmosphere from the different natural surfaces like ocean, river, lake, pond, soil and plant etc. It is difficult to measure the evaporation from the natural surfaces. To measure the evaporation from open water surface is easy and is normally in practice. The rate of evaporation is dependent on the size of the measuring surface (Pan) at constant wind speed below saturation point. Therefore a measured surface (Pan) is designed to measure the evaporation.

Dew acts as a source of moisture for the crop plants. It causes reduction in transpiration rate and saving of soil moisture reserves. Dew fall helps in insect-pest and diseases spread in crop plants. 14.1 Dew and Conditions for Dewfall The radiational cooling during night causes the moisture to saturate and further cooling results into condensation and depositing this moisture in the form of tiny droplets on the grasses, vegetative and other non-conducting surfaces. This deposition form is known as dew. Clear nights, low wind velocity in the order of 1.0 to 3.0 m/s, presence of moderate to high moisture (RH = 75%) and sufficient radiational cooling are the four necessary and conductive conditions for the occurrence of dew.

Pressure of air at a given place is the force exerted against a surface by continuous collision of gas molecules. Pressure may be defined as the force per unit surface area.

Photosynthetically active radiation (PAR) is the radiation in the 400-700 nm wave band. For plants, 400-700 nm wave band of the light spectrum is very useful because of photosynthesis activity in this band. A simple integral relationship exists between the number of molecules changed photo-chemically and the photons absorbed within the requisite wave bands regardless of photon energy. Net photosynthesis rates are found to be almost linearly proportional to the radiation intercepted, which can be measured by line quantum sensor.

The most established method for detecting crop water stress remotely is through the measurement of a crop’s surface temperature. The correlation between surface temperature and water stress is based on the assumption that as a crop transpires, the evaporated water cools the leaves below that of air temperature. As the crop becomes water stressed, transpiration will decrease, and thus the leaf temperature will increase. Other factors need to be accounted for in order to get a good measure of actual stress levels, but leaf temperature is one of the most important.

Leaf area is an important attribute in bio-productivity studies, as it bears an important and direct relationship to photosynthesis and transpiration. Leaf area is also important in determining the percentage of solar radiation intercepted by an individual plant or crop canopy. Photosynthesis centers on individual leaf. However, productivity depends not only on individual leaves but on stand of vegetation and canopy structure. The photosynthetic productivity of a canopy depends on both the amount of leaves present and the configuration of the leaves making up the canopy. Thus, leaf area has direct bearing on plant growth and final yield.

Water loss from a plant in the form of vapour is known as transpiration. Loss of water vapour mainly from leaf cells through pores but also from the leaf cuticle and through lenticels of the stem. The resistance to diffusion of water vapour through a plant’s stomata is called as stomatal resistance. Whereas, stomatal conductance is a numerical measure of the maximum rate of passage of either water vapour or carbon dioxide through the stomata, or small pores of the plant.

The information on soil-water content is necessary from the point of view of efficient water management of crop production as water required by crops is to be met from the water stored in their root zone. Soil water content changes periodically as a result of changes in field water balance components namely, precipitation, irrigation, evapotranspiration, runoff and deep percolation. Crop growth is adversely affected if the soil water depletes below certain level. For selection of crops, deciding of irrigation level and frequency of irrigation, the assessment of water content of soil is important.

Evapotranspiration (ET) is the combined loss of water from vegetation and soil. When water is not a limiting factor at a site, the rate of ET is primarily controlled by meteorological factors like solar radiation, temperature, wind and relative humidity. The measurement of ET under natural conditions is of great importance in the water resource management and irrigation planning. Potential evapo-transpiration (PET) is the evaporation from a surface if water was never a limiting factor. PE-can also be calculated using empirical formulae.

Real time weather data at different time intervals is very important for the crop weather studies, but through the agro-meteorological observatories such real time weather data are generally not collected. The acquisition of agro-meteorological data through conventional observatories requires services of trained personnel to collect, compile and creation of weather data base. With the advances in technology, weather observations are automatically recorded, stored and can be transmitted to the remote place with the help of Automatic Weather Station (AWS).

Weather phenomena are not localized, it moves and varies over space and time. The weather over a place or area is the result of far away weather events/conditions. Acquisition of global scale observation, therefore, becomes a pre-requisite for understanding the physics of the weather and also to represent the weather condition over an area at a particular time. Complex three-dimensional models of weather systems can be made by collecting weather data at multiple levels in the atmosphere. Upper-air data are important to know present weather over a place and crucial for weather forecasting.

Weather has profound influence on agriculture, which demands that the changes in weather conditions are regularly monitored. In qualitative terms, weather conditions are often expressed on the basis of final condition created by interaction of different components. For example, too hot, too cold, sunny or cloudy, humid, dry weather conditions refer to specific atmospheric conditions but are least useful in analyzing the impact of weather on agriculture.   The measurements taken at a weather station can include any number of atmospheric observables. The WMO has laid down specifications, procedures and practices in respect of various meteorological matters such as instruments, observations, telecommunications, processing of data, analysis etc.

The agro-meteorological data after the quality control is used to establish a relationship between the crop and weather or to understand their interactions. In agrometeorological data analysis, statistical tools are used. The frequent use of statistics terms and their computational formulas are given below for ready reference:

Heat units also called growing degree-days (GDD), effective heat units, or growing degree units, are a heuristic tool in phenology. GDD is a simple means of relating plant growth, development and maturity to air temperature. The concept is widely accepted as a basis for building phenology and population dynamic models. Degree day units are often used in Agrometeorology, Agronomy, Entomology and Pathology essentially to estimate or predict the lengths of the different phases of development in crop plants, insects, pathogens etc.

Glossary Radiometric Terminology Albedo                                        The portion of incoming radiation which is reflected by a surface Azimuth angle                             Angle in horizontal direction (0-360º) Angle of incidence                      Incident angle from zenith (vertical) Cosine response                        Detector response according to the cosine law Diffuse solar irradiance              Solar radiation, scattered by water vapor, dust and other particles as it passes through the atmosphere Direct solar irradiance                Radiation that has traveled a straight path from the sun Global solar irradiance               Total irradiance falling on a horizontal surface  Irradiance                                   Radiant flux density (W/m2)