Preface This book Encyclopedia of Agricultural Meteorology is prepared keeping in view the new syllabus of Agricultural Scientists Recruitment Board and other competitive exams in India. This book is meant to provide basic knowledge about Agricultural Meteorology for all Post Graduate Students and scientists who are interested to carryout scientific research in this field. Agricultural meteorology is an applied science which is in the process of rapid development, spurred in particular by the need of many countries of the world for scientific advice and technical guidance to accelerate food production for a rapidly increasing population.  Agricultural Meteorology is concerned with the interactions between meteorological, soil and hydrological factors, on one hand, and agriculture, in the wider sense, including horticulture, forestry and animal production, on the other. Scientists engaged in these disciplines have felt a need for a book that would describe necessary terminology and other basic concepts in a nutshell which will be understandable not only to the practicing agrometeorologists but also to the field workers, researchers, students and specialists in the related areas. It is being very long time that an updated and scientific book having the above features available in the market. Encyclopedia of Agricultural Meteorology is an initial attempt to pool together in one compact volume definition and terms that are considered useful to the agrometeorologists and others concerned. The book is being prepared using reviewer comments from different discipline. It is hoped that this publication will promote the further collaboration and understanding among specialists.  In addition to purely Agricultural Meteorology terms, the book also includes other relevant soil scientific, agronomical, physiological  and hydrological terms, terms pertaining to Agricultural Meteorology aspects of crop and animal production. Pure Meteorological terms,  which are found in the meteorological glossary, and of interest to agrometeorologists, have also been included in this concise initial edition.

Abiotic Factors : These are physical, chemical and other non-living components of the environment such as light, temperature, moisture, nutrients and other edaphic factors.   Absolute Drought : It is defined as a period of consecutive days none of which had rainfall of 2.5 mm or more OR when actual rainfall is less than twice the mean deviation OR actual rainfall is less than half the normal weekly rainfall.   Absolute Growth Rate : The rate of increase in size of a growing plant (or part of it) in a given time under specific growing conditions. Absolute Humidity : Absolute humidity is defined as the actual mass of water vapour present in the given volume of air. It is measured in grams per cubic meter or in terms of partial pressure of water vapour in air in mb or mm.

Back-Door Cold Front : A front moving south or southwest along the Atlantic seaboard of the United States. Backing : A change in wind direction in a counterclockwise manner in the Northern Hemisphere and a clockwise manner in the Southern Hemisphere.   Ball Lightning : A rare form of lightning that may consist of a reddish, luminous ball of electricity or charged air.   Banner Cloud : A cloud extending downwind from an isolated mountain peak, often on an otherwise cloud-free day.   Bar : A unit of pressure, (usually the atmospheric pressure on) one is equivalent to a pressure of one million dynes per centimeter square one hundred kilopascals; 29.53 inches or 750 millimeters of mercury at zero degree Celsius.

C3 Plant : A plant in which the first product of CO2 fixation is the 3-carbon compound; phosphoglyceric acid and is comparatively photosynthetically less efficient than C4 plants e.g. wheat, rice, barley etc.  C4 Plant : A plant in which the first product of CO2 fixation is the 4-carbon compound oxalo acetic acid (OAA) and is photosynthetically more efficient than C3 plants e.g. sugarcane, maize etc. Calibration : Calibration with respect to models refers to the adjustments made to model parameters so as to give the most accurate comparison between simulated results and results obtained from field measurements.  Calm : Absence of appreciable wind : On the Beaufort Scale of wind force calm is accorded the figure 0 and has a wind speed equivalent or less than 1 knot.

Daily Maximum Temperature : As per the IMD guidelines, the maximum temperature recorded in a 24 hour period at any given place or location. Daily Mean Temperature : The average of the values observed during a day (at any regular intervals or maximum and minimum) of temperature. Daily Minimum Temperature : As per the IMD guidelines, the minimum temperature recorded in a 24 hour period at any given place or location. Daily Range of Temperature : The range of temperature (between maximum and minimum) recorded as per IMD guidelines in 24 hour period.

Earthquake : A sudden, transient motion or trembling of the earth’s crust, resulting from the waves in the earth caused by faulting of the rocks or by volcanic activity.      Easterlies : Usually applied to the broad patterns of persistent winds with an easterly component, such as the easterly trade winds. Easterly Wave : A wave like disturbance, of the tropospheric easterlies of low latitude. These are synoptic scale and occur in the region of intertropical convergence zone. Ecoclimate : Climate under crop canopy is called eco-climate.  Edaphic : A term pertaining to the influence or relationship of soil or other similar media to plant growth in contrast to atmospheric influences. 

Fahrenheit Temperature Scale : A temperature scale having freezing point of water as 32 degrees and the boiling point equal to 212 degrees at standard atmospheric pressure.      Degree F = (Degree C x 1.8) + 32     Degree C = (Degree F - 32)/1.8 Fair Weather : Fine sky. Appearance of cirrus or cumulus cover is in fair amount but not covering the sky totally. Fetch : The distance over which a wind of nearly constant direction has blown, usually over a consistent surface such as water or a forest or a field. Most often the fetch is applied to a distance over water and used to determine the height of wind-generated waves.   Fine Weather : A blue and cloudless sky. Even if a few cirrus clouds are noticed, they do not express any vertical development.

Gale : Wind of force 8 or above blowing continuously and damaging trees, houses etc. General Circulation : The average general distribution of wind in motion on the Earth. This general circulation includes the scientific description of atmospheric dynamics not only statistically but also from the point of view of physics. The global wind systems like westerlies, trades, etc., are the examples of general circulation. General Circulation Model : At present, there are different models in vogue, to explain the general circulation of the atmosphere. If the general circulation is modelled in a three dimensional procedure, then it is known as a general circulation model. Geopotential : The potential energy per unit mass of a body due to the earth’s gravitational field referred to an arbitrary zero.

Haboob : Sudanese name for duststorm or sandstorm with strong winds that carry small particles of dirt or sand into the air, particularly severe in areas of drought.   Habitat : A place where a plant or a community of plants or a crop is existing. Hadley Cell : A mean thermally driven atmospheric circulation in which warm air rises and flows away from the equator at upper level and descends at the tropics. Wind moves from the tropics back to equator poleward. In many cases this term is also referred to as ‘Hadley circulation’. Hail : Solid precipitation in the form of balls or pieces if ice (hailstones) with diameter ranging from 5 mm to 50 mm or even more.  Hailstorm : A storm, often too long and severe, consisting largely of hail or frozen rain drops ranging in diameter from 5 mm to 10 mm or more. 

Ice Caps : A perennial ice/snow cover over an extensive area of land or sea, found only around the two geographic poles. Ice Columns : Ice crystals resembling stubby pencils rather than the delicate branched snowflake shape. Columns typically form in the temperature ranges -5° C to -8° C (23° F to 18° F) and below -25° C ( -13° F). They are long in comparison to their hexagonal cross-section. Ice Crystals : 1) Hexagonal (6-sided) crystals that form upon freezing of water, may be in one of several shapes : stars, needles, plates, columns or combinations of these forms. 2) Precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates, may be called diamond dust. Ice Fog : Fog composed of minute ice particles that occur in very low temperatures (typically - 30°C or - 22°F & below) under clear, calm conditions in the polar latitudes. 

January Thaw : A period of mild weather, usually following a cold spell, which often occurs in late January in the eastern regions of North America, particularly around the Great Lakes region, New England and the Canadian Maritime Provinces. The thaw is associated with a strong flow of warm air northward from the Gulf of Mexico on southerly winds on the back of a high pressure system. Jet Stream : A relatively narrow band of high-speed winds, generally greater than 50 knots (57 mph or 93 kmph), found in the upper troposphere above regions of strong horizontal temperature contrasts such as fronts. The major jet streams are the subtropical jet and the polar jet stream. Joule : The unit of work or energy. In mechanical systems, this is the work done when a force of one Newton displaces the point of application by a distance of one metre in the direction of force. In electrical systems, a joule is the equivalent of moving one coulomb through a potential difference of one volt. One joule is equal to one watt per second

Katabatic Wind : Downslope gravitational flow of the colder, denser air beneath the warmer, lighter air results and comprises the katabatic wind. It is also known as the drainage wind or mountain wind. Kelvin Temperature Scale : An absolute temperature scale in which the zero point is absolute zero. The scale is based upon the triple point of pure water and the size of the degree is same as on the celsius scale. So the 273.16 K = 0°C. That means the freezing point of water is 273.16 K and boiling point of water is 373.16 K, etc. This is more commonly called as the absolute.

Lake Effect : The effect of any lake in modifying the weather and climate along its shore and inland which depends on the size of the lake. In some areas such as the North American Great Lakes, lake effect refers to snowfall generated by the lake on the downwind shore. Lake Effect Snowfall : Snowfalls along the lee shore of a lake or downwind some distance from the shore caused by the modification of cold, subfreezing air by the relatively warmer lake water. The intensity of the lake-effect snowfall depends upon several factors : the temperature contrast between the lake surface and the air passing over it, the over-water distance the air has traversed (the fetch), and the regional weather situation. 

Macroclimate : It deals with the study of atmosphere over large areas of the earth and with the large scale atmospheric motions that cause weather. It helps in forecasting weather. OR Climate of a continent or over the complete globe.   Macrometeorology : A branch of meteorology concerned with the study of large scale meteorological phenomena over a continent or the entire globe. Macroscale : The meteorological scale covering an area ranging from the size of a continent to the entire globe.   Marine climate : Almost similar and on many occasions used as a synonym to maritime climate. The climate is basically influenced by adjacent seas and oceans.   Maritime Climate : The climate experienced in areas and regions near the oceans and seas. It is characterised by hot dry summers and mild wet winters. The diurnal variations of temperature are small and air is with high relative humidity.

Nanometre : The unit of length specifically applied to wavelengths of radiation. This is equal to ten Angstrom units or 10-9 meters. Net Assimilation Rate (NAR) : It is increase in dry weight of plant per unit leaf area per unit time. NAR is calculated from the following equation : 

Oasis effect : Vertical energy transfer from the air to the crop. It is the exchange of heat between a growing crop and hot air whereby air over the crop is cooled.   Observation : In meteorology, the evaluation of one or more meteorological elements, such as temperature, pressure, or wind, that describe the state of the atmosphere, either at the earth’s surface or aloft. An observer is one who records the evaluations of the meteorological elements.   Occluded Front : Also known as an occlusion, it is a complex front formed when a cold front overtakes a warm front. It develops when three thermally different air masses conflict. The type of frontal boundary they create depends on the manner in which they meet.

Pan Coefficient (Kp) : It is the ratio between reference evapotranspiration (ETo ) and water loss by evaporation from an open water surface of a pan (Epan ).     Kp = ETo / Epan Peat soil : An organic soil containing more than 50 per cent organic matter. Pan Evaporation : is the value of evaporation from an open pan evaporimeter (usually from standard USWB, class ‘A’ pan evaporimeter) during a day or specified period at a place. Parcel : A volume of air small enough to contain uniform distribution of its meteorological properties and large enough to remain relatively self-contained and respond to all meteorological processes.

Q10 : The number of times the rate of a process (physical, chemical or physiological) increases with a 10ºC increase in temperature. If the rate of the process is doubled Q10 is 2. Q10 for respiration is 2 to 3. Quantum : Unit of energy emitted by a photon. Quantum Flux Density : The number of quanta from a source, incident on a unit area of a small surface situated in a perpendicular direction. Quantum Irradiance : The number of quanta from a source incident on a unit area of a surface. This differs from the ‘quantum flux density’ as far as direction is concerned.

Rabi : The winter crop season in India from November to April. Main crops grown in rabi comprise wheat, mustard, etc. Rad : The ‘rad’ is the unit of radiation dose and is defined as the total absorbed dose of 100 ergs of energy per gram of tissue in a unit time. Radar : An electronic instrument used to detect objects (such as falling precipitation) by their ability to reflect and scatter microwaves back to a receiver. The radar is used for detecting tropical cyclones and monitoring their course as they approach the coast.   Radar sonde : A system in which both meteorological and wind data may be obtained, as in radiosonde. Radiance : Radiant flux per unit area per unit solid angle. Radiance has unit of watts per meter per meter per steradian. The radiance of a perfectly diffuse surface (Lambertian) is independent of the viewing angle.

Saffir-Simpson Scale : A scale relating a hurricane’s central pressure and winds to the possible damage it is capable of inflicting.   Sand Storm : An ensemble of particles of dust or sand energetically lifted to great heights by a strong and turbulent wind.   Santa Ana Wind : A warm, dry wind that blows into southern California from the east off the elevated desert plateau. Its warmth is derived from compressional heating.   Satellite Imagery : Images formed from data collected by a weather satellite that reveal visual information on the nature of the atmosphere such as the flow of water vapour, the development and movement of weather systems, or the distribution of the temperatures of cloud tops or the earth’s surface. Saturated Adiabat : Any statistically valid thermodynamic diagram on a graph related to adiabatic change under saturated conditions of an air parcel. 

Taiga (Boreal Forest) : The open northern part of the coniferous forest. Taiga also refers to sub-polar climate.   Tail Wind : The wind that assists the further movement of an object by blowing in the same direction in which the object is actually moving.   Tangent Arc : An arc of light tangent to a halo. It forms by refraction of light through ice crystals.   Tcu : An abbreviation sometimes used to denote a towering cumulus cloud (cumulus congestus).   Teleconnections : A linkage between weather changes occurring in widely separated regions of the world. 

Ultraviolet Radiation : Ultraviolet radiation (UV) is one form of radiant energy coming from the sun. The sun emits a range of energy known as the electromagnetic spectrum. The various forms of energy, or radiation, are classified according to wavelength (measured in nanometers (nm) where one nm is a millionth of a millimeter). The shorter the wavelength, the more energetic is the radiation. In order of decreasing energy, the principal forms of radiation are gamma rays, X-rays, UV (ultraviolet radiation), visible light, infrared radiation, microwaves, and radio waves. Ultraviolet, which is invisible, is so named because it occurs next to violet in the visible light spectrum. Ultraviolet radiation contains the wavelengths between 30 and 400 nm. The UV band is divided into three sub-bands in regard to its biological impacts :

Vacuum : A space in which there is no air or gas. This is more an imaginary situation. This term indicate the air or any other gases at the minimum possible pressure in a space.   Validation : Model validation in its simplest form is a comparison between simulated and observed values. Validation is determining whether the model works with totally independent data sets or not and whether it accurately predicts growth, yield and processes by comparing the values predicted by the model with those obtained from the real experiment.

Walker Circulation : A thermally driven longitudinal cellular circulation over equatorial oceans, particularly Pacific Ocean, Peruvian coast, etc. These circulations rise in the West and sink in the East.   Warm Air : If the temperature of air is relatively warm as compared to an underlying surface or another adjacent air column or within itself, then the air is said to be warm air. Warm Front : A front whose movement is such that the warmer air mass is replacing colder air mass.   Warm Sector : The state of lower troposphere in between a warm front and the following cold front. 

Xlokk : The name of a hot, dry wind that blows regularly in Malta. X-rays : The electromagnetic radiation in the wave length region below 100 angstroms.

Year : The time taken by the earth to revolve once in its orbit round the sun. Yellow Snow : Snow which has a yellow or golden colour due to the presence of pine or cypress pollen in it.

Zenith : The point directly overhead on the celestial sphere.   Zenith Angle : The angle formed by the local line perpendicular to the surface on which the radiation falls, and the position of the Sun in the sky from which the incident ray falls. 

Did You Know? 1.    During the summer, rainbows are occasionally seen after a thunderstorm. Because of this fact, the Shoshone Indians viewed the rainbow as a giant serpent that would sometimes rub its back on the icy sky and hurl pieces of ice (hail) to the ground. 2.    The total amount of precipitation to fall to earth in one year is 5,000 million million tones. 3.    The amount of water held in the atmosphere at any time is sufficient to produce about 2.5 cm (1 inch) of rain over the surface of the earth. 4.    The driest place on earth is Wadi Halfa in Sudan, with an annual average rainfall of less than 2.5 mm (less than one-tenth of an inch). 5.    The most rain to fall in a single 24 hour period is 1,850 mm  (74 inches) at Cilaos (on the Indian Ocean island of Reunion) between 15 and 16 March 1952. 6.    The wettest place in the world (based on the yearly average total) is Mawsynram, India, which receives an average of 11,870 mm (474.8 inches) of rain each year. 7.    The world’s greatest temperature range at a single location is 105°C, from minus 68°C to 37°C recorded at Verkhoyansk, Siberia.