Indian Monsoons: Rhythm

According to the meteorologists, an annual cycle of seasons recognise the following four seasons:
  1. the cold weather season
  2. the hot weather season
  3. the southwest monsoon season
  4. the retreating monsoon season

We will dwell into each of these seasons one by one.

The Cold Weather Season

Usually, the cold weather season sets in by mid-November in northern India. The coldest months in the northern plain are December and January.

Temperature

Northern Plains

The mean daily temperature remains below 21°C over most parts of northern India. The night temperature sometimes goes below freezing point in Punjab and Rajasthan. There are three main reasons for the excessive cold in north India:

  1. It is far away from moderating influence of sea resulting a continental climate.
  2. The snowfall in the nearby Himalayan ranges creates cold wave situation.
  3. Cold winds from Caspian Sea and Turkmenistan around February.

Peninsular India

The Peninsular India does not have any well-defined cold weather season.
  • Coastal areas: There is hardly any seasonal change in the distribution pattern of the temperature. For example, the mean maximum temperature for January at Thiruvanantapuram is as high as 31°C, and for June, it is 29.5°C.This is because of two reasons:
    1. moderating influence of the sea
    2. proximity to equator
  • Western Ghats: Temperatures at the hills of Western Ghats remain comparatively low.

Pressure and Winds

Northern Plains

On 22nd December, the sun shines vertically over the Tropic of Capricorn in the southern hemisphere. So, the weather is characterised by feeble high pressure conditions over the northern plain.

Sometimes the pleasant weather conditions of winter get disturbed by shallow cyclonic depressions originating over the east Mediterranean Sea and travelling eastwards across West Asia, Iran, Afghanistan and Pakistan before they reach the northwestern parts of India. On their way, the moisture content gets augmented from the Caspian Sea in the north and the Persian Gulf in the south. The Westerly Jet Streams have a very important role in steering these depressions in India.

South India

In southern India, the air pressure is slightly lower. The position of isobar 1019 mb passes through northwest India and that of 1013 mb through far south. 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 low pressure gradient, the light winds with a low velocity of about 3-5 km per hour begin to blow outwards. By and large, the topography influences wind direction.

  • Ganga Valley: They are westerly or northwesterly down the Ganga Valley.
  • Ganga-Brahmaputra delta: They become northerly in the Ganga-Brahmaputra delta.
  • Bay of Bengal: Free from the influence of topography, they are clearly northeasterly over the Bay of Bengal.

Rainfall

Winter monsoons do not cause rainfall because:

  • they move from land to sea
  • they have little humidity
  • the anti cyclonic circulation on land, as rainfall from them has less possibility

So, most parts of India do not have rainfall in the winter season.

However, there are some exceptions to it:

  • Northwestern India: Weak temperate cyclones from the Mediterranean sea cause rainfall in Punjab, Haryana, Delhi and western Uttar Pradesh. Although meagre, it is highly beneficial for rabi crops. The precipitation is in the form of snowfall in the lower Himalayas. The precipitation goes on decreasing 
    • from west to east in the plains
    • from north to south in the mountains. 
  • Central India: Along with the central part of India, northern Peninsula also get occasional winter rainfall. 
  • North-eastern India: Arunachal Pradesh and Assam in the northeastern parts of India also have sporadic rains during winter months. 
  • Southern India: During October and November, northeast monsoon while crossing over the Bay of Bengal, picks up moisture and causes torrential rainfall over the Tamil Nadu coast, southern Andhra Pradesh, southeast Karnataka and southeast Kerala.

The Hot Weather Season

With the apparent northward movement of the sun towards the Tropic of Cancer in March, temperatures start rising in north India. April, May and June are the months of summer in north India. 

  • In March, the highest day temperature of about 38°C occurs in the Deccan Plateau.
  • In April, temperature ranging between 38°C and 43°C are found in Gujarat and Madhya Pradesh. 
  • In May, the heat belt moves further north.

Temperature

North India

In May, the heat belt moves north and in the north-western part of India, temperatures rise even upto 48°C.

South India

The hot weather season in south India is comparatively mild and not so intense as compared to that of the northern India.

  • Peninsula: The moderating effect of the oceans keeps the temperatures low in the Peninsula. 
  • Western Ghats: Due to altitude, the temperatures in the hills of Western Ghats remain below 25°C. 
  • Coastal regions: The isotherms run parallel to the coast in north-south direction. It means that temperature does not decrease from north to south rather it increases from the coast to the interior.

Pressure and Winds

North India

The hot summer months witness falling air pressure resulting in the northwards shift of ITCZ. This low pressure monsoon trough occupies a position centred at 25°N in July and attracts a surface circulation of winds. These winds are:

  • southwesterly on the west coast, the coast of West Bengal and Bangladesh.
  • easterly or southeasterly over north Bengal and Bihar. 
These currents are in reality ‘displaced’ equatorial westerlies which ultimately bring a rainy season. 

  • In the northwest, dry and hot winds known as ‘Loo’, blow in the afternoon and often continuing till midnight. 
  • Dust storms in the evening are very common during May in Punjab, Haryana, Eastern Rajasthan and Uttar Pradesh. These temporary storms bring a welcome respite from the oppressing heat since they bring with them light rains and a pleasant cool breeze
  • Occasionally, the moisture-laden winds are attracted towards the periphery of the trough. A sudden contact between dry and moist air masses gives rise to local storms of great intensity. These local storms are associated with violent winds, torrential rains and even hailstorms.

The Southwest Monsoon Season

By early June, the low pressure conditions over the subcontinent are powerful enough to attract the trade winds of Southern Hemisphere. These southeast trade winds cross the equator and pick up abundant moisture. Then they follow a southwesterly direction and enter the Bay of Bengal and the Arabian Sea. That is why they are known as southwest monsoons. 

The rain in the southwest monsoon season begins rather abruptly. First rain brings down the temperature substantially. This sudden onset of the moisture-laden winds associated with violent thunder and lightening, is often termed as the “break” or “burst” of the monsoons.

  • In the coastal areas of Kerala, Karnataka, Goa and Maharashtra, the monsoon may burst in the first week of June.
  • In the interior parts of the country, it may be delayed to the first week of July. 
The day temperature declines 5°C to 8°C in this period.

Two Branches of the Southwest Monsoon

As the southwesterly winds approach the land, their direction is modified by the relief and thermal low pressure over the northwest India. The monsoon approaches the landmass in two branches: (i) The Arabian Sea branch (ii) The Bay of Bengal branch

The Arabian Sea Branch

This branch is further split into three sub-branches: (i) First sub-branch is obstructed by the Western Ghats. (ii) Second sub-branch strikes the coast north of Mumbai. (iii) Third sub-branch strikes Saurashtra Peninsula and Kachchh.

  1. Western Ghats branch
    • Windward side: As these winds climb the slopes of the Western Ghats, they become cool and the windward side of the Sahyadris and Western Coastal Plain receive very heavy rainfall ranging between 250 cm and 400 cm. 
    • Rain-shadow area: 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. Hence this region of low rainfall is known as the rain-shadow area. 
  2. North coastal branch: 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. Saurashtra Peninsula and Kachchh branch: After striking Saurashtra Peninsula and Kachchh, it passes over west Rajasthan and
    • Along the Aravalis, it causes a scanty rainfall
    • In Punjab and Haryana, it joins the Bay of Bengal branch. These two branches, reinforced by each other, cause rains in western Himalayas.

The Bay of Bengal Branch

This branch strikes the coast of Myanmar and 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.

  1. First branch moves westward along the Ganga plains reaching as far as the Punjab plains. 
  2. Second 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 heavy rainfall in the north-eastern states can be attributed to their hill ranges and the Eastern Himalayas.
The Tamil Nadu coast remains dry during this season. There are two factors responsible for it:
  1. The Tamil Nadu coast is situated parallel to the Bay of Bengal branch of southwest monsoon.
  2. It lies in the rainshadow area of the Arabian Sea branch of the south-west monsoon.

Characteristics of Monsoonal Rainfall

  • It is seasonal in character, which occurs between June and September. 
  • It is largely governed by relief or topography.
  • It has a declining trend with increasing distance from the sea.
  • It occurs in wet spells of few days duration at a time, interspersed with rainless interval known as ‘breaks’
  • The 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.
  • The summer rainfall comes in a heavy downpour leading to considerable run off and soil erosion.
  • Three-fourths of the total rain in country is received during southwest monsoon season. Hence it plays a pivotal role in India's agrarian economy.
  • Its spatial distribution is uneven which ranges from 12 cm to more than 250 cm.
  • The beginning of the rains sometimes is considerably delayed over the whole or a part of the country.
  • The rains sometimes end considerably earlier than usual, causing great damage to standing crops and making the sowing of winter crops difficult.

Season of Retreating Monsoon

The months of October and November are known for retreating monsoons.

Sequence of Retreat

  • By the end of September, southwest monsoon becomes weak as the low pressure trough of Ganga plain starts moving southward in response to the southward march of the sun. 
  • By the first week of September, monsoon retreats from the western Rajasthan. It withdraws from Rajasthan, Gujarat, Western Ganga plain and the Central Highlands by the end of the month. 
  • By the beginning of October, the low pressure covers northern parts of the Bay of Bengal. In the second half of October, the mercury begins to fall rapidly, particularly in northern India.
  • By early November, the low pressure moves over Karnataka and Tamil Nadu. 
  • By the middle of December, the centre of low pressure is completely removed from the Peninsula. 

October Heat

The retreating southwest monsoon season is marked by clear skies and rise in temperature. The land is still moist. Owing to the conditions of high temperature and humidity, the weather becomes oppressive which is commonly known as the ‘October heat’.

Rainfall in Southern and Eastern Peninsula

The weather in the retreating monsoon is dry in north India but it is associated with rain in the eastern part of the Peninsula. Here, October and November are the rainiest months of the year. The widespread rain in this season is associated with the passage of cyclonic depressions which originate over the Andaman Sea and manage to cross the eastern coast of the southern Peninsula. These tropical cyclones are very destructive. The thickly populated deltas of the Godavari, Krishna and Kaveri are their preferred targets. Every year cyclones bring disaster here. A few cyclonic storms also strike the coast of West Bengal, Bangladesh and Myanmar.

A bulk of the rainfall of the Coromondal coast is derived from these depressions and cyclones. Such cyclonic storms are less frequent in the Arabian Sea.