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The Tenth Five Year Plan (GoI 2002), for 2002 through 2007, has put emphasis on natural resource management through rainwater harvesting, groundwater recharging measures and controlling groundwater exploitation, watershed development, treatment of waterlogged areas. With regard to application of agricultural inputs like fertilizer and pesticides, the Plan stated that factors such as imbalanced use of nitrogenous (N), phosphatic (P) and postassic (K) fertilisers, increased deficiency of micronutrients and decreased soil organic carbon would be addressed through a holistic agri-environmental approach stressing Integrated Plant Nutrient and Pest Management. Further, the Tenth Plan document recognizes organic farming as a ‘thrust area’ in the sustainable use and management of resources in agriculture.

The trajectory of Indian agriculture and its associated environmental problems has brought about recognition that future agricultural growth and productivity will have to occur simultaneously with environmental sustainability. The environmental challenges, especially in terms of land degradation and groundwater depletion, water logging and excessive use of chemical inputs are posing problems for the future of Indian agriculture. To address the problems, policies have laid emphasis on promoting sustainable agriculture including organic farming. Differential approaches and policy instruments, however, will be required to address these problems. The shift from input-intensive to sustainable, particularly organic farming is a difficult task as it involves a number of policy measures dealing with a variety of issues ranging from the transfer of information and technology to the development of markets. Another difficult task, and perhaps more difficult, relates to marginal and small farmers – which comprise a substantial part of Indian agriculture. Although these marginal and small farmers have been considered organic by ‘default’, severe resource constraints make a shift to the modern sense of organic farming prohibitive.

The Indian government has been undertaking measures to promote organic farming with the aim to improve soil fertility and help to double the farmers’ incomes by the year 2022. The Prime Minister had visited Sikkim—which is India’s first organic state—and encouraged other states to replicate the “Sikkim model”. Some of the policy initiatives to promote organic farming and exports include development of an organic regulation for exports by the Agricultural and Processed Food Products Export Development Authority (APEDA), removal of quantitative restriction on organic food exports, providing subsidies to farmers under the Paramparagat Krishi Vikas Yojana (PKVY) in partnership with the state governments, and other schemes such as the Mission Organic Value Chain Development for North Eastern Region. Despite these initiatives, a recent survey-based study covering 418 organic farmers across different states of India suggests that a move to organic farming methods may not be that easy and organic farmers are not getting the expected premium price for their produce.

Agriculture in India is not of recent origin, but has a long history dating back to Neolithic age (7500–6500 B.C). It changed the life style of early man from nomadic hunter of wild berries and roots to cultivator of land. Agriculture is benefited from the wisdom and teachings of great saints. The wisdom gained and practices adopted have been passed down through generations. The traditional farmers have developed the nature friendly farming systems and practices such as mixed farming, mixed cropping, crop rotation etc. The great epics of ancient India convey the depth of knowledge possessed by the older generations of the farmers of India. The modern society has lost sight of the importance of the traditional knowledge, which had been subjected to a process of refinement through generations of experience. The ecological considerations shown by the traditional farmers in their farming activities are now a days is reflected in the resurgence of organic agriculture.

Historical Perspective of Organic Farming

The available ancient literature includes the four Vedas, nine Brahnanas, Aranyakas, Sutra literature, Susruta Samhita, Charaka Samhita, Upanishads, the epics Ramayana and Mahabharata, eighteen Puranas, Buddhist and Jain literature, and texts such as Krishi-Parashara, Kautilya’s, Artha-sastra, Panini’s Ashtadhyahi, Sangam literature of Tamils, Manusmirit, Varahamihira’s Brahat Samhita, Amarkosha, Kashyapiya-Krishisukti and Surapala’s Vriskshayurveda. This literature was most likely to have been composed between 6000 B.C. and 1000 A.D. The information related to the biodiversity and agriculture (including animal husbandry) is available in these texts. Rig-Veda is the most ancient literary work of India. It believed that Gods were the foremost among agriculturists. According to ‘Amarakosha’, Aryans were agriculturists. Manu and Kautilya prescribed agriculture, cattle rearing and commerce as essential subjects, which the king must learn. According to Patanjali the economy of the country depended on agriculture and cattle-breeding. Plenty of information is available in ‘Puranas’, which reveals that ancient Indians had intimate knowledge on all agricultural operations. Some of the well known ancient classics of India are namely, Kautilya’s‘Arthashastra’; Panini’s ‘Astadhyayi’; Patanjali’s ‘Mahabhasya’; Varahamihira’s ‘Brahat Samhita’; Amarsimha’s ‘Amarkosha’ and Encyclopedic works of Manasollasa. These classics testify the knowledge and wisdom of the people of ancient period. Technical book dealing exclusively with agriculture was Sage Parashara’s ‘Krishiparashara’ in 1000 A.D. Other important texts are Agni Purana and Krishi Sukti attributed to Kashyap (500 A.D.).

Organic farming system in India is not new and is being followed from ancient time. It is a method of farming system which primarily aimed at cultivating the land and raising crops in such a way, as to keep the soil alive and in good health by use of organic wastes (crop, animal and farm wastes, aquatic wastes) and other biological materials along with beneficial microbes (biofertilizers) to release nutrients to crops for sustainable production.

Presently the farming situation urges need to develop farming techniques, which are sustainable from environmental, production, and socio-economic points of view. Modern agricultural production throughout the world does not appear to be sustainable in the long run.

Sustainable agricultural development “is the management and conservation of the natural resource base and the orientation of technological and institutional change in such a manner to assure the attainment and continued satisfaction of human needs for the present and future generations”. Such sustainable development in the agriculture, forestry and fishery sectors, conserves land, water, plant and animal genetic resources, is environmentally non-degrading, technically appropriate, economically viable and socially acceptable. Consequently these concerns imparted a way to organic farming. It is the need of the day to understand the prospects and problems of organic farming to launch a successful and flawless organic production programme in the farm environment.

There are many explanations and definitions for organic agriculture but all converge to state that it is a system that relies on ecosystem management rather than external agricultural inputs. It is a system that begins to consider potential environmental and social impacts by eliminating the use of synthetic inputs, such as fertilizers and pesticides, veterinary drugs, genetically modified seeds and breeds, immunity booster preservatives, additives and irradiation. These are replaced with site-specific management practices that maintain and increase long-term soil fertility and prevent pest and diseases.

The science of agrometeorology has great practical utility in protection against, or avoidance of adverse climate risks of crop production. Agrometeorological information can be used in efficient land use planning, determining suitable crops for a region, risk analysis of climate hazards and profit calculations in farming, production and harvest forecasts, and in adoption of farming methods and choice of farm machinery. Meteorological instruments are the equipments used to sample the state of the atmosphere at a given time. Meteorology does not use much laboratory equipments but relies more on field-mode in situ observation and remote sensing equipments. Rain was one of the first weather parameter to be measured historically. Two other accurately measured weather-related variables are wind and humidity.

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.

Climate, soil, input and management factors influence the crop yield. Weather is the most important natural input that affects the agricultural production and productivity. The agricultural activities starting from the land preparation to harvest and post-harvest storage are greatly influenced by the weather. The effects of weather on crops may be direct such as those of precipitation, solar radiation, humidity, temperature, etc., or indirect such as those of pest, diseases, weed competition, etc. Thus, the success or failure of a crop is determined by the prevailing weather conditions. Although influence of weather conditions on yield is well established, the quantitative assessment is not always straight forward and simple. The quantitative assessment of the effect of weather on crops is an important application of agrometeorology. The relationship between weather and crop production has been understood through crop weather modelling and study on this aspect through a systematic approach started a century ago. The aim of developing crop weather models is to ensure better utilisation of resources and hence a more environmental friendly and sustainable agriculture.

A model is a schematic representation of the conception of a system or an act of mimicry or a set of equations, which represents the behaviour of a system. Its purpose is usually to aid in explaining, understanding or improving performance of a system. Agricultural models are mathematical equations that represent the reactions occurring within the plant and the interactions between the plant and its environment. However, it is a difficult task to represent any system completely in mathematical terms because of the complexity of the system. Universal models do not exist for the agricultural systems and agricultural models are built for specific purposes and the level of complexity is accordingly adopted. Different models are developed for different subsystems and several models may be built to simulate a particular crop or a particular aspect of the production system. Crop weather models may be defined as a simplified representation of the complex relationships between weather or climate on one hand and crop performance such as growth, yield or yield components, on the other hand by using mathematical and/or statistical techniques

The word ‘remote sensing’ was coined by Evelyn L. Pruitt, an American Geographer in 1960. Remote sensing is defined as collection and interpretation of information about a target without being in physical contact with it. Remote sensing is the science and art of obtaining information about an object, area or phenomenon through the analysis of data acquired by a device that is not in contact with the object, area or phenomenon under investigation (Lillesand and Keifer, 1994).

The information needs a physical carrier to travel from the objects/events to the sensors through an intervening medium. Any force field such as gravity, magnetic or electromagnetic could be used for remote sensing. However, the electromagnetic radiation is conventionally used as an information carrier in remote sensing. Remotely sensed data may be gathered by different devices, including sensors, film cameras, digital cameras and video recorders. Instruments capable of measuring electromagnetic radiation are called sensors.

Remote sensing is nothing but opposite of astronomy. In astronomy, man looks from the earth to the sky to understand and gain more knowledge about the celestial objects. But remote sensing refers to going upward in the sky to look downwards to understand and gain more knowledge about terrestrial objects. In remote sensing, the aerial sensor technologies are used to detect and classify objects on earth by means of electromagnetic radiation.

Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. Weather forecasts are made by collecting quantitative data about the past and current state of the atmosphere on a given place and using scientific understanding of atmospheric processes to project how the atmosphere will evolve on that place. Weather warnings are important forecasts because they are used to protect life and property. 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. Failure of rains and occurrence of natural disasters such as floods and droughts can lead to crop failures, food insecurity, famine, loss of property and life, mass migration, and negative national economic growth (Sivakumar, 2006).

Farming in many parts of the world, especially in the arid and semiarid regions is risky, because climate is highly variable. In some developing countries as much as 80% of the variability in agricultural production is due to the variability in weather conditions. This production variability in agriculture has immediate and important macroeconomic impacts. 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. Also, a timely forecast could give the marketing system and downstream users, such as the food processing industry, time to prepare for a bountiful harvest (Arndt et al., 2000). It is important to remember that under rainfed conditions, even a less reliable, but earlier forecast may be more valuable than an accurate but late forecast (Mjelde et al., 1998).

Many aspects of the global climate are changing rapidly. Evidence for changes in the climate system abounds, from the top of the atmosphere to the depths of the oceans. 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. It can involve both changes in average conditions and changes in variability, including, for example, changes in extreme conditions.

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 and climate have 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 (nonliving) components of the animals’ environment. Abiotic components of an animal’s environment include temperature, humidity, oxygen, wind, soil composition, day length and elevation. Knowledge of how potential environmental stressors (ambient temperature, humidity, thermal radiation, air speed, etc.) can directly and adversely affect animal performance, health and wellbeing is also required for a successful animal farming. The indirect consequences of weather include their impact on feed quality and availability.

Birds and mammals are warmblooded (endotherms or homeotherms), possessing a thermoregulatory system that maintains a stable body temperature and uniform internal environment (homeostasis) by converting food energy into heat. All metabolic processes generate substantial heat and much of this is retained within the body of homeotherms by insulating layers of fat, feathers or fur. Dairy cattle, like other warmblooded animals, function most efficiently in environments where they can maintain their body temperature at around 38° C. Tissue and cellular metabolism and the underlying biochemical reactions that sustain life and productive functions need body temperature to be maintained within very narrow limits. Relatively small increases in body temperature result in the breakdown of body protein and a significant depression in production.

Climate change, particularly global warming has complex impacts on domestic animal production system affecting feed supply, challenging thermoregulatory mechanism resulting thermal stress, emerging new diseases due to change in epidemiology of diseases and causing many other indirect impacts. So, climate change will have farreaching consequences for dairy and meat production, especially in vulnerable parts of the world where it is vital for nutrition and livelihoods. Due to poor availability of good quality feed and fodders in India, animals are generally maintained on poor quality grasses available in the pastures or are stallfed, mainly on crop residues. India is already deficit in feed and fodder. These shortages would be further aggravated by the adverse effects of global warming on fodder crop production.

Many diseases, nematodes, insects and weeds affect crop production, causing severe yield loss. Weather is the most important factor that determines the geographical distribution of these crop pests. The local weather patterns have a significant impact on the abundance of diseases and insect pests in the crops. The severity of disease and pest development depends on the combination of weather conditions during the growing season and is variable from season to season. The weather conditions include temperature (both the maximum and the minimum), light, rain, humidity, dew, cloudiness, sunshine, wind, air currents, evaporation, snow and atmospheric pressure. However, among these weather parameters, temperature and humidity play the major roles in infection of crops due to insect pests and diseases.

Different crops are grown over a wide range of agroecosystems. Though the crops are raised in varied climates, ranging from very hot to very cold, and very wet to very dry, across temperate, tropical and semiarid zones, they 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. Crop management is therefore very much about managing climate risk so as to have financially viable and sustainable agricultural systems. Professor Henry Nix of the Australian National University posed three questions that need to be addressed when considering what crop can be grown where (Nix, 1981).

Climate classification systems are ways of classifying the world’s climates. One purpose of climatic classification is to group together places with similar climatic characteristics. That means if an area or region is categorised under a particular climatic type, it possesses broadly uniform characteristics with respect to climate. Classifications have been based on climatic elements such as temperature and precipitation, which are measurable, and on differences in tree species, soil characteristics, topography, etc. The varied nature of earth’s surface and the many interactions that occur among atmospheric processes give every location on the earth a distinctive climate.

Although the delineation of distinct climates may seem likely a very straightforward job, establishing the criteria by which climates are delineated requires considerable subjectivity. There are many ways of classifying the climate depending on the user and the level of understanding of the climate system at a given time. Different regions of the same climate type constitute isoclimatic regions. Identification of isoclimates helps in the transfer of agro-technology and suggesting new cropping pattern. Climatic classification is based on two approaches, viz., generic or empirical approach and genetic approach. In the generic approach the classification is based on the observed climatic elements or their effects on the phenomena, usually vegetation or humankind. The genetic classification is based on climatic controls such as general circulation, net radiation and moisture fluxes.

The concept of climate classification was originally brought out to delineate natural vegetation, which is dependent on rainfall and temperature. Climate classification is unique tool to find out different climate types across the globe, out of which isoclimates or homoclimates can be traced out.

The climate of a particular place is the function of a number of factors such as latitude and its influence on solar radiation received, air mass influences, location of global high and low pressure zones, heat exchange from ocean currents, distribution of mountain barriers, pattern of prevailing winds, distribution of land and sea, and altitude.

Meteorological hazards are severe and extreme weather and climate events that occur in all parts of the world, although some regions are more vulnerable to certain hazards than others. Meteorological hazards include cyclones, droughts, floods, hurricanes, hailstorms, heat waves and tornadoes. 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. Meteorological hazards are often called as natural hazards. The natural hazards become natural disasters when people’s lives and livelihoods are destroyed.

Hydrological hazards are the result of either excess or a severe lack of water, i.e., flood or drought. Hydrometeorological hazard is a process or phenomenon of atmospheric, hydrological or oceanographic nature that may cause loss of life, injury or other health impacts, property damage, loss of livelihoods and services, social and economic disruption, or environmental damage. Human and material losses caused by natural disasters are a major obstacle to sustainable development. 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.

Evapotranspiration is the combination of two separate processes named evaporation and transpiration. Evaporation accounts for the movement of water to the air from sources such as the waterbodies and soil. Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapour through stomata in its leaves. So evapotranspiration is a bio-physical process.

Evaporation

The change of state of water from liquid to the vapour and its diffusion into the atmosphere is referred to as evaporation. During evaporation, water molecules absorb energy which gives them the motion required to escape from the surface of liquid and become a gas. Since a certain amount of energy is consumed in the process of evaporation, the remaining liquid is cooled by an equivalent amount. Thus, the process of evaporation is always accompanied by a cooling effect.

As evaporation proceeds, the surrounding air becomes gradually saturated with water vapour and the process will slow down and might stop if the wet air is not transferred to the atmosphere. The replacement of the saturated air with drier air depends greatly on wind speed. Hence, solar radiation, air temperature, air humidity and wind speed are meteorological parameters to be considered when assessing the evaporation process.

In agrometeorology, evaporation is defined as the maximum possible loss of moisture from a wet, horizontal, flat surface exposed to weather parameters, which exist in the vicinity of plants. Evaporation is an essential part of the water cycle. It is the primary pathway through which water moves from the liquid state back into the water cycle as atmospheric water vapour. The oceans, seas, lakes and rivers provide nearly 90 per cent of the moisture in the atmosphere via evaporation, with the remaining 10 per cent being contributed by plant transpiration. A very small amount of water vapour enters the atmosphere through sublimation, the process by which water changes from a solid (ice or snow) state to a gaseous one, bypassing the liquid phase.

The word ‘monsoon’ is derived from the Arabic word mawsim meaning season. The Arabs during their travel towards India recognised that by June strong southwesterly winds commence over the Arabian Sea and bring a lot of rains to the subcontinent. The term was first used in English in British India and neighbouring countries to refer to the big seasonal winds blowing from the Bay of Bengal and Arabian Sea in the southwest bringing heavy rainfall to the area. In the Indian subcontinent, monsoon is one of the oldest weather observations, an economically important weather pattern over June through September every year, and the most anticipated weather event and unique weather phenomenon. 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. It has significant impact on the overall well-being of subcontinent residents.

The term ‘monsoon’ is customarily used in India to refer to the rainy season during the northern hemisphere summer from June to September and 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. Now the term monsoon has been broadened to include almost all of the phenomena associated with the annual weather cycle within the tropical and subtropical land regions of the earth.

Precipitation is water in liquid or solid forms, falling on to the earth. In other words, precipitation is the deposition of atmospheric moisture on earth. Formation of clouds does not mean precipitation. Precipitation involves condensation, growth of hygroscopic nuclei, cooling of nuclei at different stages and their fall on to the earth’s surface. There may be several clouds but no precipitation. If the cloud particles are too small and remain suspended in the sky, they may not fall on the earth. 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 more remain floating due to buoyancy of air, do they fall as precipitation.

Precipitation is the major source of water to all the living organisms on the earth. All the water sources on the earth depend on precipitation only.

Forms of precipitation

Precipitation form is the state that the moisture is in, i.e., liquid, freezing, or frozen (solid). Different forms of precipitation may occur together, such as mixed rain and snow; but such an occurrence is simply a mixture of forms, not a separate form of precipitation. Because atmospheric conditions vary greatly both geographically and seasonally, several different forms of precipitation are possible. All forms of precipitation are collectively termed hydrometeors. Hydrometeors have been classified into 50 specific types. Out of these precipitation forms, only rain (liquid form) and snow (solid form) make significant contribution to total precipitation. In many parts of the world, the term rainfall is used interchangeably with precipitation.

Clouds are visible mass of little drops of water or frozen crystals suspended in the atmosphere above the surface of the earth. 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.

Clouds are the most important form of suspended water droplets caused by condensation. 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 millions. On hygroscopic particles such as sea salts, condensation can begin before the air is saturated. Supersaturation in clouds rarely exceeds one per cent. On an 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.

Water vapour changes to the liquid water through condensation process. Condensation is the direct cause of the various forms of precipitation. It occurs as a result of changes in air volume, temperature, pressure or humidity. Water vapour can also be converted directly into the solid form through a process called sublimation. The latent heat expended in evaporation is liberated in the process of condensation. Four mechanisms may lead to condensation.

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. While humidity itself is a climate variable, it also interacts strongly with other climate variables. The humidity is affected by winds and by rainfall. Moisture content is determined by the local rate of evaporation, air temperature and the horizontal transport of moisture. Humidity indicates the likelihood of precipitation, dew or fog. It also plays a major role in the heat budget and the weather phenomenon of the atmosphere.

A big layer of air called the atmosphere surrounds the earth. The air within this layer moves from place to place when it warms up or cools down. This moving air is known as wind. In other words, air in motion is called wind. Winds move moisture and heat around the world and also produce much of our weather.

The strength of the wind can vary enormously. Sometimes air moves slowly and the wind is barely noticeable. When the weather is clear there may be a gentle breeze. When the wind is still very light, but it can be felt and there may be rustlings by leaves. At other times, the air can move very quickly and become a gale or hurricane, blowing down trees and damaging electric poles and buildings.

Winds are part of a global air circulation system that acts to balance temperature and pressure around the world. Different parts of the world receive different amounts of heat from the sun. This differential heating in turn results in differences in temperature and air pressure around the world, which drives the world’s winds.

As equatorial areas are heated most, the air above them warms and rises as it becomes lighter than the surrounding air, causing an area of low pressure. In cooler areas, the air sinks because it is heavier and results in an area of high pressure. Winds will blow as air is squashed out by the sinking cold air and drawn in under the rising warm air. Any difference in temperature like this will always cause a difference in air pressure, and therefore winds will blow. These movements result in a global wind pattern, with air moving between different areas around the world and also at different heights in the atmosphere. Colder air from the poles tends to sink and move towards the equator closer to the surface of the earth. In contrast, warm air from the equator rises and moves towards the poles high in the atmosphere because it is lighter.