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The earth has two motions. (1) Rotation and (2) Revolution. The earth rotates about its axis (west to eastward) with a speed of 464 m/s or 1669 km/hr. Its angular speed of rotation is about 7.29 x 10^-5 radians/sec or 0.26244 radians per hour. This rotation causes day and night and diurnal variations of weather.

The rotating earth revolves round the sun in an elliptical orbit (with the sun at one of its Foci) with a speed (orbital speed) of about 1.073 x 10⁵ km/hr or 29770 m/s or 29.8 km/s. This revolution causes seasons. It has been stated earlier that the apparent path of the sun with respect to the celestial sphere is called ecliptic.

Crop yield is influenced by genetical (heredity), external (environmental) and management factors that occur during the crop growing period. The external environment is the climate which regulates and determines the growth and development and final output of crop plants. But, man has no control over weather, hence it is dominance over the success or failure of agricultural enterprises. According to the World Meteorological Organization, the weather induced variability in crop yield is as high as 50 per cent. Therefore, weather should be taken as one of the inputs in agricultural planning.

Variation in field crops between years is associated with many factors. This is mainly due to weather, soil and management factors. There is complex reaction of weather variables among themselves as well as with other factors. Therefore, many attempts were made to study the effect of weather variable on crop yield through simulation modeling.

Environment in which crops are grown dictates their final yield. Of these environmental factors, climate and weather the uncontrolable factors, have maximum influence on crop productivity. Vagaries of weather subject a crop to different ecological situations from year to year leading to differential responses of crops to input resources. This situation limits the use of costly inputs for realizing optimum yield. While weather itself cannot modify, the hazardous weather can often be mitigated by agronomic management. Therefore, the primary requirement for initiating agronomic measures against weather hazards is foreknowledge of weather situation that is likely to develop in an area.

Climate, in general, is the totality of weather during a longer period and over wider area. Agro climate can be defined as the conditions and effects of varying weather parameters like solar radiation, rainfall etc., on crop growth and production.

Climatic classification is a method of arranging various data of climatic parameters to demarcate a country or region into homogenous zones i.e., places having similar conditions.

Season is a period in a year, comprising few months during which the prevailing climate does not vary much. Growing season for a crop is more important for its yield and other management practices, to be followed.

Indian Meteorological Department has divided the year into four seasons.

Plant growth is resultant of all the environmental factors – climatic, physiographic, edaphic and biotic factors.

For a particular field – it is primarily a function of climate with temperature and light – being the most important factors. – Close relationship exists between plant phenology and both latitude and altitude.

El Nino means ‘The Little One’ or ‘Christ Child’ in Spanish. It was originally recognized by fisherman of the coast of South America as the appearance of usually warm water in the Pacific Ocean, occurring near the beginning of the year i.e. around Christmas. That is why the name is used. El Nino, the periodic warming of Pacific Ocean waters that affects the weather worldwide. C. Fred and T. Andrus of the University of Georgia studied the ancient fish bones from refuse left about 6000 years ago by ancient peoples in Peru and found that ocean catfish lived in water that averaged 60 to 70 warmer than now and that there was little variation in temperature. He concluded that if El Nino was occurring at the modern rate, once every two to seven years, then the bones from the fish would have reflected the temperature variation.

The march of weather factors during crop period is never smooth and regular. Agriculture, because of its dependance on climate is inevitably subjected to unpredictable long and short term fluctuation characteristic of atmosphere conditions. Climate introduces an element of risk in farming which makes it particularly hazardous occupation.

A Weather hazard in this context can be defined as any type of extreme weather condition that can damage the crops and animals leading to abnormal decrease in production and income. The extent to which these hazards create risk depends on the time of occurrence, intensity and duration of the hazard besides the age, stage of development and inherent resistance of the crop or livestock involved. Major abnormalities affecting the crop production are:

Cloud usually consists of very small droplets of water which cannot fall appreciably below the cloud base without evaporation. In this condition clouds are stable. Rain drops are about a million times heavier than cloud drops and rain develops only if the cloud droplets grow by some mechanism. Cloud Seeding mechanism depends on the difference in vapour pressure over ice and super cooled cloud, water evaporates of the droplets and migrate to the ice crystal, which grows and begins to fall increasing their growth rate by coalescence with other droplets and eventually melting and falling as rain drops. Presence of ice nuclei determines whether ice crystals form in a supercoold cloud or not. Ice crystals form on naturally occurring nuclei at temperatures of – 10⁰ to 30⁰ C. At warmer temperatures, either the efficient ice-forming nuclei are often deficient in the atmosphere or the cloud may occur with tops supercooled but not to an extent which activates the ice nuclei available. It should be possible to initiate precipitation by introducing ice crystals or by supplying artificial nuclei.

The change of state of water from solid and liquid to the vapour and its diffusion into the atmosphere is referred as Evaporation. In the meteorology, evaporation is defined as the maximum possible loss of moisture from a wet, horizontal, flat surface exposed to weather which exist in the vicinity of plants.

FACTORS AFFECTING EVAPORATION

1. Those affecting water supply at the evaporating surface. i.e., soil and plants including soil storage capacity, rainfall and irrigation and

2. Those affecting energy supply to the evaporating surface like solar radiation.

Hydrological cycle involves four major steps, Viz., Evaporation, Transpiration, Condensation and Precipitation. Though the cycle has neither a beginning nor an end, the concept of cycle begins with the water of oceans, since it occur nearly ¾ of the Earth’s surface. Radiation from the Sun, evaporates water as water vapour from the oceans into the atmosphere. The water vapour rises and collects to form clouds. Under certain condition, the cloud moisture condenses and falls back to the Earth as rain, snow, hail etc. Precipitation reaching the Earth surface may be intercepted by vegetation or enter into the soil, may flow as run-off or may evaporate.

Evaporation may be from the surface of the ground or from free water surface. Transpiration may be from plants.

Condensation: The physical process by which a vapour becomes a liquid or solid – opposite of evaporation.

Cloud has been defined as a visible aggregation of minute water droplets and / or ice particles in the air, usually above the ground level.

CLASSIFICATION OF CLOUDS

Though confusion apparently arises from the number of kinds or species, the genera seems reasonably clear out, if we are able to recognize their main characteristics.

Clouds are usually classified according to their height and appearance. For convenience, we list them in descending order. High clouds, middle clouds and low clouds are some forms do not fit in any of these categories. But fortunately their particular characteristics make them easily identifiable as vertical development clouds. We must exercise some caution in studying on height. There is some seasonal as well as latitudinal variation and there is some overlapping from time to time. However, the appearance of clouds is quite distinctive for each height category.

Air Constantly moves from place to place due to pressure differences which are caused by difference in temperature.

Wind has been defined as air in horizontal motion. According to Byers, “The wind is simply air in motion, usually measured only in its horizontal component”. According to Trewaitha, “Wind is simply air moving in a direction which is essentially parallel with the Earth’s surface.

There are two types of movement of wind in the atmosphere.

1. Horizontal movement

2. Vertical movement (Currents)

The primary wind circulation in Northern and Southern hemispheres is seen in figure on page 51.

Moisture present in the atmosphere plays a significant role in weather and climate of the region. There are three major components in the atmospheric moisture.

  I. Humidity

 II. Precipitation

III. Evaporation.

Humidity: The terminology related to humidity and concerned with gaseous form of water i.e. water vapour, several expressions of the amount of water vapour in the air is used. They are as follows.

The atmospheric pressure is weight of the air which lies vertically above an unit area centered at a point. Normal pressure of air is 1.034/cm².

Pressure is defined as the force per unit area.
At the surface of the earth the atmosphere air exerts a pressure of about 10⁵ Newtons/m² or 10⁵ Pascals, which is called one bar or one atmosphere. Evangelista Torricelli (1608-1647) devised a method for measuring the atmospheric pressure by mercury (Hg) barameter in 1643.

The Sun is the primary source of heat to the Earth and its atmosphere. The heat received from other celestial bodies as well as the interior of the Earth is rather too significant. The distance that separates the Earth from the Sun is about 149.0 million km. The diameter of the Sun measures roughly about 1.3824 million km. The surface temperature of the Sun is estimated between 5500^o C and 6100^o C. (or 5762^o K).

The weight of the sun is about 1.9 X 10 30kg. The radius is 6.97 X 10 8 m or the diameter of the sun is 14 X 10 5 km. The mass of the earth is 6 X 10 24 kg, the radius is 6.4 X 10 6 m. The gravitational force of the earth is 9.81m/s². Whereas the sun‘s attractive force is 274m/s². This is 25 times higher than earth’s gravitational force.

In climatology, terms “Weather” and “Climate” have different connotations. Weather refers to the state of atmosphere at any given time denoting the short- term variations of atmosphere in terms of temperature, pressure, wind, moisture, cloudiness, precipitation and visibility. Weather is highly variable. It is constantly changing, sometimes from hour to hour and at other times from day to day. The aforementioned properties of the atmosphere are subject to constant change and their state at any time determines the state of the weather. However, weather elements are not separate rather they are closely related with each other.

David I. Blumenstock, an eminent climatologist has defined the weather in more restrictive sense. According to him, weather is the behaviour of the lower atmosphere with particular emphasis on the atmospheric behaviour which affect the lands and oceans and have a marked influence on the organisms that live upon the lands, within the waters of the earth or in the lower air. For the geographer, this definition of weather is more satisfactory. Since, he is interested more in the working of the lower atmosphere which affects the life of man as well as natural environment directly. Thus, we come across a great many varieties of weather covering a wide range of conditions.

Geological evidence suggests that the age of the earth is about 4,50,00,00,000 (4.5 billion) years and life began on earth between 0.6 to 1.0 billion years after earth has formed. That is life began on earth some where 4 to 3.5 billion years ago.
The plant kingdom over the globe produce oxygen about 30,00,00,000 (3 million) kg per second which is consumed by the living beings on the earth. Air which is present in the atmosphere is a mechanical mixture of gases. The various gases, solid and liquid particles in the air that envelope the earth are bound to it by the gravitational attraction. This complex content of gases, solid and liquid particles is called atmosphere. The atmosphere extends above the surface of the earth to great heights. There is no sharp boundary between the atmosphere and the outer space. For our convenience, the height of the atmosphere is taken as 1000 km. The lowest one km which is called planetary boundary layer, contains 10 per cent of the mass of the atmosphere. All biological and human activities are confined to this planetary boundary level. The mass of the atmosphere is about 5.6 x 1018 kg (mass of earth is 6 x 1024 kg). The mass of the oceans water is about 1.4 x 1021 kg. This shows that the mass of the oceans is more than 250 times the mass of the atmosphere. The density of the dry air at the earth surface is 1.225 kg/m3 and at an altitude of 5.5 km this drops to 0.66 kg/m3 and at 30 km altitude it is about 0.013 kg/m3.