Term Paper on Weather and Climate | Geography

Here is a compilation of term papers on ‘Weather and Climate’ for class 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short term papers on ‘Weather and Climate’ especially written for school and college students.

Term Paper on Weather and Climate

Term Paper Contents:

1. Term Paper on the Introduction to Weather and Climate
2. Term Paper on the Elements of Weather and Climate
3. Term Paper on the Factors Determining Weather/Climate
4. Term Paper on the Global Circulation of Atmosphere
5. Term Paper on the Recent Extension of Tropical Belt and Hadley’s Circulation by Global Warming
6. Term Paper on Disasters as a Result of Weather and Climate Change
7. Term Paper on the Role of Weather and Climate
8. Term Paper on the Importance of Weather and Climate

Term Paper # 1. Introduction to the Weather and Climate:

Weather is defined as the atmospheric conditions at a given time and place with reference to temperature, pressure, moisture, wind speed and direction, cloud and sky conditions. The weather changes from hour to hour and from day to day. The weather may be for one day or one week or for one season or for one year.

Climate is the average weather conditions taken over a long period of time (20, 30 or 50yrs). It also includes the extremes and mean weather conditions. Climate is the index of vegetation. Climate varies from place to place over the earth surface. The type of the crops grown in a given region depends on the climate e.g. wheat is grown in cold climate and rice is grown in warm climate.

Weather occurs in troposphere, the near surface layer of the Earth’s atmosphere. Weather is the current atmospheric conditions with respect to prevalent humidity, wind, precipitation, and temperature in the troposphere at a particular place. Climate is an average of the weather over a long period.

The humidity, temperature, precipitation and pressure of the atmosphere are the key factors, which determine the climate. Water vapor in humid atmosphere forms clouds, and the rainfall depends on type of cloud. The atmospheric temperature affects humidity, pressure and precipitation and thus plays a major role in determining the climate in a particular location.

The temperature of a given atmosphere is determined by the amount of solar beam it receives from the sun. Solar beam striking at 90° at equatorial region produces maximum atmospheric temperature. With increasing latitude the distance between sun and earth’s surface increases resulting in six different temperature or climate zones from equator to poles.

From equator to poles there are three distinct patterns of wind circulation cells, which are around 30° apart in latitude from each other in both sides of the equator. The wind circulation loops are driven mainly by the temperature gradients across the loop. The wind circulation within a loop tends to equalize the average temperature.

The three wind circulation cells receive different quantum of heat from the solar beam. The increase in temperature in a loop leads to extension of the circulatory cell, thus affecting the other two cells. Thus the atmospheric temperature rise due to excess greenhouse gas emissions in one region and consequent rise in temperature can be felt across the globe.

The atmospheric temperature rise depends, amongst other factors, on the capabilities of the greenhouse gases to absorb heat (infra-red) from solar beam. Recent observation of the expansion of the tropical zone (Hadley’s circulation cell) by few degrees has been ascribed as due to the increase in the amount of heat absorbing gases in the atmosphere.

As the Earth rotates on its tilted axis (23.5°) around the sun, different parts of the Earth receive higher and lower levels of radiant energy at different periods. This creates the seasons. Apart from normal seasonal variations in weather, the change in wind circulation resulting from major fluctuations in temperature and pressure, can cause abrupt changes, such as, storms, cyclone, tornadoes, hurricanes etc. The temperature changes can affect the ocean circulatory system leading to disasters like. El Nino and La Nina. Global temperature rise can result in more frequent occurrences of these disasters.

Term Paper # 2. Elements of Weather and Climate:

Elements of weather and climate are those meteorological parameters of which the weather or climate is constituted or formed. The main elements are solar radiation, temperature, pressure, moisture and wind etc. On the earth surface, these elements vary from place to place and season to season.

(a) Solar Radiation:

The earth is heated by sun. The solar radiation received at the earth’s surface is called insolation, which is generally measured in calories/cm² or W/m² or Langley. The solar radiation which is coming from the sun is in very depleted form as most part of it is absorbed or reflected by the upper atmosphere. About 40 per cent of the solar radiation is reflected back to the atmosphere.

Out of the remaining 60 per cent, 20 per cent changes into heat energy, which is absorbed by the atmosphere. Another 20 per cent are diffused by the dust particles and water vapours. The remaining 20 per cent enters the land and water surface.

Uneven distribution of heat energy over the earth surface leads to variation in temperature. Temperature determines the length of growing period of the crops. Temperature depends upon the amount and intensity of radiation at a given place. The actual amount of radiation received at a place on the earth varies according to the conditions of the atmosphere as well as seasons.

The radiation received at a place depends upon the following factors:

1. Angle of incidence

2. Duration of sunshine (length of the day)

3. Distance between earth and sun

4. Transparency of the atmosphere.

(b) Air Temperature:

It is a measure of the level of sensible heat of matter, whether it is gaseous (air), liquid (water) or solid (soil). When a substance absorbs heat energy, its temperature rises. When heat flows away from a substance its temperature decreases. It is the degree of hotness or coldness of a body.

It is the intensity aspect of heat, whereas heat is the energy arising from random motion of all the molecules in a body. Temperature is measured in °C or °F or °K. A line joining the places with same temperature on the globe is termed as isotherm.

Temperature is one of the most important parameters of climate. It determines the distribution of biological forms over the earth’s surface. Length of the growing period provides working conditions for nearly all the plant functions and energy processes. Uneven distribution of heat energy over the earth’s surface leads directly to the variation of air temperature on the earth’s surface and in the atmosphere.

Low temperatures and consequent snow and ice do not allow the crop production in polar and Tundra lands of the world. As we move towards the equator, the temperature rises, frost-free period becomes longer and a number of crops are grown.

Temperature is maximum at the equator and goes on decreasing as we move away towards the poles, because sun rays are falling perpendicular at the equator. So they have to cover lesser depth of the atmosphere, whereas towards the poles, as the rays are falling at a slanting angle, depth of the atmosphere increases and density goes on decreasing.

Secondly, the vertical rays of the sun heat the minimum area in the tropical region, but oblique rays spread over a large area in the polar region. Therefore, maximum amount of radiation per unit area is received in tropical region as compared to the polar region. Thus, high temperatures are common in tropical regions and low temperatures are common in polar regions.

The tropical area becomes the heat source and the polar region becomes the sink. Thus, the frost free period is maximum in lower latitudes i.e. near the equator and minimum over higher latitudes of the earth i.e. near the polar areas. Hence, temperature gradient is always acting from equator towards poles.

Diurnal Change of Temperature:

The temperature throughout 24 hours is not uniform and this temperature change during the day is known as diurnal change of temperature. Due to rotation of the earth, its every portion at one time is facing towards the sun and at another time, it is turned away from it. So nights are cooler and days are hotter whereas noon’s are the hottest.

Although insolation is most intense at noon time but the highest temperature is recorded at about local 2.00 pm, because it takes some time before the heat of the earth is emitted to the atmosphere. The time of occurrence of maximum temperature varies from one to other place, however, it occurs when the incoming radiation is equal to the outgoing radiation. Similarly, minimum temperature occurs in the morning when the incoming energy is equal to the outgoing solar energy, just after sunrise. This is known as time lag.

1. As we move from equator towards the poles, diurnal range declines

2. The structure of land and its elevation above sea level modifies the diurnal range

3. The distance from water bodies greatly affects the diurnal range

4. Clouds and water vapours modify the diurnal range

5. Snow and ice promote quick radiation, therefore the range is higher when the land is ice covered

6. Because of clear atmosphere and want of water vapours diurnal range is greater in desert areas

Seasonal Change of Temperature:

As the earth is revolving around the sun and its axis is fixed at an inclination, which remains fixed throughout the year, this gives rise to different seasons with different temperatures. This inclination of the axis of the earth determines the number of hours during which a particular place will get sunlight. In general, we can say that

1. Within the tropics at every place the sun is shining vertically twice a year, so there is no change of temperature within the tropics. At the tropic of cancer and tropic of Capricorn the sun shines vertically only once in a year.

2. In the temperate and frigid zones, the seasonal change of temperature is the greatest

Lapse Rate:

Lapse rate is defined as the rate of decrease of temperature with height. Change of temperature is associated with increase or decrease of heat energy in the air. Sometimes heat energy remains unchanged. Under such conditions, the change in temperature is followed by an adiabatic process. Adiabatic process is that process in which no heat is added or withdrawn from the ascending or descending air mass.

Adiabatic Lapse Rate:

When the temperature decreases adiabatically in the rising air mass, the lapse rate is called adiabatic lapse rate. It may be dry adiabatic lapse rate (DALR) or saturated adiabatic lapse rate (SALR). When the air mass is dry, it is called DALR and its value is 10°C/km. When the air mass is saturated, it is called SALR and its value is 5°C/km.

Temperature inversion:

Generally temperature decreases with increase in height, but under certain conditions there is an increase in temperature with increase in elevation and this state is known as temperature inversion. This condition occurs when on clear nights the ground is snow covered or when there is excessive radiational cooling during night in winter season. With the result, the air close to the earth becomes colder than that at higher altitudes. The temperature inversion during winter season is favourable for the formation of frost.

The ideal conditions for temperature inversion are:

1. Absence of vertical winds

2. Clear sky and excessive radiational cooling

3. Long winter nights

4. Cold dry air

5. Snow covered earth surface

6. Cold advection

Latitudinal and Seasonal Distribution of Temperature:

The temperature of a place is the temperature of the air at 4 feet above the ground surface at that place. To record the temperature of the air, the thermometer is kept inside a wooden box. The wooden box is kept away from the building and big trees. The wooden box is called a Stevenson screen. It is made in such a way that it protects the thermometer from direct sunrays. The air can enter the box freely. Such a box is kept at one place in the meteorological observatory.

The distribution of temperature is shown on the maps by means of isotherms. An isotherm is a line on the map along which the temperature is the same. Temperature varies with latitude and altitude. There are two sources from which the warmth of the earth’s surface may be derived.

The first of these is the sun and the second is the interior of the earth. Seasonal variations of temperature are not felt below the ground up to 25m. Below this limit the temperature increases downward. During summer season, the earth receives radiation from the sun. Whereas, it loses heat during winter season. Because of this reason, the soil temperature below the ground surface is found to be higher during winter season.

The sun is the main source of heat for the earth. The sun rays have to pass through the atmosphere. Some of the light and heat is absorbed by the atmosphere and does not reach the earth. Taking the average temperature for the whole year, the poles are colder than the equator. The amount of the heat received from the sun is greatest at the equator and decreases gradually towards the poles. But the equator is not the hottest part of the earth.

The line of maximum temperature lies northward which is away from the real equator. Gradually the temperature decreases from the equator towards the poles. The irregular distribution of land and water on the earth’s surface tends to break up the orderly latitudinal arrangement.

The distribution of temperature is greatly affected by the distribution of the continents and oceans. In general, the land masses heat up more quickly in summer and cool more quickly in winter than the oceans. The specific heat of water is much greater than that of the solid earth. The sun rays penetrate more deep into water than into earth. Water is mobile and land is fixed. After sunset, the land cools rapidly than the ocean.

When the sun shines on water, a considerable amount of heat is used in evaporating water and not in raising the temperature. Much of the heat that falls upon the water is reflected and does not raise the temperature.

On the other hand, land surfaces are poor reflector. On the whole the sky remains cloudy over the ocean than over the land. The presence of the clouds retard both the heating and the cooling of the water. Thus the land is heated and cooled more quickly than the ocean.

Annual Range of Temperature:

The sun remains between 23 ½° N and 23 ½° S throughout the year. Therefore, maximum amount of heat is received by the earth in the tropical region than in the polar regions. More heat is lost from the polar region than the tropical region. Maximum temperature is found on the land in the tropical region during summer season. The temperature decreases from equator to polar region. Lowest temperatures are found in the polar regions.

The difference between the summer and the winter temperature of any locality is known as the annual range of temperature at that locality. The maximum and minimum temperature will not be the same every year. If we take their averages for a number of years, the difference may be called the mean annual extreme range or simply the annual extreme range. The annual range is lower in low latitudes and higher in high latitudes.

Seasonal Variations:

In the Northern Hemisphere generally, July is the hottest month and January the coldest month. The season brings great changes not only in actual temperature but also in the distribution of temperature. In the Northern Hemisphere, July is the middle of the summer.

The highest temperature is found over land and the lowest temperature over the oceans. On the other hand in the Southern Hemisphere, July is the middle of winter. Beyond the tropics the land is colder than the sea. January is the middle of northern winter and the southern summer.

The prevalent winds affect the distribution of temperature on the globe. The tropical region acts as a source and the polar region acts as sink. Therefore, the heat is transported by the prevalent winds from the tropical regions to the polar regions. North of Lat. 40°N, the prevalent winds are from the southwest. They blow over the water surface of the oceans towards the east, warming the west coasts of the continents and cooling the eastern coasts.

Moreover, on the western coasts the south winds are coming from the warmest sea and on the eastern coasts they are coming from the colder region of the land. Another influence on horizontal temperature distribution is the mountain barrier. The mountain ranges tend to restrict the movement of cold air masses.

The Himalayas in Asia and the Alps in Europe protect the regions from the cold air. The topography of an area affects the distribution of temperature. In the Northern Hemisphere, north facing slopes will generally receive less heat than the south facing slopes and the temperature will be lower on north-side.

(c) Atmospheric Pressure:

Like any other material, air has weight and it exerts pressure, which is called atmospheric pressure. Air pressure is defined as the weight of an air column extending from the bottom to the top of the atmosphere falling on a unit area. Air pressure is directly proportional to the density as well as temperature.

Therefore, change in either temperature or density causes corresponding change in pressure. It is measured with barometer. Units of pressure are mb or Pascal (pa) or mm of mercury or dynes/cm². The standard sea level pressure is about 1013.2 mb. It is seldom greater than 1050 mb or lesser than 950 mb.

Lines joining places having same pressure are called isobars. The decrease of pressure between two points is called pressure gradient. The rate and direction of change in air pressure are also known as ‘barometric slope’.

The changes in pressure which often occur are less important than temperature and precipitation but these changes influence the other elements of weather. Air masses move due to changes in atmospheric pressure at different locations (i.e. from high pressure to low pressure).

The high and low pressure areas give rise to the movement of winds which carry moisture from ocean to land. Closely spaced isobars represent a strong pressure gradient and high velocity winds, while widely spaced isobars indicate a week pressure gradient and light winds. Air pressure is closely related with other elements of weather.

The pressure of a place depends upon:

I. Changes in temperature and density

II. Amount of water vapour present

III. Elevation of a place

Air pressure is directly proportional to the temperature as well as density. It can be expressed as the product of temperature and density:

Pressure = temperature x density x constant

Pressure changes from place to place and from time to time. Variations in pressure are caused by the changes in temperature. The movement in the air is caused by variations in pressure. Wind transports heat and moisture from one place to another. All the weather changes are closely related to pressure variations.

Low pressure and high pressure areas have been observed over the earth surface. The highest sea level pressure ever recorded was 1075mb (Irkutsk, in Siberia on 14^thJanuary, 1893). The lowest sea level pressure ever recorded was 877mb (observed in the eye of a typhoon).

Wind always move from high to low pressure areas. In the high pressure areas, winds move in a clockwise direction and in low pressure areas winds move in an anti-clockwise direction. The pressure gradient always acts from high pressure to low pressure areas. When temperature increases pressure decreases and vice-versa.

Bad weather is always associated with low pressure and fine weather is always associated with high pressure. On the earth there are two types of pressure belts: low pressure belts and high pressure belts. The low pressure belts are always associated with bad weather and high pressure belts are characterised by dry weather hence deserts of the world are mostly found in these regions.

Effect of Pressure on Crop Plants:

The movement of water from soil through the plants depends upon the pressure gradient. An increase in pressure gradient in the boundary layer of the leaf surface exerts an increasing pull at the water column within the plant body. Higher the pressure gradient, higher the movement of water.

Therefore, evaporation and evapotranspiration depend upon the atmospheric pressure. During summer season, higher atmospheric demand for evaporation may lead to increased evapotranspiration and result in heat stress to the crop under limited water availability to the plants. Thus, atmospheric conditions such as high temperature, dry air and low humidity enhance the vapour pressure gradient and will increase the evaporation demand of the leaf.

(d) Wind:

There is uneven distribution of solar radiation on the surface of the earth. Incoming solar radiation exceeds the outgoing radiation from the tropical areas, whereas outgoing radiation exceeds the incoming radiation in the polar areas. The tropical areas become source of heat energy and polar areas become a sink of heat energy.

Therefore, energy must be transported from the tropical areas to the polar areas. Similarly, lower parts of the troposphere are warmer than the upper troposphere. Heat energy moves from the surface of the earth to the upper troposphere.

General circulation of the atmosphere and oceans play a vital role in equalizing the heat energy over the surface of the earth. It has been estimated that 80 per cent of energy is transported by the atmosphere and the rest 20 per cent is transported by the oceans. The energy is transported mainly as sensible heat and latent heat of water vapours.

The kinetic energy of wind system is converted into sensible heat either by internal friction or by the friction of the ground surface representing an internal energy balance. The rate of kinetic energy generation within the atmosphere is counterbalanced by energy loss from the system by friction. Hence, the wind is a part of earth-atmospheric energy system.

Wind is an important element of weather. The weather of a place is largely influenced by direction and speed of wind. When the air moves in a horizontal direction over the surface of the earth, it is called wind. Winds are always caused by differences in pressure in two adjacent pressure areas.

The changes in pressure, whether small or large, are caused by changes of temperature in different pressure areas. Finally, we can say that temperature is the cause of any type of air circulation. Lines joining places having same wind speed are known as isotachs.

Winds are the means by which uneven distribution of pressure over the globe is balanced out. Winds have been considered as an essential part of the thermodynamic mechanism of atmosphere which serves as a means of transporting heat, moisture and other properties from one part of the earth to another.

When the pressure gradient is steep, the wind velocity is higher, while the weak pressure gradient causes the wind to blow at a slow speed. The warm and cold ocean currents are generally based on the direction of the wind. Cold advection decreases the temperature and warm advection increases the temperature. These winds are called hot and cold waves. Thus, the weather forecast depends upon the direction and speed of wind.

Winds are caused by changes in temperature and pressure over a certain place. Strong winds are generally associated with low pressure and light winds are associated with high pressure area. The magnitude of the wind depends on the intensity of low pressure or high pressure. As there is a close relationship between pressure and temperature, therefore, greater the temperature differences, steeper the pressure gradient and the resultant wind.

There are two types of movement of air in the atmosphere:

1. Vertical movement

2. Horizontal movement

1. Vertical Movement:

Air moving in vertical direction is called current. The air moves vertically, if it is warmer than its surroundings. Upward and downward air currents are referred to as updraft and downdraft. Vertical motions occurring in the atmosphere are of great significance for the formation of clouds, precipitation and cyclonic storms.

2. Horizontal Movement:

Like vertical movement, horizontal movement is also very important. Horizontal movement of air transports heat, moisture and dust particles from one part of the earth to other.

Winds are caused by the following factors:

1. Horizontal pressure gradient

2. Rotation of the earth (Coriolis force)

3. Frictional force

These factors are based on the principles of Newton’s laws of motion, which describe all the motions including movement of air.

These are given below:

1. An object (parcel of air) at rest remains at rest unless acted upon by an unbalanced force and an object (parcel of air) in motion continues in the state of motion in a straight line at a constant speed unless and until it is acted upon by an external (unbalanced) force.

2. The acceleration (change in velocity) of an object of unit air mass is directly proportional to the sum of the forces acting on it. There are two types of forces operating on the atmosphere to produce winds – driving forces and steering forces.

Driving Forces:

These force always exist irrespective of whether or not the air is moving.

I. Pressure gradient force and the gravity are the two vertical forces.

II. Horizontal atmospheric pressure differences generate horizontal pressure gradient force.

Steering Forces:

A parcel of air mass is acted upon by the following forces when the air mass starts moving:

I. Coriolisforce:

This force is caused by the rotation of the earth.

II. Frictional force:

These forces depend upon the degree of roughness of the earth over which the wind blows. It acts in the opposite direction of the wind.

III. Centripetal acceleration:

It is caused by the flow of the winds around curved isobars. It has the tendency to deflect the motion towards the centre of rotation to maintain the curved flow.

Gravity:

The force that attracts all the objects towards the earth is called gravity. The force of gravity is generated by the combined effect of two forces working together.

These are as follows:

(a) The force of attraction between the earth and all other objects called gravitation.

(b) A centrifugal force imparted to all the objects because of the spin with the earth on its axis.

These two forces combine to produce the force of gravity which accelerates a unit mass of any object downward at the rate of 9.8 m/sec. The force of gravity always acts downwards and perpendicular to the earth’s horizontal surface. The value of the acceleration is greater at the poles than at the equator.

Horizontal Pressure Gradient:

Winds are caused by the pressure differences in two adjacent pressure fields. Winds always move from higher to lower pressure. Greater the pressure difference, greater is the wind speed. In other words, if the pressure gradient is steep, the wind velocity is higher and vice-versa. Temperature difference causes pressure gradient and thereby creates winds. Thus, greater the temperature differences, steeper the pressure gradient and greater is the wind speed. Warm and cold advections are caused by horizontal pressure gradient. These advections affect the normal growth of crop plants.

Rotation of the Earth – Coriolis Force:

This force is caused by the rotation of the earth. This force only changes the direction but not the speed of the wind. Due to this force, the winds get deflected to the right of their path in the northern hemisphere and to their left in the southern hemisphere. This force does not seem to exist until the air is set into motion.

When the gradient force is balanced by the coriolis force, wind motion becomes parallel to the isobars. This is called geostrophic wind. The coriolis force is directly proportional to (a) Horizontal velocity (b) mass of the moving air mass and (c) latitude. This force is zero at the equator and maximum at the poles.

Frictional Force:

Frictional force works in the opposition of the pressure gradient force to reduce the wind velocity, which further reduces the coriolis effects. When an object moves on the surface, it encounters resistance offered by the surface. The frictional force is generally described by the resistance of the surface.

In the lower troposphere, the horizontal wind encounters strong frictional force when it grows on the surface of the earth. The rougher the surface, the greater is the frictional force. The forest area offers greatest frictional force to the moving air mass resulting drastic reduction in the wind speed. The frictional force becomes negligible at about 1 km height above the ground surface.

Friction is an important factor affecting the wind at the earth’s surface. This force determines the angle at which wind will blow across the isobars as well as the speed at which it will move. It may alter wind direction.

The friction force is maximum over the land and minimum over the ocean surface. Therefore, the winds in the lower layers of the atmosphere (up to 900 meters) are greatly affected by the frictional force. Wind speed increases with height.

Upper Winds in the Troposphere:

Longitudinally Averaged Zonal Winds:

When the atmospheric circulation is averaged with respect to longitude, the zonal (east- west) wind component has large magnitude as compared to the meridional (north-south) component at most of the locations of earth. It has been found that in both summer and winter hemisphere, there are westerly jets in the middle latitude at an altitude of 10-12km.

The tropospheric jet in the winter hemisphere is stronger than the one in the summer hemisphere. Over middle latitudes the wind at these levels undergo a dramatic seasonal reversal between winter westerlies and summer easterlies.

The sudden warming phenomenon is accompanied by large changes in the longitudinally averaged zonal wind at high latitudes in the winter stratosphere. Because of this warming, the westerlies disappear at stratospheric level. However, they have little effect on the stratospheric circulation.

Tropospheric Winds at Middle and High Latitudes:

The winds at middle and high latitudes tend to blow parallel to the isobars or height contours leaving low pressure to the left in the northern hemisphere. At any given latitude, wind speed tends to be inversely proportional to the spacing of the height contours. Thus, when the space between two adjacent contours decreases, the wind speed increases. This is called geostrophic relationship between wind and pressure.

The mean height contours of 500mb surface (middle troposphere i.e. 5-6km) in both seasons is highest in the tropics and lowest in the polar regions. Thus, the prevailing winds at this level are always from the west throughout the middle and high latitudes. The wind pressure fields in the middle and high latitudes display a large amount of day to day variability. Hence, the surface and upper air charts on individual days are characterised by synoptic scale features.

Tropospheric Winds at Low Latitudes:

The winds in the tropics tend to blow more steadily from day to day. Sub-tropical high pressure belts are found near 30°N-40°N and 30°S. The winds moving from these high pressure belts are called easterly waves, because they are observed to move from east to west. Tropical cyclones generally occur in these areas. These are strongest surface winds observed anywhere in the earth’s atmosphere.

Most of these storms develop during summer season. These winds are called north-east or south-east trade winds. These winds generally meet north of equator. This is called Inter Tropical Convergence Zone (ITCZ). It is the zone where NE and SE trade winds flow together. It is characterized by strong upward motion and heavy rainfall.

Effect of Wind on Crop Plants:

Wind direction and velocity are important weather elements. The effects of wind are both local and regional. It influences the configuration and distribution of plants both mechanically and physiologically. Wind affects the crop plants directly by increasing the rate of evapotranspiration.

However, this increase is only up to a certain point, beyond which either it becomes constant or begins to fall. Wind increases turbulence in the atmosphere, resulting in an increase in photosynthesis. However, increase in photosynthesis is again up to a certain wind speed, beyond which its rate becomes constant.

When the wind is hot, it accelerates the dessication in plants by replacing the humid air with dry air in intercellular spaces. When the hot and dry wind blows continuously for longer period it results in the dwarfing of plants. This is because the cells can not attain full turgidity in the absence of optimal hydration, and thus remaining at subnormal sizes.

Another severe injury to the plants caused by strong winds is lodging. This injury is very common in crops, such as maize, wheat and sugarcane. Strong surface winds may break the twigs and shed the fruits of many plants. The crop plants with trees and shallow roots are often uprooted.

Wind can cause maximum damage in association with rain at the time of flowering. Crops grown on sandy soils in areas where strong winds prevail are damaged because of abrasion. When the plant cover is not thick, strong winds remove the dry soil so as to expose the roots of the plants and kill them (Mavi, 1994).

(e) Moisture:

It indicates the amount of water vapours available in the air. These are highly variable over the earth surface. The amount of water vapours is expressed in the form of humidity. Moisture content in the air varies from place to place and even from time to time over the same place.

Moisture content in the air is more over the ocean than on the land and consequently rainfall is more over the oceans. The water in the air is found in three states, namely: solid, liquid and gaseous. When air is cooled its capacity to hold water vapours decreases and condensation takes place.

When the dew point temperature is above 0°C, the common form of condensation and precipitation are fog, cloud, rain, dew and mist. When the dew point is below freezing level i.e. 0°C, snow, frost and hails are formed. Lines joining places receiving same amount of precipitation are called isohytes.

Latitudinal and Seasonal Distribution of Precipitation:

The total amount of water that falls on any given area, whether in the form of rain or snow or hail, is known as the precipitation or as the rainfall. It is measured by means of rain gauge. The rainfall amount is expressed in inches or in millimeters.

General Distribution of Rainfall:

Rain is caused due to the cooling of air which contains water vapour. The cooling of the air is caused by two methods. In one case, air is lifted upward where both temperature and pressure are very low. In other case, warm moist air moves towards colder latitudes, giving rise to rain when it meets and rides over the colder air masses.

The earth is surrounded by a series of belts of high pressure and low pressure alternately parallel to the equator. The winds blow into low pressure and out from high pressure. These winds are permanent. In case of high pressure, air sinks downward and in case of low pressure, air rises upward where the air is rising and the winds are blowing towards the poles the climate will be wet. Where the air is sinking and the winds are blowing towards the Equator the climate will be dry. Thus, we may divide the globe into a series of alternate dry and wet belts.

Effect of Winds on the Distribution of Rainfall:

In general, the rainfall will be heavier over the sea than over the land. In the temperate rain-belt, the rain is due to cyclonic depressions. In the equatorial belt, the rain is due to the ascending air. In temperate latitudes, the prevalent winds are westerly. In these latitudes, the western coasts of the continents have an oceanic rainfall during winter season. Rainfall decreases from west coast to east coast. The interior and the eastern coasts have a continental rainfall.

Within the tropics the trade winds carry water vapours from the oceans over the eastern parts of the continents. Rain occurs over the equatorial region throughout the year. In the greater part of the temperate regions, there will be rain in all the seasons of the year. Drought prevails in the middle of the dry belts throughout the year where anticyclone winds are prevalent.

Effect of the Distribution of Land and Sea on Rainfall:

During winter season, it is observed that on the western coasts of the land, between 30°N and 40°N the prevailing winds are south westerly and blow from sea to land. On the eastern coasts the winds are northerly and blow from land to sea. Thus, during this season west coasts receive rainfall whereas eastern coasts remain dry.

During summer season high-pressure moves slightly northward, the western coasts almost remain dry whereas eastern coast receive rainfall, because the winds are from sea to land. In the temperate zone, between 40°N to 60°N the prevalent winds on the western coasts are from the south-west throughout the year and therefore, there is a rain in all seasons; but maximum is in the winter season and eastern coast receive very little amount of rainfall. During summer season maximum rainfall is received in the eastern coast.

During winter season north west and northern parts of India receive rainfall from the cyclonic circulation commonly known as western disturbances because these are embedded in the westerly belt. Eastern coast of India also receives rainfall from the north east monsoon. During summer season south west monsoon causes rain over most parts of India.

Term Paper # 3. Factors Determining Weather and Climate:

(a) Humidity:

With increasing temperature, solid ice (form at 0°C) transforms to liquid water followed by vapors (form at 100° C) at higher temperature. The water vapor (gas phase) in atmosphere can move freely just like any other gas. It can form clouds, fog or cause higher humidity at higher concentrations.

The amount of water vapor retained by atmosphere, compared to its retention capacity, is expressed as relative humidity in weather reports. The relative humidity increases with more water vapor in the atmosphere or with a decrease in temperature, when vapor retention capacity decreases.

(b) Cloud:

The water vapor in the humid atmosphere needs to get together, or coalesce, in order to form clouds as a first step to precipitation. As such water vapor molecules do not attract each other to form clouds or rain droplets. Particles in the atmosphere act as condensation nuclei around which cloud droplets can form. These particles typically originate from ocean spray, forest fires, volcanoes and smoke stacks. This is one reason why it rains a lot more near polluting factories.

The four types of clouds are as follows:

i. Cirrus:

Occur at high altitude, where air is so cold that clouds are made from thin ice crystals, resulting in the wispy effect.

ii. Stratus:

Flat and form closer to ground. They result in drizzles.

iii. Cumulus:

These are puffy, un-threatening, or gathering, rising and thunderous.

iv. Nimbus:

These are responsible for making rain.

A combination, like, cumulonimbus, consisting of cumulus plus nimbus clouds, contains rain and is responsible for making rain.

The elevation of a cloud indicates the amount of precipitation, and the possibility of storms or thunderstorms. For example, high clouds (>20,000ft) indicate arrival of storms, while clouds that are between 6,500 to 20,000ft indicate approaching rain, and low level clouds (< 6500ft) cause rain or snow. Clouds that move vertically upwards cause thunderstorms, but do not result in heavy rains.

The cloud formation and rainfall, aided by ocean and forest, are part of nature’s hydrological cycle, an essential system for sustaining life on earth. In addition to making rain, clouds reflect solar beams, thereby cooling the atmosphere that lies below it. However, clouds also trap the light reflected from earth, thus heating up the atmosphere.

(c) Precipitation:

When relative humidity reaches 100%, water vapor from the atmosphere can precipitate and fall to the earth as rain, snow, hail, and sleet. Water vapor in liquid or solid form is known as precipitate and the formation process as precipitation. The form of precipitation depends on the degree of cooling.

i. Rain:

For rain to occur, in addition to 100% humidity, the temperature must fall causing reduction in retention capacity. This requires movement of the vapor forming clouds upward in the troposphere, where the temperature becomes lower with higher altitude. Rain depends also on the velocity at which the atmosphere moves upwards.

With slow upward movement, moisture laden atmosphere can cause light drizzle, while rapid movement can cause torrential downpour, and still faster build-up of clouds can cause thunderstorms and tornadoes. The emitted gases like sulfur dioxide or nitrous oxides from factories and cars react with water vapor in the atmosphere causing acid rain, which destroys forests, poisons water, and wears away stones.

ii. Snow:

The snow is the frozen precipitation resulting from the growth of ice crystals from water vapor in the Earth’s atmosphere. At temperatures above -40°F (-40°C), moisture begin to form snow around dust particles or other nucleation sites and subsequently precipitates as snow. Snow crystals often grow in the supersaturated environment provided by a cloud of super cooled droplets. This process of precipitate formation is known as Bergeron-Findeisen process.

(d) Pressure:

The rate of upward velocity of clouds depends on pressure. The normal atmospheric pressure as measured by barometer is 760mm (30 inch) at room temperature. Higher elevations have lower pressures thus causing air to move upwards. However, downward movement of air or ‘falling’ air produces desert. For example, in Washington and Oregon, rising air and cloud over the Cascades results in rainfall creating dense forest area along the coast, while the falling dry air on the opposite side makes Desert Island.

(e) Temperature:

The increase in atmospheric temperature is due to the heat producing long wave length infra-red spectrum of solar radiations, which are absorbed by atmospheric gases. The increase in temperature can cause more vapor phase formation and vapor retention capacity.

Lower temperature reduces vapor retention capacity thus increasing relative humidity. Also atmospheric pressure decreases with the increase in temperature and increases with the decrease in pressure. The major variables affecting weather, humidity and pressure are dependent on temperature. Temperature has therefore a profound effect on climate change.

Term Paper # 4. Global Circulation of the Atmosphere:

This large-scale movement of air (together with ocean circulation) is responsible for decreasing thermal gradients across the Earth’s surface, from the equator to the poles. In the absence of atmospheric circulation, average winter temperatures at the poles would be around -100°C rather than -30 °C as at present.

Also without the atmospheric circulation, warming of one region by excessive greenhouse gas emission would have no effect on the other regions. On the other hand, the effect of excessive rise in regional temperature would have disastrous consequences on local weather.

The global warming is the average atmospheric temperature rise across the globe. The temperature averaging effect of air circulation makes the atmospheric warming as truly global in nature. Anthrospheric emissions by industrialized countries causing global warming affect nations across the globe, including the non-emitters.

Planetary rotation results in the development of three air circulation cells in each hemisphere rather than a monolithic one. The simple model incorporates, three distinct patterns of wind circulation cells, from equator to poles, & are known as the Hadley, Ferrel and Polar cells (fig 1.1).

In the 3-cell model, due to the Earth’s rotation, another factor, called Coriolis effect, causes moving wind to swing to the right in the northern hemisphere and to the left in the southern hemisphere, thus becoming westerly and easterly, rather than blowing north and south.

Modern version of the atmospheric circulation takes into account the non-uniform nature of the Earth’s surface, such as, the land and water with differences in thermal properties causing significant difference in temperature and thus producing a series of pressure cells rather a single belt in the simple model.

The driving force for circulation is the temperature gradient. The vertical lifts in the air circulation systems are confined to troposphere, the Earth’s atmosphere responsible for climate. The vertical lift can be as high as 15 km in equatorial region and gradually decreases to around 8 km at the polar region, keeping in conformity with the change in altitudes of troposphere.

1. Hadley Cell:

Hadley cell is the air circulation loop between equator and approximately 30° latitude apart in both Northern and Southern hemispheres. This area of greater heat acts as zone of thermal lows and known as the intertropical convergence zone (ITCZ). Thermal low is an area of low pressure due to the high temperatures caused by intensive local heating in deserts and other land masses in this zone.

Large-scale thermal lows help drive monsoon circulations. Equator remains the warmest location on the Earth. The wind coming from the east is called ‘trade winds’. Wind actually travels from the north east in northern hemisphere and from the south east in southern hemisphere.

The convergence of circulating trade winds from both hemispheres at the equator leads to low pressure in this region. Low pressure and rising warm air cause storms and rains around the equator.

When the subtropical air reaches the equator the air rises vertically upwards to the top of troposphere (15km) by means of convergence and convection. It then begins to flow horizontally to the North and South poles, and thus tends to level the heat difference between the equator and poles in the circulatory cell region.

At about 30° latitude, near the altitude of tropopause, Coriolis force cause deflection of the pole-wards moving air in the Hadley cell to the right in the Northern hemisphere and to the left in the Southern Hemisphere, creating the subtropical jet streams that flow from west to east. The zonal flow also causes the accumulation of air in the upper atmosphere.

Some of the air in the upper atmosphere sinks back to the surface (30° latitude), creating a subtropical high pressure zone. This zone has no rain, and plenty of sunshine, thus creating world’s great deserts at 30° latitude. From this zone, the surface air travels in two directions. A portion of the air moves back toward the equator completing the circulation system in the Hadley cell.

2. Ferrel Cell:

Ferrel wind circulation loop lies between approximately 30° and 60° latitudes in both the Northern and Southern Hemisphere. It is believed the cell is a forced phenomenon, induced by interaction between the Hadley and Polar cell. The stronger downward vertical motion and surface convergence at 30°N coupled with surface convergence and net upward vertical motion at 60°N induces the circulation of the Ferrel cell. This net circulation pattern is greatly upset by the exchange of polar air moving southward and tropical air moving northward.

This moving wind gets deflected by the Coriolis Effect, between 30° and 60° latitude towards west are called are called prevailing westerlies. Westerlies collide with cold air traveling from the poles.

This collision results in frontal uplift and the creation of the sub-polar lows or mid-latitude cyclones. A small portion of this lifted air is sent back into the Ferrel cell, after it reaches the top of the troposphere. Most of this lifted air is directed to the polar vortex where it moves downward to create the polar high.

3. Polar Cell:

In the poles, winds come from the east and are called the ‘polar easterlies’. The Coriolis force causes them to arrive at slight angles. Surface air flow is from the poles to the equator. When the air reaches the equator, it is lifted vertically by the processes of convection and convergence.

When it reaches the top of the troposphere, it begins to flow once again horizontally. However, the direction of flow is now from the equator to the poles. At the poles, the air in the upper atmosphere then descends to the Earth’s surface to complete the cycle of flow.

Seasons:

The Earth rotates about its axis, with a tilt at 23.5 degrees. As the Earth rotates on its tilted axis around the sun, different parts of the Earth receive higher and lower levels of radiant energy. This creates the seasons. Four seasons in western world are known as spring, summer, winter and fall.

Tropical regions may have one or two more seasons, including the rainy season. The six normally occurring seasons in India (excepting South with two rainy seasons) are known as grishma (summer), varsha (rainy season), sharat (fall), hemanta (prewinter), sheeth (winter), vasanta (spring).

Climate Zones:

Areas with consistent climates are grouped together in climate regions. Climate influences ecosystems, which are the communities of plants and animals. Specific associations of organisms therefore often characterize many climate regions.

The original classification by Waldimir Koppen, which is known as the Koppen Climate classification System, divides the Earth’s surfaces into five major climate types, based on the annual and monthly average of temperature and precipitation. The five major climate types, are known as, Tropical (A), Dry (B), Temperate(C), Cold (D) and Polar (E).

Koppen’s system is the most widely used and forms the basis of other classification systems used today. Koppen’s division of the Earth’s surface into climatic regions based on temperature and humidity has generally coincided with world patterns of vegetation and soils.

The Koppen-Geiger classification system, a modification of Koppen classification, recognizes six climate regions on the basis of average monthly temperatures, average monthly precipitation, and total annual precipitation values.

The six climate regions are as follows:

i. Tropical /Megathermal climate (A)

ii. Dry/Arid (or Semiarid) climate (B)

iii. Mesothermal (C)/[Koppen’s Temperate]

iv. Microthermal/Continental climate [Koppen’s Cold] (D)

v. Polar climate (E); and a new one

vi. Highland /Alpine climate (H); which is not in Koppen’s list.

Two subgroups, S – semiarid or steppe, and W – arid or desert, are used with the B climates. Further subgroups are designated by a second, lower case letters, such as, f, m, s, & w which distinguishes specific seasonal characteristics of temperature and precipitation. To further denote variations in climate, a third letter was added to the code, such a, b, c, d, h & k. The different climate zones alongside with the climatic conditions, biomes and global positioning are shown in tab.2.1.

The relationship between climate zones and their geographical locations are shown in fig.2.2. The climate zones and biomes in different regions have distinct temperature, humidity and precipitation and are maintained by the patterns of wind movements. Below polar region, the climate zones are basically divided into two, dry and wet areas.

The wet area has three zones based on temperature, viz., tropical, mesothermal and microthermal with increasing latitude from equator to poles (fig.2.1).However the rise in temperature due to global warming has a profound effect in making changes in the climate zones.

Term Paper # 5. Recent Extension of Tropical Belt and Hadley’s Circulation by Global Warming:

A recent finding indicates that the tropical belt covering the Hadley’s circulation has extended by 2 to 4.5° in latitude (NS), i.e., 145 to 330 miles (225 to 530km), beyond the 1979 figures of 23.5°(NS) in latitude (Fig.2.3). Global warming and depletion of ozone layer are found to be responsible for extensions of Hadley’s circulation cell.

The expansion in Hadley circulation cell has resulted in the extension of tropical belt by 2 to 4.5° in latitude (NS) in Ferrell cell. The net effect is the rise in average global temperature. The major climate types in the tropical belt from 23.5° N to 23.5°S latitudes are as follows (fig.2.3).

The shift shall result in climate changes not only the tropical belt but also in mesothermal and microthermal zones. India’s plane lands belong to tropical belt and the extension in northern latitude would cover the highland zone, such as Himalayan regions in tropical belt. The possible effects on the climate shall include, an expansion of drier and hot climate areas and a fundamental shifts in ecosystem and human settlements.

Term Paper # 6. Disasters as a Result of Weather and Climate Change:

Apart from normal seasonal variations in weather, the change in wind circulation resulting from major fluctuations in temperature and pressure, can cause abrupt changes leading to disasters, such as, storms, cyclone, tornadoes, hurricanes, El Nino, La Nina and others.

i. Storm Fronts:

Storm Fronts can occur when air masses meet. An air mass is an enormous body of air with almost uniform humidity and temperature. The air masses are named according to regions, where they originate. Arctic and polar are cold; tropical and equatorial are warm; maritime is moist; continental is dry.

When the air masses meet, a front is formed. This is the eye of the storm. Fronts can be cold or warm. Cold fronts create the most violent storm. Cold heavy air lifts the lighter warm, wet air causing an increase in humidity and formation of tall cloud, leading to storm. The vertical motion of the air is the key to the storm.

ii. Cyclone:

A frontal cyclone is a large spinning weather system that occurs along turbulent fronts. Coriolis Effect causes the wind to veer right in the north and left in southern hemispheres, it also causes spinning motion. Frontal cyclones generally occur at the junction of the two fronts.

iii. Tornadoes:

Most tornadoes occur in the US because of two opposing air masses, the cool air mass from Canada, and the warm, tropical air mass from south. In spring, when cold air mass still lingers, the warm tropical air mass from south moves in. The light warm air moves fast over the cold mass creating a spinning wind vortex, called tornado. Tornadoes wind can exceed 300 mph (480kph). One of the effects of global warming can be attributed to increased occurrences of tornadoes.

iv. Hurricanes:

Hurricanes are tropical cyclones whose wind speeds exceed 73 mph (160km/h). Hurricanes are called typhoons in the western Pacific and cyclones in the Indian Ocean. Unlike frontal cyclone, hurricanes do not occur along the fronts of contrasting air masses.

The terms “hurricane” and “typhoon” are regionally specific names for a strong “tropical cyclone”. A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation.

v. El Nino:

‘El Nino’ (Spanish; the Christ Child) was a term coined by Ecuadorian and Peruvian fishermen. It indicated the warm ocean phenomena, which occurs every 2 to 7 years around Christmas off the coast of Ecuador and Peru.

In the El Nino year, an easterly trade wind around the equator weakens. This allows warm water in the eastern Pacific, around Australia and Indonesia to flow eastwards. It flows towards Peru and Ecuador. The thermocline layer of water is the area of transition between the warmer surface waters and the colder water of the bottom.

El Nino condition (fig.2.4) results from weakened trade winds in the eastern Pacific Ocean near Indonesia, allowing piled-up warm water to flow toward South America. This causes up-welling of warm water by about 500fit, pushes the thermocline (boundary between warm and cold water), to a lower level and results in disaster. The big circulatory loop moving the upper atmosphere from Australia & Indonesia to South America and then back along the surface is known as the Walker circulation.

El Nino events are linked with a change in atmospheric pressure known as Southern Oscillation (SO). The Southern Oscillation is the see-saw pattern of reversing surface air pressure between the eastern and western tropical Pacific. When the surface pressure is high in the eastern tropical Pacific it is low in the western tropical Pacific, and vice-versa. Because the ocean warming and pressure reversals are, for the most part, simultaneous, scientists call this phenomenon the El Nino/Southern Oscillation or ENSO for short.

The normal increase in water temperature is 1 to 2°C while in strong El Nino temperature can go up to 4-6°C. The temperature increase is at the higher side of the current prediction of global warming by 1.4 to 5.8°C over the next century. Thus El Nino can be used as a working model of the effect of global warming on global weather.

Impact of El Nino:

In El Nino years, the rain area that is usually centered over Indonesia and the far western Pacific moves eastward into the central Pacific. The waves in the flow aloft are affected, causing unseasonable weather over many regions of the globe. The impacts of El Nino upon climate in temperate latitudes show up most clearly during winter time.

For example, most El Nino winters are mild over western Canada and parts of the northern United States, and wet over the southern United States from Texas to Florida. El Nino affects temperate climates in other seasons as well. But even during wintertime, El Nino is only one of a number of factors that influence temperate climates.

El Nino in 1997-98 was one of the most powerful climate events of the century. According to UN estimates, massive fires, floods, frosts and droughts killed around 2100 people and caused property damages to the tune of $33 billion. Asia, Indonesia, and Japan were hit by famine and fire during the 1997-98 El Nino. Many economists believe that this was one of the reasons for the great Asian Stock Market Crash in 1997.

El Nino produces more heat, and together with the increase in global warming, it is expected to cause an intense disaster. If global warming can increase the sea water temperature in South America, the amount of increase is sufficient to kick in an El Nino, which may become a permanent feature.

According to NOAA (National Oceanic and Atmospheric Administration’s recent report (July 2009), El Nino, the warming of the eastern and central Pacific Ocean, has developed and should persist throughout the winter, which may cause damaging storm in California and more storms in the South.

Ecosystem Destruction by El Nino:

Coral reefs are sensitive ecosystems; they are a home to many plants and fish. The rise in sea temperature caused by El Nino and exposure to the Sun combine to destroy algae that protects the coral, which then bleaches white and dies. Destruction of coral from the effects of El Nino can be extensive. Recovery of the reefs may take a very long time.

vi. La Nina:

La Nina is the wicked sister of El Nino. It is characterized by unusually cold ocean temperature in the Equatorial Pacific. Therefore La Nina is the negative, or the cooling phase of El Nino. The effects are more intense from December through March, which is the same period as that of El Nino.

During La Nino, westbound intense trade wind drives more than normal amount of warm surface water westward, causing the rise of deep chilly water off the cost of Peru and Ecuador. This produces a cold tongue of water that extends from South America, all the way to Samoa (Fig.2.4). With heat (warm water) moving westward, the rising moisture in West Pacific forms an intense low pressure zone. This affects the monsoons off Southeast Asia, drenches part of Australia, and produces rains at far off places like southern Africa.

La Nina formation is shown in fig.2.5. Strong Pacific trade winds blow warm surface water westward. Cold water rises to the surface. Cooler air disrupts jet streams; northern jet stream loops into Alaska, Canada.

During La Nina, the polar jet stream moving towards Canada, goes further south, thus pushing frigid air deep into USA, causing freezing winter in Northwest & Upper Mid-west, producing less rain in southeastern states and drought in southwest deserts. In 1998, La Nina hurricane was the deadliest, killing 9,000 in Central America. A probable cause of the tornadoes which swept the Southern US on 5^th February, 2008 is La Nina, due to the cooling of tropical Pacific(2).

Natural disasters are likely to be more frequent and with greater intensities due to climate change resulting from global warming.

Term Paper # 7. Role of Weather and Climate:

Weather and climate play an important role in the pest and disease outbreaks in crops. There exists a significant relationship between the plant diseases and prevailing weather conditions. Weather is the main cause for season to season variations in the severity of disease occurrence.

Weather fluctuations are the main source of uncertainty in the crop production. Pests and diseases have direct influence on the crop plants. Every year considerable losses in the crop production are caused by the pests and plant diseases all over the world.

Therefore, the control of pest and disease is helpful for reducing the losses in the grain yield of the crops. Plant diseases are influenced by microclimate within the crop field, which itself is controlled by weather conditions. The pathogens are affected by the weather conditions through out the life cycle of the crops.

In order to reduce the magnitude of the losses by the pests and diseases, it is necessary to forecast the intensity of the diseases well in advance, so that necessary control measures may be adopted. The forecast of the pests and plant diseases can be made by using combination of various weather parameters favourable for the occurrence of pest and disease.

The forecast of plant disease is easier as compared to the pest, because the insects are mobile and can move from one place to other place, whereas plant disease continue to be subjected to the same environmental conditions for a long time.

The population of the insects undergoes diurnal changes in different positions of the plants near the ground and in the air. Similarly, the insects can move from one part of the crop field to the other part, where the environmental conditions are favourable for their survival.

The insects are dependent on temperature variations and air movements. Most of the insects are mobile but locusts are highly mobile and can move to distant places. These can move along with the wind from one region to other region.

Worms, caterpillars and larvae change their positions on the plants at different times of the day and night for suitable microclimatic conditions. Generally, the population of the insect is maximum in the fore-noon and in the evening. That is why, they can cause more damage in the morning as well as in the evening hours.

During noon hours, when the intensity of radiation is maximum, the shaded portion of the plants are damaged by the insects. Aphids being light, have the tendency to move vertically along with strong convective currents and they can also move along with the strong winds to far off places.

Among the different weather parameters, air temperature and atmospheric humidity are the most important factors influencing the insect and pest incidence. Higher range of humidity combined with moderately low range of temperature is highly conducive to insect occurrence and their multiplication.

Pests and diseases can grow in a better way under tropical and sub-tropical humid conditions where the temperature remains far above freezing level.

Weather has an indirect bearing on the variations of other factors, which affect the pests and disease organisms. The pests and disease organisms have the tendency to escape the harmful effects of one or more factors at one time or another during their life cycle.

Air plays an important role in the movement of the organisms. The extent and speed of the spread of the pest and disease depend upon the current weather conditions.

The growth and development of a crop depend on the weather conditions during the growing period. At different phenological stages of the crop, the growth and development of pests and disease organisms are controlled by weather conditions. Further, the growth and survival of the pests and disease are influenced by the prevailing and the coming weather conditions.

In such cases, if the expected weather conditions are favourable, then serious outbreaks of pests and disease may occur. Similarly, weather conditions are important at all the stages for the air-borne pathogens and the insects. The weather conditions prevailing during the last few weeks and in the current week indicate whether the pathogens or pests are likely to become air borne or not.

Several laboratory experiments have been conducted to assess the effect of different levels of environmental factors like temperature and humidity on the survival rate of the organism. These studies are helpful to determine the modification or the changes of the effects of one factor by that of another one.

These studies may be used to detect the presence or absence of thermal or other environmental constants. The accumulation of these above threshold values may be helpful for the passage of organism from one stage of its life cycle to another.

Laboratory studies may be helpful to delineate the environmental ranges which do not contribute to the development of the organism. These studies have been carried out under constant environmental conditions, which are different from the natural conditions of variable nature.

The diurnal variations may have less effect on the development of the organism, if they happen to be in the region where the development is linearly related to the environmental factors. However, in the higher or lower ranges where the relationship tends to be curvilinear, diurnal variations may exert a marked modifying effect.

Diurnal variations of temperature have a larger amplitude in the temperate regions as compared to the tropical areas. In the same regions, diurnal variations of temperature and relative humidity would be small in the rainy season and quite considerable in the clear and cold season.

Term Paper # 8. Importance of Weather and Climate:

The success or failure of agriculture depends on weather and climatic conditions during the life cycle of the crop. The crop growth is the result of many physical and physiological processes, each of which is affected by the environmental factors. There is a close relationship between various weather parameters and growth of agricultural crops.

Therefore, weather is one of the important inputs in the crop production. When the crops are grown under optimum weather conditions, higher yields are obtained, but if there is any deviation, the crop yield is affected.

The main factors which have strong influence on crop growth and yield are air temperature, moisture content of air, duration and quality of light, radiation from the sun, sky conditions, surface wind, cloudiness and precipitation.

So far, all available resources have been exploited to greater extent and further use of these resources may not increase the agricultural production. So, there is a great need to boost the agricultural production by evolving new techniques. Before studying the agro-climatology and agro-meteorology, it is important to know about other sciences which are concerned directly or indirectly with climate.

Different branches of sciences which are concerned with the understanding of atmospheric phenomena arc as follow:

(1) Meteorology

(2) Climatology

(3) Hydrology

(1) Meteorology:

Meteorology is the science of weather. It may be defined as the science which deals with the atmospheric phenomena and their time bound behaviour. The word meteorology is derived from two Greek words: Meteor and logus.

Meteor means events occurring above the earth surface and logus means to study. Thus, meteorology is the study of the phenomena occurring above the earth surface. Meteorology can be divided into various branches, which deal with different aspects of this science.

a. Physical Meteorology:

It is the branch of meteorology, which deals with the study of physical processes of the atmosphere. In this, we study the physical processes of the atmosphere such as solar radiation, its absorption and scattering in the earth-atmosphere system, the radiation back to space and transformation of solar energy into kinetic energy of air. Cloud physics and the study of rain processes are a part of physical meteorology.

b. Dynamic Meteorology:

It deals with the atmospheric phenomena, which are in motion. This particular branch of meteorology attempts to describe the atmospheric processes through mathematical equations which together are called a numerical model.

After defining the initial state of the atmosphere or ocean, the equations are solved to derive a final state, thus enabling a weather prediction to be made. Dynamic meteorology deals with a wide range of hydro-dynamical equations from a global scale to small turbulent eddies. The process of solving the equations is very complicated and requires powerful computers to accomplish.

c. Experimental Meteorology:

It is a branch of meteorology, which deals with the study of atmospheric processes by means of experiments in the laboratory as well as in the field e.g. cloud formation experiment in the laboratory and rain making experiments in the field.

d. Applied Meteorology:

It is the branch of meteorology, which deals with the application of meteorology to human activities e.g. transportation, town planning etc. Like agriculture, there are many human activities which are affected by weather and for which meteorologists can provide valuable inputs.

Meteorologists use weather information and adopt the findings of theoretical research to suit a specific application; e.g. design of aircraft, control of air pollution, architectural design, urban planning, exploitation of solar and wind energy, air conditioning, development of tourism etc.

e. Synoptic Meteorology:

This branch of meteorology, which deals with the analysis of synoptic observations plotted on the charts for the purpose of weather forecasting. The observations are recorded simultaneously at various places. Weather observations taken on the ground or on ships, and in the upper atmosphere with the help of balloon soundings, represent the initial state of the atmosphere at a given time.

When the data are plotted on a weather map, we get a synoptic view of the world’s weather. Hence, day to day analysis and forecasting of weather has come to be known as synoptic meteorology. It is the study of movement of low pressure areas, air masses, fronts and other weather systems like depressions and tropical cyclones.

f. Agricultural Meteorology:

It is the study of meteorology in relation to agriculture. In simple terms, agricultural meteorology is the application of meteorological information and data for the enhancement of crop yields, and reduction of crop losses because of adverse weather. This branch of meteorology is of particular relevance to India because of the high dependence of our agriculture on monsoon rainfall, which has its own vagaries.

g. Micrometeorology:

It is the study of the climate near the ground, where plants and animals can survive. It deals with the study of small scale meteorological conditions. The study involves refine measurements close to the earth surface and within the crop for understanding the exchange processes over and above the crop field.

h. Macro-Meteorology:

It deals with the study of large scale meteorological conditions. It relates to large geographical regions such as continents or even the entire globe.

(2) Climatology:

Climatology is the study of the climate of a place or region on the basis of weather records accumulated over long periods of time. The average values of meteorological parameters derived from a data base that extends over several decades are called climatological normals.

Different regions of the world have different characteristic climates which determine the crops of any region. However, it is now recognised that climate is not static and issues such as climate change and global warming are receiving increasing attention.

(3) Hydrology:

It is defined as the science, which deals with the study of water bodies and water cycle in the earth atmosphere system.