Why is there an asymmetry in the heating of land and water bodies?
- Water has a relatively high heat capacity ie the amount of heat required to raise the temperature of a substance by one degree, is higher for water than land. So a given amount of heat can cause a greater temperature over the land than oceans.
- Convective activities, ocean currents and winds-induced water movement cause displacement of water (both vertical and horizontal) and the heat present in it. But in land there only conduction and heat is confined to a few centimetres from the surface.
- Some amount of heat is used by the water for evaporation (including change of state), so effective heat for raising temperature is less. No such mechanism on the land.
So the temperature variability is higher over land than oceans. Thus land is heated faster and it also cools faster compared to oceans.
Due to the apparent northward movement of the Sun, temperature starts rising from March. By the end of the May a low pressure trough is developed in the Indian sub-continent due to intense heating and consequent strong convective activities.
Due to this, dry and hot winds blow, locally known as Loo. Dust storms are very common during May in the north-western part of the country. Sometimes they are accompanied by light rains and cool breeze which give temporary relief from the oppressive heat. These are called pre-monsoon showers.
In the eastern and North-Eastern parts, violent thunderstorms accompanied by heavy showers and hails are experienced.They cause damage to standing crops, trees, buildings, livestock and human lives in West Bengal, Assam and Odisha. The Norwesters occur in the month of Baisakh and thus are locally known as Kalbaisakhi in Bengal and Bardoichila in Assam.
Mango Showers or
Early ripening of mangoes
Deficiency of these rains
affects the mango
harvest in South India
Useful for the
cultivation of tea, jute
Monsoon is derived from the Arabic word Mausim meaning season. Summer monsoon in India is from the South-west direction while winter monsoon is from the North-east.
When the Sun (apparent position) is in the northern hemisphere, there is a northward shift in the Equatorial Low pressure belt and other pressure belts too. In the summer season, the sun shines vertically over the Tropic of Cancer and thus the circum-polar whirl, sub-tropical westerly Jetstream (STWJ) and the ITCZ too shift northwards. ITCZ is in the Indo-Gangetic Plain is often called the Monsoon Trough in this position. Equatorial Low pressure belt is found in the interiors of Indian sub-continent upto 20°- 25° N latitude. They are super-imposed to form an intense low pressure in the region.
The STWJ is placed at around 200-300mb level (pressure) ie. 9 to 12 km in the upper troposphere. In winter STWJ flows along the southern slopes of the Himalayas but in summer it shifts northwards and flows along the northern edge of the Tibetan Plateau.
Tibet is at an altitude of around 5,000 m above sea level. It gets heated in summer and is warmer than the air over the adjoining regions. As a source of heat for the atmosphere, it generates rising air. During its ascent the air spreads outwards and gradually sinks over the equatorial part of the Indian Ocean. The ascending air is deflected to the right by the earth’s rotation and moves in a clockwise direction leading to anticyclonic conditions in the upper troposphere over Tibet around 9 to 12 km.
Upper tropospheric low pressure prevails over North-west of India. Intense heating of land causes low pressure at the surface level too. The combined effect of all these leads to an intense low pressure over the sub-continent.
Temperature contrast between the warm air (due to heating of the land and adjacent air column) and the overlying cool air at the upper troposphere leads to the formation of Tropical Easterly Jetstream (TEJ) at around 12km. It flows from north-east to south-west, and feeds the Mascarene high pressure belt with winds. TEJ thus determines the severity of the monsoon.
The intense low pressure over India is sufficient to attract trade wind components from the Southern hemisphere. Winds from the Mascarene high are deflected as they cross the equator due to coriolis force, and flow from South-west to North-east direction, in the name of South-west (SW) Monsoon.
The front where the south-west monsoons meet the north-east trade winds is known as the Monsoon Front. These winds pickup moisture from the oceans and cause precipitation over the land, far into the interior too.
Somali/Findlater jet causes the sudden outbreak/ bursting of monsoon.
Usually monsoon hits India first around May 25 (Andaman & Nicobar Islands), and mainland India by Jun 1 (Kerala). It proceeds further in branches, causing rainfall in different parts of India progressively.
Indian Peninsula divides SW monsoon current into 2 branches
1. Arabian Sea branch
It is further split into three branches
i) One branch is obstructed by the Western Ghats
- windward side of the W.Ghats & Western Coastal Plain receive very high rainfall (250-400 cm)
- After crossing the W.Ghats, they descend and get heated up which reduces their humidity
- So they cause little rainfall east of the W.Ghats. This is the rain-shadow area
- But few parts to the east of the ghats receive rainfall due to the presence of gaps, as moisture laden winds enter through the gaps. Eg : Pal Ghat ( Palakkad gap), Thal ghat (Kasara ghat), Bhor ghat
ii) Another branch moves along the Narmada and Tapi River valleys
- rainfall in extensive areas of central India.
- Thereafter, they enter the Ganga plains and merge with the Bay of Bengal branch.
iii) Third moves north through Kachchh, Saurashtra & western Rajasthan
- Low rainfall due to the absence of mountain barrier in Kachchh & parallel position of the Aravalli ranges
- It is also due to the shutting effect of the hot and dry air, obstructing the upward movement of the moisture by the overlying dry air.
- 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
2. Bay of Bengal branch
The Bay of Bengal (BoB) branch moves parallel with the eastern ghats & produces no rainfall until it strikes the coast of Myanmar and parts of South-East Bangladesh. But the Arakan and Pegu Yoma Hills along the coast of Myanmar deflect a big portion towards the Indian subcontinent. From here, this branch splits into 2 under the influence of the Himalayas
i) One branch moves westward along the Ganga plains reaching as far as the Punjab plains
ii)The other branch moves up the Brahmaputra valley in the North and the North-East, causing widespread rains.
- It strikes the Garo and Khasi hills of Meghalaya.
- Cherrapunji/ Mawsynram lie on the windward side of the Khasi hills and receives rainfall from this branch while Shillong / Dispur / Guwahati lie on the leeward side, so they receive less rainfall being in the Rain Shadow area.
NOTE : The Arabian Sea Branch is more powerful than the Bay of Bengal Branch due to two factors :
1. The Arabian Sea is a larger sea than the BoB
2. The entire Arabian Sea Branch goes over India whereas only a part of the BoB Branch enters India, the rest going towards Myanmar and Thailand.
[A] The Tamil Nadu coast remains dry during this season. Reasons :
i. It is situated parallel to the BoB branch of SW monsoon.
ii. It lies in the rain shadow area of the Arabian Sea branch
[B] Distributional pattern of rainfall : Main trends
i. It declines from East to west & north-west in the Great Indian plains.
ii. It declines from West coast towards the interior parts of the Peninsula.
[C] Direction of winds
General direction in this season is from South-West to North-East in the major parts of the country. But due to the presence of the Himalaya, these are south-easterly and easterly in the North-East and Ganga Plain.