The Distribution of Rainfall, a key determinant of a region’s climate, exhibits significant variability across the globe. Various factors, including geographical location, topography, proximity to water bodies, and atmospheric circulation patterns, contribute to the non-uniform dispersal of rainfall. Equatorial regions generally experience high and consistent precipitation due to the convergence of trade winds and the presence of the Inter-Tropical Convergence Zone. In contrast, areas closer to the poles tend to receive lower rainfall as a result of cold air masses and limited atmospheric moisture. Mountainous regions often demonstrate an orographic effect, causing increased rainfall on the windward side and creating rain shadows on the leeward side. Understanding the distribution of rainfall patterns is crucial for assessing water availability, managing ecosystems, and supporting agricultural practices worldwide.
Rain Bearing Systems and Distribution of Rainfall
- The southwest monsoon splits into two branches, the Arabian Sea Branch and the Bay of Bengal Branch near the southernmost end of the Indian Peninsula.
- It arrives in India in two branches: the Bay of Bengal branch and the Arabian Sea Branch First originate in the Bay of Bengal causing rainfall over the plains of north India. Second is the Arabian Sea current of the southwest monsoon which brings rain to the west coast of India. The latter extends toward a low-pressure area over the Thar Desert and is roughly three times stronger than the Bay of Bengal branch.
The monsoon winds originating over the Arabian Sea further split into three branches:
- One branch is obstructed by the Western Ghats. These winds climb the slopes of the Western Ghats and as a result of orographic rainfall phenomenon, the windward side of Ghats receives very heavy rainfall ranging between 250 cm and 400 cm. 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.
- 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.
- A third branch of this monsoon wind strikes the Saurashtra Peninsula and the Kutch. 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, caused rains in the western Himalayas.
The intensity of rainfall over the west coast of India is, however, related to two factors:
- The offshore meteorological conditions.
- The position of the equatorial jet stream along the eastern coast of Africa.
- The Bay of Bengal branch strikes the coast of Myanmar and part of southeast 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 south and southeast instead of from the south-westerly direction. From here, this branch splits into two under the influence of the Himalayas and the thermal low is northwest 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.
- Mawsynram, located on the crest of Khasi hills, receives the highest average annual rainfall in the world.
- The Tamil Nadu coast remains dry during this season because it is situated in the rainshadow area of Arabian Sea branch of the south-west monsoon and lies parallel to the Bay of Bengal branch of south-west monsoon.
- Frequency of tropical depressions originating over the Bay of Bengal varies from year to year. As the axis of the monsoon trough oscillates with the apparent movement of sun between Tropic of Cancer and Tropic of Capricorn, there are fluctuations in the track and direction of these depressions, and the intensity and the amount of rainfall vary from year to year.
- The amount of rainfall in north India varies with the frequency of the tropical depressions. On an average, one to three depressions are observed every month and the life span of one depression is about one week .
- The rain, which comes in spells, displays a declining trend from west to east over the west coast, and from the southeast towards the northwest over the North Indian Plain and the northern part of the Peninsula.
- Rajasthan desert receives low rainfall in spite of being in the path of Arabian Sea branch of monsoon. This branch blows parallel to Aravallis mountain chain without obstruction and thus, does not release moisture here.
Break in the Monsoon
- During the southwest monsoon period after having rains for a few days, if rain fails to occur for One or more weeks is known as the break in the monsoon. These dry spells are quite common during the rainy season. These breaks in the different regions are due to different reasons:
- 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.
Retreat of Monsoon
- Monsoon starts retreating on the first of September. It starts retreating from the north-western part of India. This day is the last day of the rainy season in Jaisalmer and Barmer in Rajasthan. By 15th September, monsoon leaves Punjab, Haryana, Rajasthan, and Gujarat. The area under the monsoon influence shrinks slowly and the monsoon retreats from all parts of India except the southern peninsular region. Monsoon winds in most parts of the country are replaced by the north-easterly trade winds. These winds blowing over the Bay of Bengal picks up moisture from there and causes rainfall in Tamil Nadu.
Features of Monsoon Rainfall
- Monsoon rain is seasonal and occurs between June and September. The spatial distribution of rainfall is largely governed by relief or topography. For example, The windward side of the Western Ghats registers a rainfall of over 250 cm.
- Again, the heavy rainfall in the northeastern states can be attributed to their hill ranges and the Eastern Himalayas. Rainfall ranges from 20 cm in western Rajasthan to more than 400 cm in certain parts of Western Ghats and North-East India.
- The monsoon rainfall has a declining trend with increasing distance from the sea. Rainfall decreases from east to west in the plains as one branch of the monsoon enters from the eastern side. Kolkata receives 119 cm, Allahabad 76 cm and Delhi 56 cm only.
- Breaks (discussed above) 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.
- The rains sometimes end considerably earlier than usual, causing great damage to standing crops and making the sowing of winter crops difficult.
Monsoons and the Economic Life in India
- Monsoon is that axis around which revolves the entire agricultural cycle of India. It is because about 64 percent people of India depend on agriculture for their livelihood and Agriculture itself is based on the southwest monsoon.
- Except for the Himalayas all the parts of the country have temperatures above the threshold level to grow the crops or plants throughout the year.
- Regional variations in monsoon climate help in growing various types of crops.
- The agricultural prosperity of India depends very much on timely and adequately distributed rainfall. If it fails, agriculture is adversely affected mainly in areas where irrigation is not developed.
- Sudden monsoon bursts create a problem of soil erosion over large areas in India.
Seasons
- Seasons are a special feature of the Indian climate. Temperature, pressure, wind direction, and the amount and duration of rain vary from one season to the other. Meteorologists identify four seasons in India.
The Cold Weather Season (Winter)
- The cold weather season commences in northern India from mid-November and persists until February. December and January mark the coldest months in the northern region. During this period, temperatures generally decrease from south to north. Days are relatively warm, while nights turn cold. Frost is a common occurrence in the northern plains, and the higher reaches of the Himalayas witness snowfall.
- In contrast, the peninsular region of India, owing to its proximity to the equator and the moderating influence of the sea, lacks a well-defined cold weather season. Coastal areas here experience minimal seasonal temperature variation.
- North-east trade winds dominate the country during the winter season, resulting in dry conditions for most parts. However, these winds bring rainfall to the Tamil Nadu coast as they blow from the sea to the land.
- Around December 22nd, the sun is directly overhead the Tropic of Capricorn in the Southern Hemisphere. In the northern part of India, a weak high-pressure system forms, ushering in clear skies, low temperatures, low humidity, and gentle variable winds due to the relief provided by the Ganga valley. The cold weather season also brings cyclonic disturbances from the west and northwest, originating in the eastern Mediterranean Sea and traveling across West Asia, Iran, Afghanistan, and Pakistan before reaching northwestern India. These disturbances, while bringing limited rainfall, significantly benefit the cultivation of Rabi crops.
The Hot Weather Season (Summer)
- The northward movement of the sun towards the Tropic of Cancer in March leads to rising temperatures in northern India. April, May, and June constitute the summer months in this region, with temperatures reaching 38°C in March, 42°C in April (in Gujarat and Madhya Pradesh), and 45°C in May (in northwestern India).
- In contrast, the peninsular region maintains relatively lower temperatures due to the moderating effect of the oceans, with temperatures ranging between 20°C and 32°C. The Western Ghats’ hilly areas remain below 25°C due to their higher altitude.
- The Inter Tropical Convergence Zone (ITCZ) shifts northward with increasing temperatures, creating an elongated low-pressure monsoon trough extending from the Thar Desert in the northwest to Patna and the Chotanagpur plateau in the east and southeast. Southwesterly winds along the west coast and the coasts of West Bengal and Bangladesh, as well as easterly or south-easterly winds over north Bengal and Bihar, characterize this period.
- Notably, the hot weather season features strong, gusty, hot, dry winds known as “loo,” primarily affecting north and northwestern India. Dust storms and localized thunderstorms are common during May, bringing brief relief from the scorching heat and benefiting crops like tea, jute, and rice.
- Towards the end of the summer, pre-monsoon showers, referred to as “Mango showers,” occur in Kerala and coastal areas of Karnataka, aiding in the ripening of mangoes.
The South-West Monsoon Season (Rainy Season)
- The south-west monsoon season unfolds as low-pressure conditions intensify over the northwestern plains due to rising temperatures. By early June, the south-east trade winds from the Southern Hemisphere, crossing the equator and adopting a southwesterly direction, usher in moisture-laden air from the Indian Ocean. This onset of moisture-laden winds, accompanied by thunder and lightning, is termed the “burst” of monsoons.
- The monsoon approaches India in two main branches: the Arabian Sea branch and the Bay of Bengal branch.
- Arabian Sea branch: This branch splits into three parts, affecting different regions of India. One branch ascends the Western Ghats, leading to heavy rainfall on the windward side and a rain shadow area on the leeward side. Another branch moves through the Narmada and Tapi river valleys, providing rainfall to central India and the Chotanagpur plateau. The third branch affects the Saurashtra Peninsula, west Rajasthan, Punjab, and Haryana, eventually contributing to rainfall in the western Himalayas.
- Bay of Bengal branch: This branch hits the coast of Myanmar and parts of southeast Bangladesh. It splits into two further branches, one heading westward along the Ganga plains and the other moving north and northeast along the Brahmaputra valley, resulting in heavy rainfall in the northeastern region.
- The Tamil Nadu coast remains dry during this season due to its location parallel to the Bay of Bengal branch of the south-west monsoon and its position in the rain shadow area of the Arabian Sea branch.
- The monsoon season is punctuated by breaks, during which there are intermittent rainless periods. These breaks are related to the movement of the monsoon trough and the frequency and intensity of tropical depressions. Monsoons are crucial for India’s agrarian economy, as they bring over three-fourths of the country’s annual rainfall.
The Retreating Monsoon Season
- In October and November, as the sun appears to move southward, the monsoon trough over the northern plains weakens. It gradually gives way to a high-pressure system, causing the south-west monsoon winds to weaken and withdraw.
- By early October, the monsoon withdraws from the northern plains, marking a transition from the hot rainy season to the dry winter season. This period is characterized by clear skies, rising temperatures, and oppressive daytime conditions known as “October heat.”
- In late October, temperatures start to decrease rapidly in north India. The low-pressure trough over northwestern India shifts to the Bay of Bengal, leading to the formation of cyclonic depressions originating in the Andaman Sea. These cyclones often affect the eastern coasts of India and can cause extensive damage, particularly in the deltas of the Godavari, Krishna, and Kaveri rivers.
- These cyclonic storms may also impact Bangladesh, West Bengal, and Odisha, with the Coromandel coast receiving a significant portion of its rainfall from depressions and cyclones. Cyclonic disturbances are less frequent in the Arabian Sea during this season.
| Season | Duration | General characteristics | Temperature | Wind, disturbances | rainfall |
| Winter season | Mid-November to February | Clear skies, fine weather, low humidity | Mean daily temperature below 21oC in North India.Some partexperiencetemperaturebelow freezing point. Temperature increases from north to south. | High pressure over north-western India. Winds blow from northwest to southeast. Around four or five westerly disturbances are carried by the westerly jetstream. | Westerly disturbances cause rainfall in the northern plains. Rainfall decreases from to east in the plains but increases in the north-east again it catch water from the Bay of Bengal. North-east monsoon causes winter rainfall in southern Andhra Pradesh, Tamil Nadu etc. |
| Summer season | April, May, June | Excessive heat, hot loo, duststorms and dryness | Excessive heat, hot loo, duststorms and dryness | Low pressure over the north-western part of India and high pressure over the southern part of the Bay of Bengal. ITCZ shifts to Ganges Plain. Wind direction varies from onepart of India to the other. Duststorms are frequently experienced in the afternoon in the northern plains. | Completely dryseason. Duststorms and thunder provide some rainfall. Eastern regions receive more rainfall comparatively. |
| South-west monsoon | June –September | Whole of India under south-west monsoon. India faces severe cyclones and thunderstorms. | June is the hottest month. The temperature remains low during July and August which rises high in September increasing precipitation. | Winds south-westerly over mainland India. | India receives 80% precipitation in this season. There is a decline of rainfall from east to west plains. Details are discussed under ‘monsoon above. |
| Retreating monsoon | October-November | Monsoon winds are retreating gradually and sudden rise of temperature with Octoberheat. | Day temperature is high and nights are cool and pleasant. The average minimum temperature falls below 20oC. | Winds are north-easterly. Clear skies and Gentle breezes are characteristics of this season. | SouthernPeninsularregion (Tamil Nadu, Kerala, and SouthernAndhra Pradesh)receives rain. Cyclonic activities are more frequent in the Peninsular region. |
Traditional Indian Seasons
- In the Indian tradition, a year is divided into six two-monthly seasons. This cycle of seasons, which the common people in north and central India follow, is based on their practical experience and age-old perception of weather phenomena. However, this system does not match with the seasons of south India where there is little variation in the seasons.
| Season | Months according to Indian calendar | Months according to English |
| Vasanta | Chaitra-Vaisakha | March-April |
| Grishma | Jyaistha-Asadha | may-june |
| Varsha | Sravana-Bhadra | july-august |
| Sharada | Asvina-Kartika | September-October |
| Hemanta | Margashirsa-Pausa | November-December |
| Shishira | Magha-Phalguna | January-February |
Distribution of Annual Rainfall
- A glance on this map indicates that the distribution of rainfall in India is uneven. On the basis of the distribution of rainfall, India can be divided into the following four regions as shown below in table ::
| Category | Rainfall in cms | regions |
| Heavy Rainfall | More than 200 | Western coast, western ghats,sub-Himalayan region of north-east, Garo, Khasi and Jaintia hills of Meghalaya. In some parts, rainexceeds 1000 cm. |
| Moderate rainfall | Between 100 to 200 | 100 cm isohyet extends fromGujarat to south up toKanyakumari is parallel to western ghats. Northern Andhra Pradesh,eastern part of Maharashtra,Madhya Pradesh, Odisha, someparts of Jammu and Kashmir |
| Low rainfall | Between 60 to 100 | Most parts of Tamil Nadu,Karnataka, Andhra Pradesh,eastern Rajasthan, south-westernUttar Pradesh |
| Inadequate rainfall | Less than 60 | Punjab, Haryana, north-westernRajasthan, Kachchh, Kathiawar |
Variability of Annual Rainfall
- Variability of rainfall refers to variations in rainfall from the average amount. The variability of rainfall is computed with the help of the following formula:
C.V. = (Standard Deviation / Mean) x 100; where C.V. is the coefficient of variation.
- The rainfall in India is highly variable. The actual rainfall of a place in a year deviates from its average rainfall by 10 to over 60 percent.
- The mean annual rainfall variability of rainfall in India has been plotted in Figure 14. The description of annual rainfall’s variability is detailed as:
- It may be noted from figure 13 and figure 14 that the highest variability is found in the areas where the average annual rainfall is the lowest such as desert areas of Rajasthan. Here, variability of rainfall is around 60 per cent.
- Contrary to this, in the areas where the average annual rainfall is over 200 cm (Meghalaya plateau, Western Ghats), the annual variability of rainfall is less than 10 per cent.
- A very large part of India falls in the category of 15 to 30 per cent annual variability of rainfall. Tamil Nadu, Karnataka, Andhra Pradesh, Maharashtra etc. fall in this category
- Variability of annual rainfall increases from the western coast to the interior of the Peninsular region from West Bengal and Odisha towards north and north-west.
FAQs
Q. What are the major factors that influence the distribution of rainfall across the world?
Ans. The distribution of rainfall across the world is primarily influenced by global wind patterns, proximity to water bodies, topographical features, and the positioning of major climate zones such as the equatorial belt, tropics, and temperate zones.
Q. How is the rainfall distributed across different regions of India?
Ans. The distribution of rainfall in India varies from region to region, with the southern and northeastern parts experiencing heavier rainfall due to the influence of the southwest and northeast monsoons, while the northwestern and western regions receive relatively lesser rainfall.
Q. How is the general distribution of rainfall categorized across different climatic regions in India?
Ans. The general distribution of rainfall in India is classified into various climatic regions, including areas with heavy monsoonal rainfall, regions with moderate rainfall, and arid or semi-arid zones that receive relatively low precipitation.
Q. What are the key factors contributing to the uneven distribution of rainfall in India?
Ans. The uneven distribution of rainfall in India can be attributed to factors such as the influence of the monsoons, geographical features like mountain ranges and plateaus, the impact of regional wind patterns, and the influence of climate change on precipitation levels.
Q. How does the monsoon contribute to the distribution of rainfall in India?
Ans. The Indian monsoon, characterized by the southwest and northeast monsoons, plays a pivotal role in the distribution of rainfall across the country, bringing heavy precipitation to specific regions during the monsoon season.
Q.What is the significance of seasonal rainfall for agricultural activities and ecosystems in India?
Ans. Seasonal rainfall is crucial for agricultural activities, as it provides the necessary moisture for crop cultivation, influences the planting and harvesting seasons, and contributes to the overall health and productivity of ecosystems in India.
In case you still have your doubts, contact us on 9811333901.
For UPSC Prelims Resources, Click here
For Daily Updates and Study Material:
Join our Telegram Channel – Edukemy for IAS
- 1. Learn through Videos – here
- 2. Be Exam Ready by Practicing Daily MCQs – here
- 3. Daily Newsletter – Get all your Current Affairs Covered – here
- 4. Mains Answer Writing Practice – here
