Essay on Monsoon

Essay on Monsoon!

Essay on the Definition of Monsoon:

I. The term monsoon is applied to a wind system in which there is a reversal of prevailing wind direction between the winter and summer seasons. The name ‘monsoon’ is derived from an Arabic word “mausam” i.e. season and the term was first applied for the winds over Arabian sea which blow approximately for six months from north-east and of another six months from south-west.

II. Monsoon winds are confined to the tropics between 20°N and 20°S on both sides of the equator.

III. South-west monsoon during summer season over India may be thought of as the south-east trade winds of the southern hemisphere, which on crossing the equator are deflected to the right by the coriolis force (rotation of the earth). As a result, these approach the land from a south-westerly direction.

Essay on the Types of Monsoon:

i. Summer Monsoon:

Summer monsoon causes rainfall over India from June to September. The occurrence of monsoon is based on the principle of differential heating. During the day, large amount of solar radiation is received. As a result, land becomes warmer more quickly than the sea.

Because of difference in thermal energy, wind is generated and starts blowing from sea to land. This is called sea breeze, the intensity of which depends on the thermal gradient between the land and the sea.

During night, solar radiation is absent. Therefore, land mass loses heat energy more rapidly than the sea. Land cools quickly, resulting breeze from land to sea. This is known as land breeze. Similarly tropical regions receive maximum radiation during summer season.

As a result, strong thermal gradient is set up between land and sea resulting monsoon in the tropics. In fact, these are land and sea breezes on a large scale seasonal wind systems moving across vast areas of the globe, persistently in the same direction. But these are reversed with change of season.

ii. Asiatic Monsoon:

The monsoon circulation is well developed in east (China, Japan) and south-east (India, Sri Lanka, Burma, Indonesia). The first and main cause is the size of the continent and second is the effect of Tibetan plateau and Himalayas, which are extensive high land areas extending from east to west direction forming an effective barrier between tropical and polar air masses.

In summer the Asiatic monsoon circulation is more complex, but it is of great importance as it is the source of precipitation in many parts of Asia including India. In this case, the winds blow from high pressure area over the seas towards the low pressure area over the land.

These winds first of all develop over southern China, then progress towards Burma and then start towards India more than a month later. The delayed start of south-west monsoon over India is linked with the upper air circulation i.e. about 6-8 km above in the troposphere.

All the air masses constituting Asiatic summer monsoon are humid and unstable because of their long journey over the tropical oceans and therefore they give large amounts of precipitation.

Essay on Other Areas of Monsoon:

Areas with monsoon circulation are found in lower latitudes with the exception of south America, where the monsoon circulation is poorly developed due to its small size as well as very small area found in northern hemisphere.

Another reason for the poor monsoon circulation is because the cold weather current flows on the western sides of the continents, therefore, land is relatively warmer than the ocean throughout the year and hence the formation of high-pressure cell over the continent is prevented during winter season.

i. Australian Monsoon:

The circulation is an extension of Asiatic monsoon with seasonal characteristics reversed. During the Southern Hemisphere winter, south-easterly winds blow from the high-pressure areas of the southern Pacific Ocean. These winds bring dry weather over north Australia.

During the southern hemisphere summer, the Asian winter monsoons reach the northern part of Australia as the north-east monsoon which bring precipitation and bring warm and humid weather.

ii. African Monsoon:

Monsoon circulation over Africa is smaller than Asian monsoons. In Africa, these are entirely surface winds as they rarely reach the level higher than 5 km.

In west Africa, there are noticeable differences between two monsoon winds (summer & winter), but in east Africa, the two monsoon winds are very much similar in their humidity and thermal characteristics and are only differentiated by their directions. The regional variations in African monsoon are due to the form of the continent.

Conditions before the Onset of Summer Monsoon:

1. Status of solar radiation:

During summer season, the sun moves northward across the equator in the northern hemisphere. A large amount of radiation is received during summer season. Maximum amount of radiation is received during the month of June whereas minimum radiation is received during the month of December.

Because of lag, June, July and August are the warmest months in the northern hemisphere, while December, January and February are the coolest months.

North-west India becomes hot during the months of April, May and June. As a result, a low pressure area is established over this region and adjoining parts of Pakistan. This low is also known as seasonal low pressure area. This is a semi­-permanent feature, which remains active over north-west India during the monsoon period (June to September).

All the rainfall activities over India are directly associated with seasonal low pressure area. This is the main driving force for the movement of monsoonal air from Bay of Bengal and Arabian sea toward north and north-west India.

2. Location of Inter tropical Convergence Zone (ITCZ):

An inter tropical convergence zone is also known as trough of low pressure area, where north-east and south-east trade winds converge. During winter season, Its location remains around the equator.

With rise in temperature over central India, its location starts shifting towards north and gets established over indo-gangetic plains during the month of June and remains active during the monsoon period. This feature (ITCZ) is known as monsoon trough over indo-gangetic plains.

3. Wind Pattern over India:

The wind pattern over India is controlled by the monsoon trough, which always remains in motion. The location of the monsoon trough varies from June to September. During the month of June, it passes through Lahore, Allahabad and Head Bay of Bengal.

Generally during the month of June, southerly winds prevail over Gangetic West Bengal. But over rest of India, south to south-westerly winds are prevalent. During the month of July, monsoon trough passes through Hisar, Gaya and Silchar. To the north of this line, easterly winds get established. These winds bring huge amount of moisture form the sea, resulting clouds and precipitation over many parts of India.

4. Behaviour of Upper Air Winds:

Over north India and indo-gangetic plains, easterly winds become strong at the surface and in the lower and middle troposphere. In the upper troposphere, subtropical westerly jet stream, a strong core of westerly winds at 12 km height, remains active along 29°N latitude during winter season. But during summer season, it shifts to the north of Himalayas.

On the other hand, in the lower troposphere, westerly winds prevail over peninsular India, whereas easterly winds are strong in the upper troposphere. A strong belt of easterly winds known as tropical easterly jet stream appears at a height of 6 km.

5. Anticyclone over Tibetan Plateau:

Tibet plateau is a vast high land over Himalayan region with length about 200 km and width ranging from 600 km in the west to 1000 km in the east. During summer season, it becomes very hot, resulting low pressure area at the surface with an average height of 4 km.

The ascending air from the surface spreads outwards both to the north and south of the plateau at an elevation between 9 and 12 km. As a result, an anticyclone is formed, which is known as Tibetan high.

Because of the presence of Tibetan high, easterly winds over gangetic plains of India get intensified resulting vigorous monsoon. Whenever, the formation of an anticyclone circulation is delayed, then the formation of monsoon circulation is also delayed.

Thus, the onset of monsoon depends upon the shifting of subtropical westerly jet stream to the north of Himalayas and the formation of anticyclone over Tibet plateau.

Onset and Advance of Summer Monsoon:

The onset of monsoon takes place over south India and withdrawal takes place from north India, therefore, the duration of monsoon in north India is shorter than that over the peninsular India.

Over the extreme peninsular India, the withdrawal of the south-west monsoon often coincides with the onset of north-east monsoon because there is no break of dry period between the two monsoons. The duration of monsoon over India varies from two to four months.

The change over from the winter pattern to the summer pattern is considered rather abrupt than the change over from the summer to winter pattern. Many workers have shown that the seasonal pattern changes abruptly from winter to summer with shift of the jet stream south of Himalayas to north of the Himalayas.

The change may take place within a week or two, but no distinct date could be given to the onset of monsoon over Kerala.

Attempts have been made to relate the onset (period/date) with that of the pre- monsoon activity of westerlies. It was observed by the earlier workers that the activities of the westerlies in the pre-monsoon period over north India are negatively con-elated with the onset of monsoon. However, this has not been found to be true in all the years.

Anant Krishnan and others found a method of declaring the onset of monsoon over south Peninsula on the basis of rainfall distribution at seven stations along the west coast of India. Out of these seven stations, at least five stations must receive rainfall for three days continuously.

They further established that after 20^th May the onset of South-west monsoon can be declared, if five stations receive 1 mm or more rainfall continuously. If there is discontinuation of such a feature for two days, a temporary withdrawal of monsoon is indicated.

Monsoon advancement into Kerala is not only based on the rainfall on the west coast but also on the formation of shear line in the lower and middle tropospheric winds.

El-nino and Summer Monsoon Circulation:

El-nino is one of the important teleconnections of monsoon which is associated with global circulation. It has the capability to disturb the global wind circulation. It has been observed that a remarkable disturbance of ocean and atmosphere occurs at an interval of about three to eight years.

Eastern pacific ocean areas are considered to be the origin of el-nino and its effects are experienced all over the world. This disturbance may last for more than a year causing droughts, heavy rainfalls, severe spells of heat and cold, or a high incidence of cyclonic storm in various parts of the pacific and its eastern coasts. This phenomenon is called el-nino.

Now recently scientists have recognised a new event called La-nina (the girl child). This event represents a situation roughly opposite to El-nino. During La-nina period, sea surface temperatures in the central and western pacific ocean becomes lower than the average. South Pacific sub-tropical high becomes very strong.

As a result, strong south-east trade winds are established. These winds shift warm surface water westward bringing cooler water to the surface off western continental coasts. La-nina conditions were identified during 1988. During the summer of 1988, some parts of North America experienced a drought which may be related to La-nina conditions.

Now computer models have been developed using surface temperature along with air temperature and pressure which can predict El-nino events reasonably some months before they occur.

Now it has been well established that “El-nino” refers to all the conditions associated with warmer water than normal temperature in the tropical pacific from the international dateline to South America. On the other hand when the water in this area becomes colder than normal, it is called La-nina has global effects but not as striking as “El-nino.”

Withdrawal of Summer Monsoon:

The change from summer to a winter type of circulation is of shorter duration than the reversal from winter to summer. It is much more gradual than the abrupt changes associated with the onset of the summer monsoon. The monsoon begins to withdraw from northern India around mid September.

By the end of October, it has usually withdrawn from the region of north of 15°N, and from Bangladesh and Burma. Finally it withdraws from the extreme south of the Indian peninsula and Sri Lanka by December.

Winter Monsoon/North-East Monsoon:

Winter monsoon is a land origin wind system, which moves from land to sea. In this case, the land surface is colder than the surrounding sea surface. Therefore, a shallow induced high pressure cell develops over land with low pressure belt over the adjacent sea surface.

A land to sea pressure gradient is set up, which causes cold dry continental air masses to flow seaward. This is cold and dry air, which does not cause much rainfall. However, when it moves across some water bodies, it picks up moisture and can cause light to moderate rainfall over adjoining areas. The winter monsoon causes less rainfall as compared to summer monsoon.

During winter season polar cold air accumulates over Siberia (Russia) resulting in high pressure area. It is a semi-permanent feature which is called Siberian high. Winds blowing out of this high pressure area across south-east Asia, are northerly to north-easterly.

These winds are dry and extremely cold over land, but while crossing Himalayas, they get heated due to adiabatic warming. They form north-easterly monsoon during winter season. When these winds move across Bay of Bengal, they become moist and cause rainfall over coastal Andhra Pradesh, Tamilnadu and Kerala.

During winter of the northern hemisphere, radiation losses in the northern parts of Asian continent form an extensive high pressure belt over there. Therefore, the winds move southwards or south-eastwards. They start from the zone of high pressure. On crossing the equator, these winds curve into westerlies over Indonesia.

Most of the south and south-east India are dominated by the winter monsoons. North India is rarely affected due to the barrier effect of Himalayas which prevents the polar air from moving over the gangetic lowlands or Indo-gangetic plains. Winter monsoon is known as north-easterly monsoon over south-east coast of India and causes rain over Andhra Pradesh, Tamilnadu and Kerala.

During summer monsoon period there are two regions where scanty rainfall occurs. These regions are Rajasthan and Tamilnadu. A large scale subsidence over Rajasthan does not allow rainfall to occur. Heavy rainfall occurs on windward side of western ghats but scanty rainfall occurs over Tamilnadu as it lies on the leeward side.

But Tamilnadu and adjoining parts of coastal Andhra Pradesh receive rainfall during north-east monsoon. After the withdrawl of south-west monsoon from India and adjoining sea areas. The wind direction changes from south-west to north-east over Bay of Bengal, India Peninsula and the Arabian sea. This is known as winter monsoon or north-east monsoon.

Winter monsoon winds are weak as compared to summer monsoon winds. Therefore, light to moderate rainfall occurs in coastal Andhra Pradesh and many parts of Tamil Nadu and Kerala. Rainfall activities continue to persist during the period from October to December.

Besides winter monsoon winds, these parts receive moderate to heavy rainfall under the influence of cyclonic storms and depressions, which move towards peninsular India from Bay of Bengal.

Apart from these weather systems, there are low pressure systems, which move across extreme south-peninsular India. These are weak systems, as they cover large area and cause rain over a narrow coastal belt. Rainfed crops are raised in these areas.

The success or failure of the crops depend upon the amount and distribution of rainfall. Hence, the crop productivity directly depends upon the frequency and the strength of these weather systems.

Impact of Summer Monsoon on Agriculture:

Summer monsoon plays an important role in agricultural production. The success or failure of agricultural crops depends upon the behaviour of monsoon. In fact, the farmers living in the tropical monsoon climate are at the mercy of the monsoon rains.

Rainfed crops cannot be raised without the timely arrival of monsoon rains. Rice is the major crop, which is cultivated during rainy season. This crop not only depends upon the total amount of rainfall but also upon distribution of rainfall during the life cycle of the crop.

The Indian economy is associated with the agricultural production. The arrival and withdrawal of monsoon determines the agricultural production. If the arrival of monsoon is delayed by a few weeks, then the sowing of kharif crops is delayed leading to the reduction in agricultural production. Summer monsoon remains active over India from June to September.

Early withdrawal of monsoon decrease the duration and total amount of rainfall, which decreases the agricultural production. Moreover, abnormal behaviour of the monsoon may cause floods in one region, whereas droughts are experienced in other regions of the country. As a result, every year large variations are witnessed in grain production. Hence, the summer monsoon has a direct impact on the national economy.

There is a strong link between monsoon variability and food production in India. Summer monsoon season is of paramount importance as it causes more than 75 per cent of the annual rainfall of the country. Every year the behaviour of the monsoon activity undergoes large variations. It is a well known fact that monsoon variability does not occur uniformly across the country.

When some part of India experience drought while others experience floods. The length of the growing season depends upon the length of the monsoon season. The late onset and early withdrawl of monsoon cause unusual stress on the health of the crops.

The climatic conditions are modified by the arrival of the monsoon. Late arrival can increase the temperature. Similarly, break monsoon conditions can increase the temperature and evapotranspiration.

Agriculture based economy of India can be destabilised by the recurrence of floods and droughts. These phenomena not only cause immense hardships to the farmers, but also influence the health of the government. The Indian agriculture is still dependent on the behaviour of the monsoon, therefore, failure or pulsating character has serious repercussions on our national economy and political stability.

Rainfed agro-ecosystem occupies a prominent place in the Indian agriculture. The importance of rainfed crops will continue to be so in future. It has been estimated that about 63 per cent of agricultural crops in India are raised under rainfed conditions.

This system contributes about 44 per cent food production to the national food basket and supports 40 per cent of the human and 60 per cent of the livestock population. Even after full exploitation of irrigation potential, about 50 per cent cultivated area will continue to depend on rainfall.

The average monsoon rainfall of India is around 850 mm with a standard deviation of 10 per cent. About 30 per cent of the country (109 m ha) is drought prone and as such suffers from critical water shortages. Besides uncertainties in rain water availability, the swings in the onset, continuity and withdrawal pattern of monsoon make crop production a risky proposition in rainfed areas.

The coefficient of variation of the monsoon rainfall in the areas located in the rainfall zones of less than 500 mm, 500-700 mm, 700-1000 mm and more than 1000 mm is in the range of 50-55, 40-50, 30-40 and 20-30 per cent, respectively.

Timely arrival of the monsoon rains has great impact on the food production. If the monsoon rains fail to occur at their normal time or delayed by a few weeks, widespread drought may appear resulting economic disaster. Drought conditions can also appear during break monsoon period, when the strength of monsoon winds become weak and rainfall activities disappear over most parts of the country.

Generally break monsoon conditions appear in the first week of August and such conditions may exist for about one week. However, break monsoon period may prolong for more than one week. During this period, cloudiness and rainfall are absent and air becomes dry.

Because of the absence of cloudiness, temperature increases which enhances the rate of evapotranspiration. If these conditions persist for a longer period, drought conditions are expected to appear. Under such conditions, available soil moisture is insufficient to meet the increased demand of evapotranspiration.

During the month of August, the growth rate of rice crop is very high, therefore, the water demand of the crop increases manifold. The condition of kharif crops deteriorates further, if the intensity of drought increases under enhanced evapotranspiration due to low relative humidity and high temperature conditions.

Drought becomes intensified in those areas, where soil lose moisture rapidly due to high rate of evaporation. Under drought conditions, crops grown in heavy soils may suffer less damage as compared to light soils.

Under dry conditions, the air and soil temperature affect all the growth processes of the crop plants. Likewise, leaf temperature, which depends on the absorption of radiation and transpiration, is very important for the growth of crop plants.

If soil moisture is available, then high temperatures are not as serious as the low temperature in arresting the plant development. But under very high temperature conditions, crop growth is slowed or even stopped regardless of moisture supply.

The crop production in India is greatly influenced by monsoon rains. India is one of the few countries of the world, where the rainfall is seasonal. We know that every year monsoon rains undergo large variations, therefore the regional distribution of rainfall over the country is uneven. Because of this failure, large coefficients of variation can be depicted.

Coefficients of variation are very high in those areas, where rainfall is scanty. In such areas, agricultural crops cannot be raised without irrigation. However, failure of rains is less common in those areas where variability is very small.

The Indian economy is associated with the agricultural production. The arrival and withdrawal of monsoon determines the agricultural production. If the arrival of monsoon is delayed by a few weeks, then the sowing of kharif crops is delayed leading to the reduction in agricultural production. South-west monsoon remains active over India, which adversely affects the normal growth of the crops.

The erratic behaviour of monsoon has detrimental effect on the Indian economy because some parts of the country experience floods, whereas other parts experience drought causing heavy losses to the crops. Although there are several states like Punjab, Haryana and Uttar Pradesh, where paddy crop is raised under irrigated conditions, even then monsoon rainfall has favourable impact on the normal growth of crops.

Keeping in view the significance of monsoon in the national economy, several models have been developed to predict the arrival of monsoon. Since 1988, power regression model have been used to predict the monsoon arrival and total rainfall over the country.

This model is being upgraded to improve the accuracy of prediction so that the severity of the floods and droughts may be assessed accurately in different parts of the country.

It is a well-known fact that every year summer monsoon has erratic behaviour. Monsoon variability has a great impact on food production of India. The farmers have to depend on the timely monsoon rainfall for the sowing of kharif crops. The onset and advance of monsoon over many parts of India is a regular feature, however, the rainfall distribution undergoes large variations.

The delay in the onset of monsoon by one week or more can destabilise the economy of India because normal sowing of kharif crops cannot be carried out.

Sometimes the normal onset of monsoon is there, but later on it be comes weak, therefore sufficient water may not be available for the normal growth of crop plants. Hence, seasonal total rainfall and its distribution during different penological stages of the crops has a great impact on crop growth and production.

Essay on Break Monsoon Conditions:

Monsoon trough remains as a quasi-permanent feature during the active period of monsoon. Normally it is situated over the gangetic plains. Monsoon trough is not a fixed line but it remains oscillating northward and southward about its normal position. The axis of the monsoon trough shows a southward inclination with height up to about 6 km.

When the monsoon trough is in its normal position, widespread rains occur up to about 500 km on either side of the axis of the trough. The rainfall occurs due to the low level convergence (vortex) embedded in the trough. These low pressures have the tendency to move in west-north-westerly direction. These vortices dissipate after causing rainfall.

When the monsoon trough moves northward of its normal position, the monsoon activities decrease. The rains are confined to the foot hills of the Himalayas. South- easterilies are replaced by dry westerlies. As a result, rainfall almost ceases over most parts of the country. When the monsoon trough shifts southward from its normal position then heavy rainfall occurs across the station.

The activity of the monsoon rains is decreased for certain periods, which are known as breaks in the monsoon. During these periods, there is little or practically no rainfall over most parts of the country, West coast receives some amount of rainfall but in very small quantity. Heavy rainfall is concentrated along the foot hills of Himalayas.

Strong westerlies spread over the whole country with a trough line along the foot hills while the easterlies are absent. Tibetan high or high pressure area over Tibet is replaced by a deep trough of the mid-latitude westerlies. Monsoon activities over most parts of the country disappear.

Essay on Monsoon Depression:

The activity of south-west monsoon is often associated with the activity of the seasonal trough in the lower and middle troposphere. The monsoon trough is activated from time to time due to the movement of vortices (low pressure) within trough. The vortices intensify into depression stage and heavy rainfall generally occurs in the south-western sector of the depression.

Most of the monsoon rainfall is generated by the westward movement of the depressions or low pressure systems in the Bay of Bengal. Generally on an average, one to three such systems can occur every month, especially in July and August. The horizontal dimension of a ‘low’ or a depression may be around 500km. Such system can remain active for one week.

The conditions which are favourable for the formation of monsoon depression are:

(i) The upper air cyclonic circulation descends downward to form a low pressure or depression at the surface. They cause the extension of the seasonal trough into Head Bay. Therefore, the monsoon depression develops over Bay of Bengal.

(ii) Waves in the upper tropospheric easterlies can intensify the low level trough in the region of divergence associated with them in higher levels and cause formation of depression in the low levels.

(iii) Monsoon depression can also develop when cyclone over pacific ocean moves westward and becomes weak later on. These cyclones change to low pressure system and move westward. When such low pressures come over north Bay of Bengal, they get intensified into monsoon depressions.

(iv) After a break period, incursion of fresh monsoon air due to vortex formation and its movement in the Bay increase the vorticity in the north Bay and can cause the formation of depression.

Orography of the Land:

Orography plays an important role in the mechanical uplift of the moisture laden monsoon winds. Heavy rainfall occurs on the windward sides of the mountains whereas rain disappears on the leeward side. The intensity of precipitation depends on the strength of the monsoon current and its moisture content.

The orographic influence can be noticed on the western ghats where moist laden monsoon winds cause heavy rain on the windward side and no rain occurs on the leeward side, called rain shadow area. Tamilnadu and adjoining parts come under rain shadow area, therefore, no rain or scanty rains are noticed during South-West monsoon season.

Similarly Himalayas provide necessary mechanical uplift to the moisture laden monsoon airmass. That’s why heavy to very heavy rain occurs on the windward side of the mountains. Sometimes even the clouds burst over the mountain areas. Several cases of cloud burst are noticed during monsoon period in areas of Himachal Pradesh and Uttaranchal etc.

Cloud Burst:

Cloud burst is a localized phenomenon, which affects small area within a short period of time. The rainfall intensity may be around 100 mm or more. Sudden increase in the intensity of rainfall can generate flash floods causing great havoc to life, property, transportation and communication systems.

Normally cloud bursts are confined to hilly areas, but sometimes it may occur in plain areas also, where excessive moisture laden air mass accumulates and becomes unstable due to steep environmental lapse rate. The orientation of the Kulumountains is favourable for the cloud burst.

It has been observed that the human casualities and loss of animals are the results of flash floods. Landslide is a major problem in the hilly areas, which is always associated with cloud burst. A series of severe cloud bursts have been experienced in several hilly areas of Uttarakhand in June 2013, which caused flash floods and landslides of highest magnitude and created havoc in the state.

Topographic features in Himachal Pradesh are favourable for the cloud burst during monsoon period. This is one of the major disaster prone states in India w.r.t. flash floods triggered by cloud bursts.

A cloud burst is a devastating weather phenomenon representing sudden burst of highly concentrated rain associated with thunderstorm over a small geographical area. Cloud bursts are common to all hilly areas but the states of Himachal Pradesh and Uttarachal etc. are most affected due to topographical conditions. Such type of weather hazards cause extensive damage to the agricultural land.

Essay on the Forecast of Monsoon:

The Indian economy is directly associated with monsoon behaviour. The agricultural production is a major component of the Indian economy. Every year large variability in agricultural production is observed due to erratic behaviour of monsoon. It is a well known fact that south-west monsoon is capable to destabilise the economy of the country.

The farmers start planning their kharif crops after the harvesting of rabi crops in the month of April. They are interested to know the long range forecast of monsoon well in advance before the actual sowing of the crops. It is an uphill task to predict the erratic behaviour of monsoon.

Every year in the month of April the same question is raised. Will the onset of monsoon be early, normal or late? The delay in the onset of monsoon affects the sowing of kharif crops. Above all, the onset of monsoon, total amount of monsoon rain (June to September) and its distribution during the life cycle of the crops always haunt the weatherman and the farmers.

The long range forecast of monsoon rainfall was attempted in 1886 by using the statistical correlations between past and future weather in the same or some other regions of the globe. The relationships thus developed are used to predict the behaviour of large scale system several months ahead.

Heavy snowfall over Himalayas during January to May was found to cause abnormal pressure conditions which were favourable for the advance of monsoon over the Indian subcontinent.

Sir Gilbert Walker was the first scientist who realized the need to establish statistical relationship between monsoon rainfall and certain meteorological parameters from selected places over the globe.

In 1924, he made and important discovery of “Walker Circulation” and “Southern Oscillations”. Both these phenomenon are associated with monsoon rainfall. He introduced the concept of correlation coefficient for long range forecasting.

It is well known fact that atmosphere is a thermodynamic heat engine driven by solar energy received from the sun. Similarly monsoon circulation is a thermally driven low pressure are which forms due to the thermal contrast between a vast Asian continent and the Indian ocean to its south.

Differential heating is contributed by the sensible heat due to elevated land mass such as Tibetan Plateau and Himalayan ranges and the latent heat released due to the condensation of monsoon rainfall over south-east Asia. Greater the temperature contrast, better it is for the monsoon rainfall. The temperatures are, therefore, considered to be directly associated with the monsoon rainfall.

Several statistical techniques have been developed. But these models could not become operational due to lack of information. Then a power regression model was developed by using 16 meteorological related parameters. For the long range forecast of monsoon rainfall, India Meteorological Department has been using this model since 1988.

This model enabled the India Meteorological Department to issue long range forecast by April 16. Some of the parameters used in the model are global and others are regional in nature. The main parameters which are physically associated with monsoon rainfall are temperature, pressure, wind pattern and snow cover over the Himalayas.

Recently a new term “El-nino” has been identified as one of the important telecommunications of monsoon which is global in nature. It has the capability to distort global wind pattern and has far reaching effects on the world trade. The historical data on the behaviour of monsoon indicated that during El-nino year south­west monsoon becomes weak leading to drought condition over India.

Many studies have indicated that the weak monsoon of 1982 and 1987 may be attributed to the “El- nino effect”. Now the use of “El-nino effect would improve the accuracy of long range forecast of south-west monsoon in the near future.

Long range weather forecasting techniques are based on 16 parameters. Out of these, there are six predictors related to the pressure field, five related to the temperature and upper level winds and finally five depend on the snow cover and atmospheric oscillation.

Predictors for long range forecast of monsoon are given below:

1. Pressures:

(a) Location of a 500hpa ridge over India in April.

(b) Port Darwin Pressure in April.

(c) Pressure over Argentina in April.

(d) Indian ocean equatorial pressure.

(e) Northern hemispherical pressure anomalies from January to May.

(f) 50hpa ridge trough pattern over northern hemisphere during January and February.

2. Upper winds and temperatures:

(a) 10hpa westerly winds over Balboa.

(b) Central India minimum temperature in May.

(c) Northern India minimum temperature in March.

(d) Minimum temperature over the east coast of India March.

(e) Northern hemispherical temperature in January and February.

3. Snow cover and atmospheric oscillations:

(a) Eurasian snow cover in previous December.

(b) Himalayan snow cover from January to March.

(c) The ENSO Index (Tahiti-Darwin pressure) from March to May.

4. The El-Nino category of current year (October of previous year to May of current year).

5. The El-Nino category (measured by the sea surface temperature over the equatorial south-eastern Pacific) of previous year.