What is Agroforestry?

Agroforestry is a sustainable management mechanism for land that:

• increases overall production,
• combines agricultural crops, tree crops, forest plants and animals simultaneously, and
• applies management practices that are compatible with cultural practices of the local population.

It is a type of social forestry in which an individual farmer undertakes tree farming and grows fodder plants, grasses and legumes on his own land. In agroforestry trees are considered a crop and become a part of crop combinations.

There is an increasing number of farmers who plant trees either along the boundaries or in the whole field/farm as a crop. In fact, in the northern plains of India, trees are planted by most of the farmers irrespective of the size of their holdings. The large farmers and absentee landlords generally put a part of their holdings or total agricultural area under tree crops.

The main advantage of agroforestry is considered to be its positive impact on the soil. Let’s understand the impact of trees on the soil.

How trees improve soils:

1. Litter or Mulch:

During the lifetime of a tree, leaves, twigs and branches die and fall to the ground as litter. Litter contributes to the organic-matter content of the soil after it decomposes. A soil that is rich in organic matter has a better capacity to absorb and retain water and thus is also more resistant to erosion. A good cover of litter or mulch can also be very effective in suppressing weeds.

2. Increase nutrient content of soil:

• Nutrient Uptake from deeper layers of soil: Tree roots normally penetrate deeper into the soil than the roots of crops. Though not experimentally verified, trees are generally considered more efficient than crops in taking up nutrients released by weathering deep in the soil.
• Nutrients from the atmosphere: The presence of a tree reduces the wind speed and creates good conditions for the deposition of dust. Nutrients in the atmosphere are conveyed to the soil when they are dissolved in rain or settle with dust. Rain water dripping from leaves and flowing along the branches carries the nutrients to the ground, together with those released from the tree itself and associated plants growing on it. It is known that the amounts of nutrients reaching the ground in this way are substantial.
• Nitrogen Fixation: Many leguminous trees and a few non-leguminous ones have the ability to fix atmospheric nitrogen through symbiosis with bacteria or fungi in root nodules. The fixation of nitrogen has been proven and found to be a significant factor in soil fertility. Also, the litter from nitrogen-fixing plants is often rich in nitrogen which is added to the soil when the litter or mulch decomposes.

3. Protection from soil erosion

Tree roots and stems reduce surface run-off, nutrient leaching and soil erosion.

Source

Some Other Benefits of Agroforestry:

1. Environmental Benefits:

• Reduction of pressure on natural forests. It is also recognized that Agroforestry is perhaps the only alternative to meeting the target of increasing forest or tree cover to 33 percent from the present level of less than 25 per cent, as envisaged in the National Forest Policy (1988).
• Agroforestry is known to have the potential to mitigate the climate change effects through:
  • microclimate moderation and natural resources conservation in the short run, and
  • carbon sequestration in the long run. Agroforestry species are known to sequester as much carbon in below-ground biomass as the primary forests, and far greater than the crop and grass systems.
• When strategically applied on a large scale, with an appropriate mix of species, agro-forestry enables agricultural land to withstand extreme weather events, such as floods and droughts, and climate change.

2. Economic benefits:

• A greater output of food, fuel wood, fodder and timber.
• Increase in levels of farm income due to improved and sustained productivity.
• Agroforestry leads to a reduction in the incidence of total crop failure, which is common to single cropping or monoculture systems. This is because such systems are structurally and functionally more complex than monoculture.
• The absentee landlords go for agroforestry to retain the title of the land and to increase their income. It provides an alternative to land owners to manage their agricultural land in absence of family labour.
• Agroforestry has significant potential to provide employment to the rural and urban population through production, industrial application and value addition ventures. Current estimates show that about 65 % of the country’s timber requirement is met from the trees grown on farms.

3. Social benefits :

• Improvement in rural living standards from sustained employment and higher income.
• Makes the villagers self-dependent at the village-level.

Despite the numerous material and geo-climatic benefits, agroforestry if not carefully planned can have some adverse effects on agricultural land. Some of these are:

• In the fields along which trees have been planted, the productivity per unit area decreases, as in at least about two meters from the trees the moisture content in the soil is significantly reduced. Also, the combination of trees and crops must be so chosen so as to minimise the competition between their roots for moisture.
• Unscientific planning can also negatively impact the agricultural productivity of the land. E.g. growing trees with photophilic i.e. light-loving plants.
• Pests and Diseases: Mixtures of trees and crops make a more diverse environment than monocropping. A diverse environment enables a greater variety of species of all kinds of organisms — both desirable and less desirable — to thrive. With increased diversity, the risks of pests and diseases may also increase. On the other hand, a greater diversity of species also allows for better regulatory mechanisms which may reduce the seriousness of the pests and diseases. Careful planning is necessary so that the organisms promoted by one component in the system and are not harmful to the other component or components.

Thus, for an agroforestry system to be successful, a careful and scientific approach is required which incorporates knowledge from both agriculture and forestry.

Source

Apart from these, certain shortcomings have been observed in the case of agroforestry programs in India:

• The tendency of farmers to opt for market-oriented trees rather than the trees which are more ecologically suited or are locally needed (fuelwood/fodder).
• Agroforestry has benefitted the big farmers more than the marginal and small farmers. Many of the absentee landlords plant commercial trees in their agricultural landholdings to save their land from dispossession. This programme is said to have encouraged absentee landlordship.
• The diversion of good agricultural land from cereal and commercial crops may create the scarcity of food and industrial raw material.
• Possible negative impact on crop production because of planting trees on fertile lands.

Thus, while the potential benefits of agroforestry are well documented, particular aspects of these systems need to be adapted to suit the areas where they are introduced. These adaptations primarily involve the selection of the ideal combinations of trees, shrubs and crops that will benefit each other, the environment and the income of the small-scale farmers.

National Agroforestry Policy 2014:

In February 2014, India became the first nation in the world to adopt an agroforestry policy. Agroforestry was earlier covered as a part of other policies e.g. the National Forest Policy, the Green India Mission etc.

The Policy aims to improve coordination, convergence and synergy between various elements of agroforestry, scattered across various existing missions, programme and schemes under different ministries—agriculture, rural development and environment.

Objectives of the National Agroforestry Policy 2014:

• Promote agroforestry to increase farm income and livelihoods of rural households, especially the small and marginal farmers.
• Protect and stabilise ecosystems, and promote resilient cropping and farming systems to minimise the risk during extreme climatic events.
• Simultaneously provide raw material to wood based industries. Thus create new avenues for rural employment, and reduce pressure on the forests.
• To develop capacity and strengthen research in agroforestry and create a massive people’s movement for achieving these objectives.

Cropping pattern used on a farm and its interaction with farm resources, other farm enterprises and available technology that determines its make up, is called a cropping system.

A farmer’s choice of crops is governed by several sets of factors: Physical, Social and Economic. But, by and large, most of the Indian farmers go for cultivation of a number of crops at their farms and rotate a particular crop combination over a period of 3-4 years. It results in a multiplicity of cropping systems, which remains dynamic in time and space making it difficult to precisely determine the spread of different cropping systems.

Scientists have identified more than 250 cropping systems being followed throughout the country but it is estimated that only 30 major cropping systems are prevalent (except the areas under monocropping due to moisture or heat conditions)

A large diversity of cropping systems exists under rainfed and dryland areas with an overriding practice of intercropping. This is because of greater risks involved in cultivating a large area under a particular crop. While in areas with assured irrigation only a few cropping systems are followed and they have a considerable coverage across the region and contribute significantly to foodgrains production at the national level.

Types of Cropping Systems in India:

Broadly speaking there are 3 types of cropping systems in India:

1. Sequential – In sequential multiple cropping, farmers use short duration crops and intensive input management practices. E.g.

• In Maharashtra:- Rice-Frenchbean-Groundnut
• In Rainfed Areas:- Pigeon Pea – Wheat

2. Inter-Cropping – Growing two or more crops simultaneously on the same field is called intercropping. In this case, crop intensification is in both temporal and spatial dimension. There is an inter-crop competition during all or in part of crop growth. E.g.

• Maize and Groundnut in Ranchi
• Cotton and Groundnut in Junagarh

3. Alley Cropping System – Growing of annual crops with multipurpose perennial shrubs/trees is called alley cropping. It is a way of increasing production potential under fragile environments. It is recommended to meet food, fodder and fuel needs besides improving soil fertility e.g. In the salt affected alluvial soil areas of Modipuram, alley cropping of rice-wheat sequence is done with trees like babool.

Major Cropping Systems in India:

The crop occupying the highest percentage of sown area of the region is taken as the base crop and all the other possible alternative crops sown in the region as substitutes (i.e. spatial variation) for the base crop in the same season or as the crops that fit in with the rotation in the subsequent seasons (i.e. temporal variations) are considered in the pattern.

1. Rainy Season Cropping Systems: Among the Kharif Crops – Rice, Sorghum, Pearl Millet (Bajra), Maize, Groundnut and Cotton are the prominent crops to be considered as the base crops for describing the Kharif Cropping Patterns.

1. Rice Based Cropping Patterns: Rice is grown in the high rainfall area or in the areas where supplemental irrigation is available to ensure good yields. At an all India Basis, nearly 80% of rice is sown during June to September and the rest during the remaining season. E.g.
   • With rice, other crops being cotton, vegetables and fruits (in Meghalaya)
   • Jute as an alternative to rice (in Orissa, coastal Andhra Pradesh, Bihar, Jharkhand, Assam etc.)
   • In Bihar, the alternative crops being pulses, wheat, maize, jute, sugarcane and oilseeds
2. Kharif Cereals other than rice:
   • Maize-based cropping systems: Maize is grown in high rainfall areas or on soil with a better capacity for retaining moisture, but with a good drainage.
   • Sorghum based cropping system: Grown in medium rainfall regions. These systems are popular in Maharashtra and Madhya Pradesh. The alternative crops being cotton, pulses, groundnut, and small millets in Maharashtra.
   • Pearl-Millet based cropping system –Pearl millet is a more drought resistant crop than several other cereal crops and is generally preferred in low or less dependable rainfall and on light textured soils. These are popular in Rajasthan, Maharashtra, Gujarat and Uttar Pradesh. E.g. in Rajasthan with pulses, groundnut, oilseeds, etc.
3. Groundnut based cropping systems: These are popular in Andhra Pradesh, Gujarat, Karnataka, Tamil Nadu and Maharashtra. E.g. In Andhra Pradesh and Tamil Nadu, rice forms an alternative; in Karnataka, Sorghum is the main alternative crop, whereas cotton, tobacco, sugarcane and wheat are also grown here.
4. Cotton based cropping systems: These are popular in Maharashtra, Gujarat, Andhra Pradesh and Punjab. E.g. Cotton with other crops:
   1. Sorghum (Kharif and rabi), groundnut, pulses and wheat etc;
   2. With irrigation in some places sugarcane and rice are also grown with cotton.

2. Winter Cropping Systems: Among the Rabi Crops, wheat together with barley and oats, sorghum and chickpea are the main base crops. Generally, wheat and chickpea are concentrated in the subtropical region in northern India, whereas Rabi sorghum is grown mostly in the Deccan.

1. Wheat and Chickpea based cropping systems: Wheat and Chickpea are grown under identical climate and can be substituted for each other. E.g. alternative crops in Madhya Pradesh being Kharif sorghum groundnut, oilseeds, cotton, small millets etc. In Punjab, the alternative crops are rice, mailze, cotton, pearl-millet etc.
2. Rabi Sorghum based cropping systems: These are popular in Maharashtra, Karnataka and Andhra Pradesh. E.g. in Maharashtra, the alternative crops are pearl millet, pulses, oilseeds and tobacco etc.

3. Plantation and other commercial crops:

• Sugarcane, Tobacco, Potato, Jute, Tea, Coffee, Coconut, Rubber, Spices and condiments are important crops. Some of these are seasonal, some annual, some perennial.
• Generally, the areas occupied by these crops are very limited compared with those of food and other crops. Nevertheless, they are important from commerce point of view.
• Besides the above-mentioned crops, there are certain horticultural crops like apple, mango and citrus fruits.
• E.g. in the jute growing areas, rice is the usual alternative crop; in Punjab, Bihar, Haryana and Uttar Pradesh, wheat and maize are rotation crops with sugarcane.

4. Mixed Cropping:

• Crop Mixtures, for example, pulses and some oilseeds are grown with maize, sorghum and pearl millet.
• Lowland Rice is invariably grown unmixed, but in upland rice, several mixtures are prevalent. E.g. In un-irrigated areas of the north, during rabi season, wheat, barley and chickpea are the mixtures of grain crops.
• Note: Mixed Cropping was considered by some researchers a primitive practice, but now many researchers regard it as a more efficient way of using the land. [Mixed cropping ≠Mixed Farming; Mixed Farming = Farming + Animal Rearing. Note the difference!]

Note4students: With this article, we conclude our discussion on Indian Agriculture. However, these articles only cover the geographical aspect of agriculture in India. For a comprehensive understanding and coverage of other aspects like economic, government schemes etc., do read our comprehensive article series on Indian Agriculture (Click here to read).

1. Rice:

• Geographical Conditions of Growth:
  • Rice grows best in areas of warm, humid climate; rice requires temperatures between 20°C and 35°C and a well-distributed rainfall of about 100 cm or irrigation facilities.
  • Fertile soil. Delta and valley soils are the most suitable. Soils with higher clay content are preferred for its cultivation due to their better moisture retention capacity.
• Important Producing Areas:
  • West Bengal (highest producer), Uttar Pradesh, Andhra Pradesh, Punjab (highest per hectare yield), Bihar, Orissa, Chhattisgarh, Assam, Tamil Nadu, Haryana.
  • About 25% of the arable land is used for the cultivation of rice.
• On the basis of climatic conditions, three varieties of rice are found in India:
  • Winter/ Agahani/ Aman – Planted in July-August and harvested in October-December. 86% of the total rice cultivation comes under this variety.
  • Autumnal/ Kuari/Aus – Sown in May-June and harvested in September – October.
  • Summer/Boro – It is sown in November – December and harvested in March-April. It is grown on 1% of the total rice area.

Note: All the above mentioned 3 varieties (aman, aus, boro) are grown in West Bengal and Assam.

• Important varieties: IR-8, Jaya, Padma, Hamsa, Krishna, Sabarmati, and IET 1039.
• India is the 2nd largest producer in the world after China.

2. Wheat

• Geographical Conditions of Growth:
  • It is a crop of temperate climate. The ideal temperature for its cultivation is about 15°-20°C and requires a moderate amount of rainfall of 25-75 cms. It can be grown in the drier areas with the help of irrigation.
  • Well drained loamy and clayey soils are ideal.
• Important Producing Areas:
  • Wheat is cultivated on about 14% of the total arable area of the country.
  • There are two important wheat producing zones in the country – the Ganga-Satluj plains in the north-west and the black soil region in the Deccan.
  • In north India, wheat is sown in October – November and harvested in March – April. In south India, it is sown in September-October and harvested in December – January. Wheat takes lesser time in ripening in south India than that in the north because of hotter climatic conditions in the south.
  • Uttar Pradesh (highest producer), Punjab (highest yield per hectare), Madhya Pradesh, Haryana, Rajasthan, Bihar, Gujarat, Maharashtra, West Bengal, Uttarakhand.
• Important varieties: Sonalika, Kalyan, Sona, Sabarmati, Lerma, Roso, Heera, Shera, Sonara-64.

3. Maize

• Geographical Conditions of Growth:
  • It is a crop of semi-arid climate and is used both as food and fodder.
  • A temperature of 21°C- 27°C and 50-100cm rainfall is suitable for its cultivation.
  • Alternate spells of rains and sunny weather are ideal for maize.
  • It is a Kharif Crop but in some states like Bihar, it is grown in the rabi season also.
  • It grows well in alluvial and red soils with good drainage. It can also be grown in mountainous soils.

• Important Producing Areas:
  • It is grown on 3.6% of the total sown area in the country.
  • Karnataka, Uttar Pradesh, Bihar, Rajasthan and Andhra Pradesh are important maize producing states.
• Important Varieties: Sartaz, Ganga, Deccan-103, Deccan-105, Dhawal, Prabhat, Arun, Kiran etc.

4. Millets:

Jowar, Bajra and Ragi are the important millets grown in India. Though these are known as coarse grains, these have high nutritional value. These are grown on 16.5% of the total sown area in the country.

1. Bajra
   • Geographical Conditions of Growth:
     • It thrives in areas of warm, dry climate.
     • It requires an annual rainfall of 45cm and temperature ranging between 25 to 30°C.
     • It grows well on sandy soils and shallow black soil.
   • Important Producing Areas:
     • Rajasthan is the largest producer of bajra followed by Uttar Pradesh, Maharashtra, Gujarat and Haryana.
2. Jowar
   • Geographical Conditions of Growth:
     • It is a rainfed crop mostly grown in moist areas which hardly needs irrigation.
     • This crop requires moderate rainfall of 30-100cms and high temperatures ranging from 20 to 32°C.
     • Both excessive moisture and prolonged drought are harmful.
     • Well drained light soils are ideal.
   • Important Producing Areas:
     • It is the third most important food crop with respect to area and production.
     • It is cultivated as the major food crop in semi-arid areas of the central and south India.
     • In north India, Jowar is mainly a Kharif crop whereas it is sown as both Kharif and Rabi in the southern states.
     • Maharashtra is the largest producer of jowar followed by Karnataka, Andhra Pradesh and Madhya Pradesh.
     • India is the largest producer of jowar in the world.
3. Ragi
   • Ragi is very rich in iron, calcium, other micro-nutrients and roughage.
   • Geographical Conditions of Growth: It is a crop of dry regions and grows well on red, black, sandy, loamy and shallow black soils.
   • Important Producing Areas: Karnataka is the largest producer of Ragi followed by Tamil Nadu. Other states important for the production of ragi are Himachal Pradesh, Uttaranchal, Sikkim, Jharkhand and Arunachal Pradesh.

4. Pulses

• Major pulses that are grown in India: tur, urad, moong, masur, peas and gram.
• Chief Characteristics:
  1. India is the largest producer as well as consumer of pulses in the world. About 20% of the pulses of the world are produced here.
  2. These are the major sources of protein in a vegetarian diet.
  3. Being leguminous crops, all the above-mentioned pulses (except tur) help in restoring soil fertility by fixing nitrogen from the air. Therefore these crops are mostly grown in rotation with other crops.

• Geographical Conditions of Growth:
  1. Pulses need less moisture and survive even in dry conditions
• Important Producing Areas: The major pulse producing areas are Madhya Pradesh, Uttar Pradesh, Rajasthan, Maharashtra and Karnataka. It is grown on about 11% of the total sown area in India.

5. Barley

• Geographical Conditions of Growth:
  • This is grown mainly as a dry crop.
  • This is the crop grown in the wheat producing area on the poor soil and in area of lower precipitation.
  • Although the geographical conditions required for its cultivation are similar to those for wheat, it has more tolerance for adverse environmental conditions.

• Important Producing Areas: The major producers are Uttar Pradesh, Bihar and Madhya Pradesh.

Horticultural Crops (Fruits, Vegetables, Flowers etc.)

• The contribution of horticulture in the total agricultural produce of the country is 30.4% (Year 2012-13)

• Fruits:

  • India produces 10% of the total fruit production in the world and is the second largest producer of fruits in the world.
  • India is the largest producer of mango, banana, cashew, cheekoo, and lemon in the word.
  • India produces both tropical and temperate fruits.

• Vegetables:

  • India is the second largest producer of vegetables in the world and produces about 13% of the world’s vegetables.
  • India is an important producer of potato, pea, cauliflower, onion, cabbage, tomoato and brinjal.

Source

1. Alluvial Soils

• Formation: They are mainly derived from the debris brought down from the Himalayas or the silt left out by the retreating sea. Thus they are azonal soils.
• Areas: Alluvial soils are widespread in the northern plains and the river valleys. Through a narrow corridor in Rajasthan, they extend into the plains of Gujarat. In the Peninsular region, they are found in deltas of the east coast and in the river valleys.
• Soil texture: The alluvial soils vary in nature from sandy loam to clay. These soils are more loamy and clayey in the lower and middle Ganga plain and the Brahmaputra valley. The sand content decreases from the west to east.
• Soil Colour: The colour of the alluvial soils varies from the light grey to ash grey depending on the depth of the deposition, the texture of the materials, and the time taken for attaining maturity.
• Other Characteristic Features:
  • In the Upper and Middle Ganga plain, two different types of alluvial soils have developed, viz. Khadar and Bhangar.

• • Khadar: the newer alluvium deposited by floods annually, enriches the soil by depositing fine silts, light colour, found near river beds, porous in nature.Bhangar: older alluvium, clayey, darker, has lime nodules called Kankars, found in doabs (inter-fluve areas).
  • Alluvial soils of the northern plains —> transported soils —> therefore lack humus —> lack nitrogen [That is why we need to use nitrogenous fertilisers in the northern plains!]. Exception: the Ganga-Brahmaputra delta region is rich in humus.
  • These soils lack in nitrogen, phosphorus and humus. However, they are generally rich in potash and lime.
  • The soil profile has no stratification.
  • Alluvial soils are intensively cultivated.
  • In certain areas, these soils are covered with unproductive wind-borne soil called Loess.

• Limitations:
  • Allow water to sink into lower strata, and
  • Lack nitrogen (But these soils are capable of fixing nitrogen very rapidly through leguminous crops (peas, beans, cloves etc.)
• Suitable Crops: Wheat, rice, maize, sugarcane, pulses, oilseeds, fruits and vegetables, leguminous crops.

2. Black Soil

• These soils are locally known as the ‘Regur Soil’ or the ‘Black Cotton Soil’. Internationally, these are known as ‘tropical chernozems’. These soils are famous for the cultivation of cotton.
• Formation: These have mainly formed from the Deccan Trap rocks —> Zonal Soils
• Areas: These are found in the Deccan trap region. Black soil covers most of the Deccan Plateau which includes parts of:
  • Maharashtra,
  • Madhya Pradesh,
  • Gujarat,
  • Andhra Pradesh and some parts of
  • Tamil Nadu.
• Soil Texture: Black cotton soil (regur soil) is highly argillaceous i.e. clayey. It is deep and impermeable and thus has high water retention capacity.
• Soil Colour: These soils are black in colour due to the presence of iron, aluminium compounds and humus.
• Other Characteristic Features:
  • These soils are rich in minerals and known for their fertility.
  • The soil depth varies from place to place. It is very thick in lowlands but very thin on highlands. Also, in the upper reaches of the Godavari and the Krishna, and the northwestern part of the Deccan Plateau, the black soil is very deep.
  • These soils swell and become sticky when wet and develop deep wide cracks when dry. This helps in self-aeration, which leads to absorption of nitrogen from atmosphere. Thus, there occurs a kind of ‘self ploughing’. This aeration and oxidisation to deep levels contributes to maintenance of fertility of these soils. This continued fertility is favourable in the area of low rainfall for cotton cultivation even without irrigation.
  • Due to slow absorption and loss of moisture, the black soil retains the moisture for a very long time, which helps the crops, especially, the rain fed ones, to sustain even during the dry season.
  • Chemically, the black soils are rich in lime, iron, magnesia and alumina. They also contain potash. But they lack in phosphorous, nitrogen and organic matter.
• Suitable Crops: These soils are highly productive and well suited to the cultivation of cotton, pulses, millets, linseed, tobacco, sugarcane, vegetables and citrus fruits.

Note: In the southern and eastern parts of the country where rainfall is heavy, black soils often occur in close proximity to red soils. Black soils occupy valleys and low-level areas whereas the red soils occur on higher slopes and hill tops. Mixed black and red soils occur in Coimbatore, Madurai, Tirunelveli (Tamil Nadu) and Bundelkhand region.

3. Red and Yellow Soils

• Locally called ‘Chalka’ in Andhra Pradesh.
• Formation: These are derived from granites, gneisses and other metamorphic rocks —> Zonal Soils. These are formed under well-drained conditions.
• Areas: Along the piedmont zone of the Western Ghat, a long stretch of area is occupied by red loamy soil. Yellow and red soils are also found in parts of Orissa and Chattisgarh and in the southern parts of the middle Ganga plain. They encircle the black cotton soil zone.
• Soil Colour: The soil develops a reddish colour due to a wide diffusion of iron in crystalline and metamorphic rocks. It looks yellow when it occurs in a hydrated form. Often, their upper layer is red and the lower layer is yellow.
• Soil Texture: Varies from sand to clay and loam.
• Other Characteristic Features:
  • The fine-grained red and yellow soils are normally fertile, whereas coarse-grained soils found in dry upland areas are poor in fertility.
  • Have a porous and friable structure.
  • They are generally poor in nitrogen, phosphorous and humus.
  • These soils are airy and need irrigation for cultivation.
  • Intense leaching is a menace in these soil areas.
• Suitable Crops: In places where irrigation facilities are available, the crops cultivated are wheat, cotton, pulses, tobacco, millets, oilseeds, potato, maize, groundnut and orchards.

4. Laterite Soil

• The word laterite has been derived from the Latin word ‘Later’ which means brick. These soils when wet are as soft as butter but become hard and cloddy on drying. Therefore, these are widely cut as bricks for use in house construction.
• Formation: The lateritic soils are particularly found on high flat erosion surfaces in areas of high(>200cm) and seasonal rainfall. The alternating wet and dry seasons lead to the leaching away of the siliceous matter of the rocks leaving behind the compounds of iron and aluminium. These are zonal soils.
• Areas: These soils have mainly developed in the higher areas of the Peninsular plateau. The laterite soils are commonly found in Karnataka, Kerala, Tamil Nadu, Madhya Pradesh and the hilly areas of Orissa and Assam.
• Soil Colour: Reddish brown in colour due to the presence of iron oxide.
• Other characteristic features:
  • With rain, lime and silica are leached away, and soils rich in iron oxide and aluminium compound are left behind(thus the reddish brown colour). Also, humus content of the soil is removed fast by bacteria that thrives well in high temperature.
  • These soils represent the end product of decomposition and are generally low in fertility.
  • The pebbly crust is the important feature of laterites which is formed due to alteration of wet and dry periods.
  • These soils are acidic in character due to leaching. Application of manures and fertilisers is required for making these soils fertile for cultivation.
  • These soils are poor in organic matter, nitrogen, phosphate and calcium, while iron oxide and potash are in excess.
• Suitable crops: Red laterite soils in Tamil Nadu, Andhra Pradesh and Kerala are more suitable for tree crops like cashewnut. These soils are also suitable for tea plantations.

That’s it for this part!

Chief Characteristics of Monsoonal Rainfall:

• Monsoonal rainfall is largely governed by relief or topography. For example:
  • The windward side of the Western Ghats registers a rainfall of over 250 cm.
  • The heavy rainfall in the north-eastern states can be attributed to their hill ranges and the Eastern Himalayas.
• The monsoon rainfall has a declining trend with increasing distance from the sea. E.g. consider the amount of rainfall received during the south-west monsoon period at the following places:

Kolkata (119 cm) –> Patna (105 cm) –> Allahabad (76 cm) –> Delhi (56 cm)

• The rain comes in spells and displays a declining trend from west to east over the west coast, and from the south-east towards the north-west over the North Indian Plain and the northern part of the Peninsula.
• Its spatial distribution is also uneven which ranges from 12 cm to more than 250 cm.

What causes monsoons?

Monsoon is a familiar though a little known climatic phenomenon. Despite the observations spread over centuries, the monsoon continues to puzzle the scientists. Many attempts have been made to discover the exact nature and causation of monsoon, but so far, no single theory has been able to explain the monsoon fully.

Let’s look at some of the important concepts about the origin of monsoons. We will study these along the following lines:

1. The Classical Hypothesis
   • Halley(1690’s)
   • Hadley (1730’s)
2. The role of moisture.
3. The shifting of ITCZ – Flohn

Recent theories:

1. The role of Tibetan plateau
   • As a mechanical barrier
   • As a high-level heat source
2. The role of jetstreams
   • The Sub-tropical/ tropical westerly jetstream
   • The tropical/ equatorial easterly jetstream

1. The Classical Hypothesis

Halley hypothesized that the primary cause of the annual cycle of Indian monsoon circulation was a differential heating effect between the ocean and land in South Asia.

• During April and May when the sun shines vertically over the Tropic of Cancer, the large landmass in the north of Indian ocean gets intensely heated. This causes the formation of an intense low pressure in the northwestern part of the subcontinent.
• The pressure in the Indian Ocean in the south of the landmass is high as water gets heated slowly.
• This pressure differences causes the flow of winds from the high pressure to low pressure areas

Thus the first model ever proposed about the mechanism of the Indian monsoon was Haley’s planetary scale sea-breeze – land breeze system in 1690’s.

Hadley later argued that Haley’s model was lacking in the physical ingredient of the effect of rotation of earth about its axis. Hadley reasoned that the monsoon originates in the SE trades of the southern hemisphere and as it flows across the equator towards the heated landmass, the coriolis force causes it to turn to right, thereby forming the summer monsoon or the SW monsoon.

Image Source

2. The role of moisture:

Moisture plays an important role in the atmospheric circulation which was not appreciated by either Halley or Hadley.

In this respect, the release of latent heat augments the differential heating produced by the land-sea contrast by providing a mechanism for the solar energy received over the vast stretches of the Indian Ocean to be collected, stored, concentrated and later released over the parched landscape of India by a massive atmospheric motion (See the following image to understand this transfer of solar energy) . As the moisture laden air comes over India, with its upliftment there takes place condensation releasing latent heat – augmenting the low pressure conditions i.e. the cause getting augmented by its effect.

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The shifting of ITCZ – Dr. Flohn

The existence of Asian monsoon according to Dr. Flohn is not due to the temperature contrast between land and sea but primarily due to the annual migration of thermally induced planetary winds and pressure belts. For Flohn, the winter monsoon is nothing but the tropical easterlies or the northern trade winds. According to Flohn, the SW monsoon is constituted by the equatorial westerlies which have been displaced northward (because of shift of ITCZ by 20° or more).

Recent Theories:

1. The role of Tibetan Plateau:

A remarkable aspect of the large scale circulation during the summer monsoon season over south-Asia is the upper tropospheric anti-cyclone situated over the Tibetan Plateau.

• The Tibetan Plateau located more than 4500m above mean sea-level with a length of 2000km and width of 600km in the west and 1000km in the east, is considered to be one of the key factors in the development of monsoon circulation in the region.
• The Tibetan plateau exerts its influence both as a mechanical barrier in the atmospheric flow as well as a high level heat source.
• An anti-cyclone appears in the upper troposphere over Tibet during the Indian monsoon season primarily due to latent and sensible heating over the plateau.
• This anti cyclone appears over south-east Asia in the month of May and then moves north-westwards, reaching Tibetan plateau around the height of the summer monsoon season.
• From about September, the anticyclone migrates south-eastward towards Indonesia and loses definition after October.
• Variations in the intensity and position of this High and its orientation have been found to be closely related to the monsoon circulation over South Asia.

The role of Jet streams:

Jet streams which affect India’s monsoon:

1. Sub tropical/ Tropical Westerly Jet stream (TWJS) ~27°N

• (Covered in detail in the discussion on winter season. Need to revise? Click here!)
• According to Dr. Koteshwaram, the disappearance of this jet from the south of the Himalayas paves the way for the burst of South-West Monsoon along the coast of Kerala.

2. Tropical/ Equatorial Easterly Jet stream (TEJS) ~14°N

• This jet is a prominent feature of the upper air circulation during the Indian summer monsoon season (June – September). It appears as a band of strong easterlies extending from SE Asia across the Indian Ocean and Africa to the Atlantic, generally at a height of about 14 kms. It is present over south Indian peninsula between 12° and 15°N with a mean position of 14°N. This easterly jet stream is held responsible for the burst of the monsoon in India.
• The position and speed of this jet have been found to have significant spatial and temporal fluctuations, which in turn correlate with monsoon rainfall distribution over peninsular India.

Factors which influence the South-West monsoon intensity and rainfall distribution:

• Mascarene High and the Somali Jetstream: It is the high pressure area at sea level south of the equator in the Indian Ocean near Mascarene Islands. The position and intensity of this high are considered to be closely linked to the south summer monsoon activity.

According to scientists, the broad belt of high pressure around the Mascarene Islands generates a cross-equatorial flow known as the Somali Jet which brings heavy rain to India’s west coast. A strong, low level jet usually means a strong monsoon over peninsular India.

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• Variation in the axis of the monsoon trough: The monsoon trough axis experiences considerable day to day variation in its position, which has a vital bearing on the monsoon rainfall distribution in the region. During the break monsoon periods, when there is a temporary lull in monsoon activity, the trough line shifts to the foothills of the Himalayas. Rainfall over the central parts of India decreases considerably and there is an increase in rainfall over northern India along the Himalayan foothills.

• Teleconnections (ENSO – El–Nino and Southern Oscillation Index):

A. Southern Oscillation Index (SOI)

1. • A curious see-saw pattern of meteorological changes has been observed b/w Indian Ocean & Pacific ocean. Whenever pressure is higher over Indian Ocean, low pressure prevails over Pacific Ocean & vice versa. This see-saw relationship is referred to as the Southern Oscillations.
   • To measure this pressure difference, a SOI has been developed based on the pressure data at certain representative stations (e.g. the SOI between Tahiti island and Darwin is the most widely used)

SOI (Southern Oscillation Index) = (Atm. Pressure @Tahiti) – (Atm. Pressure @Darwin)

Tahiti = Pacific Ocean, Darwin = Indian Ocean

B. El-Nino (Spanish for male child)

1. • It is an anomalous warming of the Eastern Tropical Pacific Ocean that occurs at 2 – 10 years intervals and is frequently associated with far reaching climatic and oceanic impacts around the world.

These two are believed to be inter-connected – the southern oscillation has been identified as the atmospheric counterpart to El-Nino. Both are together referred to as ENSO.

During the high southern oscillation phase (Normal situation), precipitation tends to be abundant in the Indonesian – Australian region, most of the south Asian region, SE Africa and northern coast of South America. Relatively dry conditions prevail over the equatorial pacific, east central Africa and northern Mexico.

During the low phase of the southern oscillation (El-Nino), the rainfall anomalies are approximately opposite. The El-Nino is found to be adversely correlated with the SW monsoon rain.

La-Nina (Spanish for girl child) refers to the reinforced normal situation and is found to be positively correlated with the SW monsoon rain.

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Temperature Conditions during this season:

As the sun shifts northwards, the temperature rapidly increases especially in the northern India.

The wind circulation over the subcontinent undergoes a complete reversal at both, the lower as well as the upper levels. Let’s see how:

Distribution of air pressure and winds on the surface of the earth:

• As a result of rapid increase in temperature in May over the north-western plains, the low-pressure conditions over there get further intensified.
• Because of the heating of the subcontinent, by the middle of July, the low-pressure belt near the surface [termed as Inter Tropical Convergence Zone (ITCZ)] shifts northwards, roughly parallel to the Himalayas between 20° N and 25° N. The ITCZ in this position is sometimes called the monsoon trough.

[Revision Note: ITCZ is the low-pressure zone located at the equator where the trade winds converge. It is also referred to as the thermal equator. It is a zone of generally calm, hot, rising air and low pressure.]

• Roughly, this elongated low-pressure monsoon trough extends over the Thar desert in the north-west to Patna and Chotanagpur plateau in the east-southeast.
• Meanwhile, the sea surrounding India heats up slowly, creating a zone of relatively high pressure off India’s Southern Coastline.
• Next, because winds blow from high to low pressure, warm moisture-laden air moves south-westerly direction from the sea over to India in the north and North East and is deflected by the Coriolis force as the Earth spins. It is this moist air current which is popularly known as the south-west monsoon.

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Jet Stream and Upper Air Circulation:

• By this time, the westerly jet stream withdraws from the Indian region. In fact, meteorologists have found an interrelationship between the northward shift of the equatorial trough (ITCZ) and the withdrawal of the westerly jet stream from over the North Indian Plain. It is generally believed that there is a cause and effect relationship between the two.

• An easterly jet stream flows over the southern part of the Peninsula in June and has a maximum speed of 90 km per hour. In August, it is confined to 15°N latitude, and in September up to 22° N latitudes. The easterlies normally do not extend to the north of 30° N latitude in the upper atmosphere.

Easterly Jet Stream and Tropical Cyclones: [The tropical cyclones were discussed in detail in the discussion on summer season. Remember? Revise here before moving forward!]

• The easterly jet stream steers the tropical depressions into India.
• These depressions play a significant role in the distribution of monsoon rainfall over the Indian subcontinent. The tracks of these depressions are the areas of highest rainfall in India.
• The frequency at which these depressions visit India, their direction and intensity, all go a long way in determining the rainfall pattern during the south-west monsoon period.

The onset and progress of South-West Monsoon in India:

1. Entry of Monsoon into India:

• The rain in the south-west monsoon season begins rather abruptly. This sudden onset of the moisture-laden winds associated with violent thunder and lightning is often termed as the “break” or “burst” of the monsoons.
• The mean date for the burst of monsoon along the coast of Kerala is 1 first June with a coefficient of variation of a week. It reaches the interior parts of the country by the first week of July. By mid-July, south-west monsoon engulfs the entire subcontinent.

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• Impact of this rainfall: One result of the first rain is that it brings down the temperature substantially. The day temperature registers a decline of 5°C to 8°C between mid-June and mid-July.

2. The progress of monsoons: As these winds approach the land, their south-westerly direction is modified by:

• The relief, and
• The thermal low pressure over the north-west India.

The monsoon approaches the landmass in two branches:

• The Arabian Sea branch
• The Bay of Bengal branch.

The Arabian Sea branch:

The monsoon winds originating over the Arabian Sea further split into three branches:

1. One branch is obstructed by the Western Ghats.
   • As these winds climb the slopes of the Western Ghats, they become cooler, their moisture retention capacity reduces and as a result, the windward side of the Sahyadris and Western Coastal Plain receive very heavy rainfall ranging between 250 cm and 400 cm.
   • Much of the rainfall along the Western Ghats is thus orographic, as the moist air is obstructed and forced to rise along the Ghats.
   • After crossing the Western Ghats, these winds descend and get heated up. This reduces humidity in the winds. As a result, these winds cause little rainfall east of the Western Ghats. This region of low rainfall is known as the rain-shadow area.

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2. Another branch of the Arabian Sea monsoon strikes the coast north of Mumbai. Moving along the Narmada and Tapi river valleys, these winds cause rainfall in extensive areas of central India. The Chotanagpur plateau gets 15 cm rainfall from this part of the branch. Thereafter, they enter the Ganga plains and mingle with the Bay of Bengal branch.
3. The third branch of this monsoon wind strikes the Saurashtra Peninsula and the Kachchh. It then passes over west Rajasthan and along the Aravallis, causing only a scanty rainfall. In Punjab and Haryana, it too joins the Bay of Bengal branch. These two branches, reinforced by each other, cause rains in the western Himalayas.

Note: Though this branch passes through Rajasthan, it does not cause rainfall there. This is because:

• The direction of Aravallis is parallel to these monsoon winds.
• The dry and hot winds from the Sindh region of Pakistan lessen the relative humidity of these monsoon winds and do not allow them to get saturated.

The Bay of Bengal branch:

• The Bay of Bengal branch strikes the coast of Myanmar and parts of south-east Bangladesh. But the Arakan Hills along the coast of Myanmar deflect a big portion of this branch towards the Indian subcontinent.
• The monsoon, therefore, enters West Bengal and Bangladesh from the south and south-east instead of from the south-westerly direction.
• From here, this branch splits into two under the influence of the Himalayas and the thermal low in north-west India.
  • One branch moves westward along the Ganga plains reaching as far as the Punjab plains.
  • The other branch moves up the Brahmaputra valley in the north and the northeast, causing widespread rains.
    • Its sub-branch strikes the Garo and Khasi hills of Meghalaya. The hills of Garo, Khasi and Jaintia are expanded in a funnel shape with an opening towards the sea. So the moist winds coming from the Bay of Bengal undergo a sudden rise here and cause excessive rainfall. Mawsynram, located on the crest of Khasi hills, receives the highest average annual rainfall in the world (1141cm).
    • There is relatively less rainfall on the leeward slope of the Meghalaya Plateau e.g. 143 cm in Shillong and 161 cm in Guwahati which is very less in comparison to that in Mawsynram.
    • The Brahmaputra valley to the north also experiences a rain-shadow effect; the problem is mitigated, however, by the adjacent Himalayas, which cause the winds to rise again, thereby establishing a parallel belt of heavy precipitation.

Note:

• The Arabian Sea branch is stronger than the Bay of Bengal branch. 65% of the humidity brought by the monsoon comes from the Arabian Sea whereas the monsoon coming from the Bay of Bengal contributes only 35% to the humidity.
• In the Gangetic Plains, the two branches merge into one. By the time they reach the Punjab their moisture is largely spent. The gradual reduction in the amount of rainfall toward the west is evidenced by the decline from 162 cm at Kolkata to 66 cm at Delhi and to desert conditions still farther west.
• Over the north-eastern portion of peninsular India, the two branches also intermittently collide, creating weak weather fronts with sufficient rainfall to produce patches of fairly high precipitation (more than 152 cm) in the Chotanagpur plateau.

Here it is important to know why the Tamil Nadu coast remains dry during this season.

It is because:

• The Tamil Nadu coast is situated parallel to the Bay of Bengal branch of south-west monsoon.
• It lies in the rain-shadow area of the Arabian Sea branch of the south-west monsoon.

3. Break in the Monsoon

During the south-west monsoon period after having rains for a few days, if rain fails to occur for one or more weeks, it is known as ‘break’ in the monsoon. These dry spells are quite common during the rainy season.

Reasons:

• These breaks in rainfall are related to the cyclonic depressions mainly formed at the head of the Bay of Bengal, and their crossing into the mainland. Besides the frequency and intensity of these depressions, the passage followed by them determines the spatial distribution of rainfall.
• In northern India rains are likely to fail if the rain-bearing storms are not very frequent along the monsoon trough or the ITCZ over this region.
• Over the west coast, the dry spells are associated with days when winds blow parallel to the coast.

4. The Withdrawal of the SW monsoon:

The withdrawal of the SW monsoon and the onset of the NE monsoon in the end of September are a gradual phenomenon. They take place almost at the same time and tend to merge. This explains the popularity of the phrase “Retreating Monsoon”. This will be taken up in detail in the discussion on Autumn Season.

Temperature Conditions during this season:

By the end of December (22nd December), the sun shines vertically over the Tropic of Capricorn in the southern hemisphere. So, India which is located in the northern hemisphere experiences low temperatures. January and February are the coldest months over most parts of the country.

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The isotherm of 20°C runs roughly parallel to the Tropic of Cancer.

The northern region:

This season usually begins in late-November in northern India. Lowest temperatures are observed in Punjab and Rajasthan. December and January are the coldest months in the northern plain. The night temperature may be quite low, sometimes going below freezing point in Punjab and Rajasthan.

There are three main reasons for the excessive cold in north India during this season:

• States like Punjab, Haryana and Rajasthan being far away from the moderating influence of sea experience continental climate.
• The snowfall in the nearby Himalayan ranges creates cold wave situation; and
• Around February, the cold winds coming from the Caspian Sea and Turkmenistan bring cold wave along with frost and fog over the northwestern parts of India.

The peninsular region:

The Peninsular region of India, however, does not have any well-defined cold weather season. There is hardly any seasonal change in the distribution pattern of the temperature in coastal areas because of:

• Moderating influence of the sea, and
• Proximity to the equator.

Distribution of air pressure and winds on the surface of the earth:

In winter months, the weather conditions over India are generally influenced by the distribution of pressure in the Central and Western Asia.

• A high-pressure centre in the region lying to the north of the Himalayas develops during winter.
• This centre of high pressure gives rise to the flow of air from the north towards the Indian subcontinent, south of the mountain range.
• These surface winds blowing out of the high-pressure centre over Central Asia reach India in the form of a dry continental air mass.
• These continental winds come in contact with trade winds over northwestern India. The position of this contact zone is not, however, stable. Occasionally, it may shift its position as far east as the middle Ganga valley with the result that the whole of the northwestern and northern India up to the middle Ganga valley comes under the influence of dry northwestern winds.
• In south India, the air pressure is slightly lower (due to higher temperatures). As a result, winds start blowing from northwestern high-pressure zone to the low air pressure zone over the Indian Ocean in the south. Due to a low pressure gradient, the light winds blow outwards with a low velocity of about 3-5 km per hour.
• By and large, the topography of the region influences the wind direction (See the following map):
  • They are westerly or northwesterly down the Ganga Valley.
  • They become northerly in the Ganga-Brahmaputra delta.
  • Free from the influence of topography, they are clearly north-easterly over the Bay of Bengal.
• These land bearing winds being cold and dry don’t give rain over most parts of the country. However, these winds cause rain along the Coromandel Coast since they collect moisture on their way over the Bay of Bengal.
  

  

  Image Source – NCERT ClassXI

Jet Stream and Upper Air Circulation:

Higher up in the atmosphere, a different pattern of air circulation is observed during the winter months:

• All of Western and Central Asia remains under the influence of westerly winds along the altitude of 9-13 km from west to east. These winds blow across the Asian continent at latitudes north of the Himalayas roughly parallel to the Tibetan highlands. These are known as the sub-tropical/tropical westerly jet stream.
• Tibetan highlands act as a barrier in the path of these jet streams. As a result, jet streams get bifurcated. One of its branches blows to the north of the Tibetan highlands, while the southern branch blows in an eastward direction, south of the Himalayas, later recombining into a single stream over China.
• It is believed that this southern branch of the jet stream exercises an important influence on the winter weather in India.
• The mean position of this jetstream is about 25°N.

• This jetstream first appears over the northern parts of the Indian subcontinent in the month of October after the withdrawal of the summer monsoon and shifts progressively southwards with the advance of the winter season. Thereafter, it shifts back towards the north, weakens and disappears from the South Asian region with the establishment of the South-West monsoon in June. According to Dr.Koteshwaram, the disappearance of this jet from the south of the Himalayas paves the way for the burst of South-West Monsoon along the coast of Kerala.

Western Disturbances:

During the winters, the weather in India is pleasant. The pleasant weather conditions, however, at intervals, get disturbed by shallow cyclonic depressions originating over the east Mediterranean Sea.

• These temperate cyclones travel eastwards across West Asia, Iran, Afghanistan and Pakistan and enter the north-western parts of India with the help of the westerly jetstream (discussed above).
• On their way, the moisture content gets augmented from the Caspian Sea in the north and the Persian Gulf in the south. Since these extratropical cyclones reach India from the West, they are usually referred to as the western disturbances over the Indian region.

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• Because of the high terrain, mountain ranges, most of these disturbances are in the mature stage and hence weak or irregular by the time they reach India.
• Reaching Rajasthan, Haryana and Punjab, these disturbances slow down and stagnate due the nearly closed in nature of the region with high hills.
• With moisture feed from the Arabian Sea, these disturbances may intensify and move north-east. The tracks of these disturbances come farthest south to latitudes 22/23°N in the month of February.
• An increase in the prevailing night temperature generally indicates an advance in the arrival of these disturbances.

The western disturbances result in:

• A cold spell in north-western India as these depressions are followed by cold waves.
• Winter rainfall in Punjab, Haryana, Delhi and western Uttar Pradesh, locally known as ‘mahawat’. Although the amount is meagre, it is highly beneficial for Rabi crops.
• Snowfall in the higher altitudes of Kashmir and Himachal Pradesh and sometimes hail too. It is this snow that sustains the flow of water in the Himalayan rivers during the summer months.

The precipitation goes on decreasing from west to east in the plains and from north to south in the mountains.

Note: The term western disturbance is generally applied to all disturbances which come from the West, even when a “low” may not be seen on the synoptic charts.

• South Asia
• East Asia
• Northern Australia
• West Africa

Let’s take the case of South Asia:

During winter, the large continent of Asia gets extremely cold and the Siberian high pressure develops. Air flow is offshore and dry. During the summer, the continent develops low pressure in response to heating and the airflow reverses. Moisture-laden air from the ocean is brought inland where it rises over the terrain and produces extremely large amounts of rainfall. See the following diagram and observed the marked shift in the wind direction.

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But at the same time, we know that there are great regional differences and variations experienced in India with respect to temperature rainfall etc. Let’s look at some of these regional variations:

• Variations in temperature: While in the summer the mercury occasionally touches 55°C in the western Rajasthan, it drops down to as low as minus 45°C in winter around Leh. On a December night, the temperature in Drass (Jammu and Kashmir) may drop down to minus 45°C while Thiruvananthapuram or Chennai on the same night record 20°C or 22°C.
• Daily temperature range: In Kerala and in the Andaman Islands, the difference between day and night temperatures may be hardly seven or eight degree Celsius. But in the Thar desert, if the day temperature is around 50°C, at night, it may drop down considerably up to 15°-20°C.
• Regional variations in type and amount of precipitation: While snowfall occurs in the Himalayas, it only rains over the rest of the country. Similarly, while Cherrapunji and Mawsynram in the Khasi Hills of Meghalaya receive rainfall over 1,080 cm in a year, Jaisalmer in Rajasthan rarely gets more than 9 cm of rainfall during the same period. Most parts of the country get rainfall during June-September, but in the coastal areas of Tamil Nadu, it rains in the beginning of the winter season.

These regional variations differentiate the weather and climate of different regions of India.

This brings us to the difference between weather and climate:

Weather is the momentary state of the atmosphere while climate refers to the average of the weather conditions over a longer period of time. Weather changes quickly, maybe within a day or week but climate changes imperceptively and may be noted after 50 years or even more.

Factors Determining the Climate of India:

India’s climate is controlled by a number of factors which can be broadly divided into two groups — factors related to location and relief, and factors related to air pressure and winds.

Factors related to Location and Relief

1. Latitude: The Tropic of Cancer passes through the central part of India in the east-west direction. Thus, the northern part of the India lies in the sub-tropical and temperate zone and the part lying south of the Tropic of Cancer falls in the tropical zone. The tropical zone being nearer to the equator experiences high temperatures throughout the year with small daily and annual range. Areas north of the Tropic of Cancer being away from the equator, experience extreme climate with high daily and annual range of temperature.

1. The Himalayan Mountains:

The lofty Himalayas in the north along with its extensions act as an effective climatic divide.

• The Himalayas act as an invincible shield to protect the subcontinent from the cold northern winds. These cold and chilly winds originate near the Arctic circle and blow across central and eastern Asia.
• The Himalayas also trap the monsoon winds, forcing them to shed their moisture within the subcontinent.

3. Distribution of Land and Water: India is flanked by the Indian Ocean on three sides in the south and girdled by a high and continuous mountain-wall in the north. As compared to the landmass, water heats up or cools down slowly. This differential heating of land and sea creates different air pressure zones in different seasons in and around the Indian subcontinent. The difference in air pressure causes a reversal in the direction of monsoon winds.

4. Distance from the Sea: With a long coastline, large coastal areas have an equable climate. Areas in the interior of India are far away from the moderating influence of the sea. Such areas have extremes of climate. That is why the people of Mumbai and the Konkan coast have hardly any idea of extremes of temperature and the seasonal rhythm of weather. On the other hand, the seasonal contrasts in weather at places in the interior of the country such as Delhi, Kanpur and Amritsar affect the entire sphere of life.

5. Altitude: Temperature decreases with height. Due to thin air, places in the mountains are cooler than places on the plains. For example, Agra and Darjeeling are located on the same latitude, but the temperature of January in Agra is 16°C whereas it is only 4°C in Darjeeling.

6. Relief: The physiography or relief of India also affects the temperature, air pressure, direction and speed of the wind and the amount and distribution of rainfall. The windward sides of Western Ghats and Assam receive high rainfall during June-September whereas the southern plateau remains dry due to its leeward situation along the Western Ghats.

Factors Related to Air Pressure and Wind

The following three factors are responsible for the differences in local climates of India:

1. • Distribution of air pressure and winds on the surface of the earth.
   • Upper air circulation caused by factors controlling the global weather and the inflow of different air masses and jet streams.
   • The inflow of western cyclones generally known as disturbances during the winter season and tropical depressions during the south-west monsoon period into India.

Note: The variations in the atmospheric pressure closer to the surface of the earth have no role to play in the making of upper air circulation.

The mechanism of these three factors and their impact on the Indian climate will be understood with reference to specific seasons of the year in the following sections.

India’s Climatic Calendar

The climatic conditions of India can best be described in terms of an annual cycle of seasons.

The India Meteorological Department (IMD) recognises the following four seasons:

1. • The Winter Season (January – February)
   • The Pre-monsoon Season or summer season (March – May)
   • The Southwest Monsoon Season or rainy season (June – September)
   • The Post Monsoon Season or autumn season (October – December)

   

   India’s Climatic Calendar