Monday, 16 September 2019

Indian Climate: South West Monsoon Season

Table of Contents

South West Monsoon Season – South West Monsoons – Arabian Sea branch & Bay of Bengal branch, Break in South West Monsoons, Advance & Withdrawal.

Isoline, Isobar, Isotherm & Isohyet

ISOLINE: imaginary lines joining regions with equal rainfall or any other parameter.
ISOBAR: imaginary lines joining regions with equal pressure.
ISOTHERM: imaginary lines joining regions with equal temperature.
ISOHYET: imaginary lines joining regions with equal rainfall.

Rainy Season – South West Monsoon Season

South West Monsoon Season – June to mid-September.
South West Monsoon Season is also known as hot-wet season.
Sudden onset is the important feature of South West Monsoons.
With the onset of monsoons, temperature falls drastically and humidity levels rise.

Temperature during South West Monsoon Season

Sudden onset of South West Monsoons leads to significant fall in temperature [3° to 6°C].
The temperature remains less uniform throughout the rainy season.
The temperature rises in September with the cease of south-west monsoons.
There is rise in temperature whenever there is break in the monsoons.
The diurnal range of temperature is small due to clouds and rains.
The highest temperatures are experienced at places west of the Aravali [38° to 40°C]. This is due to lack of clouds and hot continental air masses.
Other parts of Northwest India also have temperatures above 30°C.
The temperatures are quite low over the Western Ghats due to heavy rainfall.
The coastal areas of Tamil Nadu and adjoining parts of Andhra Pradesh have temperatures above 30°C as they receive little rainfall during this season.

Pressure and Winds During South West Monsoon Season

Low pressure conditions prevail over northwest India due to high temperature.
ITCZ (monsoon trough) lies along the Ganga plain. There are frequent changes in its location depending upon the weather conditions.
The atmospheric pressure increases steadily southwards.
Over the peninsular region, due to pressure gradient between north and south, winds blow in a southwest to northeast direction from Arabian sea and Bay of Bengal.
Their direction undergoes a change in Indo-Gangetic plain where they move from east to west.

Rainfall During South West Monsoon Season

Three fourths of the total annual rainfall is received during this season.
The average rainfall over the plains of India in this season is about 87 per cent.
Normal date of the arrival of the monsoon is 20th May in Andaman and Nicobar Islands.
The advance of the monsoon is much faster in the Bay of Bengal than in the Arabian Sea.
The normal date of onset of the southwest monsoon over Kerala i.e. the first place of entry in the mainland of India is 1st June.
The monsoons advance quickly accompanied with a lot of thunder, lightning and heavy downpour. This sudden onset of rain is termed as monsoon burst.
Sometimes monsoons are delayed or they come much earlier than normal.
Normally the onset occurs between 29th May and 7th June.
The earliest onset was on 11th May in 1918 and 1955, while the most delayed onset was on 18th June in 1972.

indian monsoon - rainy season - june september

South West Monsoons – Arabian Sea branch and Bay of Bengal branch

Monsoon winds beyond south Kerala progress in the form of two branches viz. the Arabian Sea branch and the Bay of Bengal branch.

South West Monsoon – Arabian Sea branch and Bay of Bengal branch

The Arabian Sea branch gradually advances northwards. It reaches Mumbai by 10th June.
The Bay of Bengal branch spreads rather rapidly over most of Assam. The normal date of its arrival at Kolkata is 7th June.
On reaching the foothills of the Himalayas the Bay branch is deflected westward by the Himalayan barrier and it advances up the Gangetic plain.
The two branches merge with each other mostly around Delhi to form a single current.
Both the branches reach Delhi more or less at the same time.
The combined current gradually extends to west Uttar Pradesh, Haryana, Punjab, Rajasthan and finally to Himachal Pradesh and Kashmir.
By the end of June the monsoon is usually established over most parts of the country.
By mid-July, the monsoon extends into Kashmir and the remaining parts of the country.
By this time it reaches Kashmir, it has shed most of its moisture.
Arabian Sea branch of the monsoon is much powerful than the Bay of Bengal branch for reasons:

The Arabian Sea is larger than the Bay of Bengal, and
the entire Arabian Sea current advances towards India, whereas only a part of the Bay of Bengal current enters India, the remainder proceeding to Myanmar, Thailand and Malaysia.

The Arabian Sea branch of the southwest monsoons is divided into three distinct streams on arriving in the mainland of India.

The first stream strikes the west coast of India and gives extremely heavy rainfall of over 250 cm. It strike perpendicular to Western Ghats causing plentiful Orographic Rainfall [400 to 500 cm annual rainfall on the windward side].
Rainfall is drastically reduced to about 30-50 cm on the leeward side of the crest.
There is a narrow belt of marked aridity on the immediate leeward side of the Western Ghats. But once it is passed, the air starts rising again and the amount of rainfall increases further east.
The second stream enters Narmada—Tapi troughs
(narrow rift valley) and reaches central India. It does not cause much rain near the coast due to the absence of major orographic obstacle across the rift. Some parts of central India receive rainfall from this stream (Ex: Nagpur).
The third stream moves parallel to the Aravali Range without causing much rainfall. Consequently the whole of Rajasthan is a desert area.
However, some orographic effect is occurs on the south-eastern edge of the Aravali Range. Mt. Abu gets about 170 cm rainfall while the surrounding plains have only 60 to 80 cm rainfall.

The Bay of Bengal Branch of the southwest monsoon is divided into two distinct streams.

The first stream crosses the Ganga-Brahmaputra delta and reaches Meghalaya. Here that the orographic effect results in intense rainfall. Cherrapunji receives an annual rainfall of 1,102 cm, major portion of which occurs from June to August.
Mawsynram (present champion) located at 1,329 m above sea level just 16 km to the west of Cherrapunji (X champion) records higher annual rainfall of 1,221 cm.
Both the stations are located on the southern slopes of the Khasi hills at the northern end of a deep valley running from south to north.
The second stream of the Bay of Bengal branch moves along Himalayan foothills as they are deflected to the west by the Himalaya and brings widespread rainfall to Ganga plain.
The rainfall by this stream is characterized by a steady decline as we move from east to west up the plain. [Previous Prelims Question]
The Tamil Nadu coast remains relatively dry during the south-west monsoon period because of

rain shadow effect of the Arabian Sea current and
Bay of Bengal current which flows parallel to the coast.

Break in the South West Monsoons

During the Monsoon season, there are periods when the Monsoon trough shifts to the foothills of Himalayas, which leads to sharp decrease in rainfall over most parts of the country but increase along the Himalayas and parts of Northeast India and Southern Peninsula.
During July and August, there are certain periods when the monsoons become weak. Rainfall practically ceases over the country outside the Himalayan belt and southeast peninsula. This is known as break in the monsoon.
Breaks are likely to occur during the second week of August and last for a week.
The breaks are believed to be brought about by the northward shifting of the monsoon trough (minimum low pressure cell in ITCZ). The axis of the trough lies at the foothills of the Himalayas during the break period.

The monsoon trough is a portion of the Intertropical Convergence Zone as depicted by a line on a weather map showing the locations of minimum sea level pressure, and as such, is a convergence zone between the wind patterns of the southern and northern hemispheres.

During the break period, heavy rainfall occurs over the sub-Himalayan regions and the southern slopes of the Himalayas.
On an average one or two breaks do occur during the rainy season. 85 out of 100 years there is a break in the monsoons.

Depressions in South West Monsoon Season

A major part of the South West Monsoon rainfall is generated by depressions [intense low pressure] originating in the Arabian Sea and Bay of Bengal. Some depressions develop over land also.
About 3-4 depressions are formed per month from June to September.
Almost all of them are sucked inward through the deltas of great rivers [They need moisture to be alive], the Ganga, the Mahanadi, the Godavari, the Krishna and the Cauvery and cause heavy rain in these areas.
The location of depressions strongly coincide with the latitudinal position of ITCZ.
Most of the depression originate to the west of 90⁰ E in Bay of Bengal and move in north-west direction.
In the Arabian Sea in June-July, the depressions move either in north-west or in northerly direction and may affect west Gujarat or Maharashtra.
Storms during August and September are rare and are formed close to Maharashtra-Gujarat coast.
Most of the rainfall in central and northern parts of the country is caused by these depressions.
The absence of depressions or a change in their tracks result in deficit or no rain.

Advance and Withdrawal of South West Monsoons

Chief Characteristics of South West Monsoon Rainfall

Major part of monsoon rains are received between June and September.
Monsoonal rainfall is largely governed by relief and is orographic in its mode.
The amount of rainfall decreases with increasing distance from the sea.
The rainless interval during south west monsoon season is known as ‘breaks’. The breaks in rainfall are related to tropical cyclones which originate in the Bay of Bengal.
There are large scale spatial variations in the distribution of rainfall.
Monsoons often fail to keep date. Sometimes the monsoons withdraw before the scheduled time causing considerable damage to the crops
.

Indian Climate: Summer & Winter Seasons

Winter Season – Summer Season – Indian Climate: Pressure, Winds, Rainfall, Temperature, Cyclones, Western Disturbances. Loo, Andhis, Norwesters.

Indian Climate – Seasons

The cold weather season or winter season,
The hot weather season or summer season,
The south-west monsoon season or Rainy season, and
The season of the retreating monsoon or cool season.

Winter Season in India

November – March. January is the coldest month.
Sun’s apparent path is to the south of equator.
Clear sky, pleasant weather, low temperature, low humidity, high range of temperature, cool and slow north-east trade winds.
The diurnal range of temperature, especially in interior parts of the country, is very high.

Temperature in Winter Season

The isotherm of 20°C runs roughly parallel to the Tropic of Cancer.
To the south of this isotherm the temperatures are above 20°C. Here there is no distinctly defined winter weather. Some parts of Kerala and Tamil Nadu typically experiences temperatures near 30°C.
To the north mean temperatures are below 21°C and the winter weather is distinct.
The mean minimum temperature is about 5°C over north-west India and 10°C over the Gangetic plains.
Dras Valley in Kashmir is the coldest place in India. The minimum temperature recorded at Dras was – 45°C in 1908.

Pressure in Winter Season

High air pressure prevails over large parts of north-west India due to low temperatures coupled with divergence induced by the ridge of the STJ.
Pressure is comparatively lower in south India.
The winds start blowing from high pressure area of north-west to low pressure area of south-east. The wind velocity is low due to low pressure gradient.
The path of the winds depend on pressure gradient and physiography.

prevailing winds cyclones January - winter

Western Disturbances in Winter Season

The spell of fine weather over north-western and northern India is often broken due to inflow of western disturbances.
They intensify over Rajasthan, Punjab, and Haryana.
They move eastwards across the sub-Himalayan belt up to Arunachal Pradesh.
They cause light rain in the Indus-Ganga plains and snowfall in the Himalayan belt.
After the passage of the disturbance, widespread fog and cold waves lowering the minimum temperature by 5° to 10°C below normal are experienced.
Fog lowers visibility and causes great inconvenience for transportation.

Tropical Cyclones in Winter Season

This is the season of least tropical cyclone activity.
The frequency of tropical cyclones decreases with the advancement of the season.
This is due to low sea surface temperature and exit of ITCZ farthest south.
The storms which are born in the Bay of Bengal strike Tamil Nadu and bring heavy rainfall.
Some of them cross the southern peninsula over to the Arabian Sea.
Some storms originate in the Arabian Sea and move towards either north or west.

Precipitation in Winter Season

The retreating winter monsoons pick up some moisture while crossing the Bay of Bengal and cause winter rainfall in Tamil Nadu, south Andhra Pradesh, south-east Karnataka and south-east Kerala (Usually in the first weeks of November).
The highest seasonal rainfall of about 75 cm between October and December.
Most of it occurs along the south-eastern coast of Tamil Nadu and adjoining parts of Andhra Pradesh. Thereafter, it gradually decreases.
The western disturbances also cause a little rainfall in north-west India.
The amount of rainfall gradually decreases from the north and north-west to east (it is opposite in rainy season).
The northeastern part of India also gets rainfall during the winter months.

rainfall india post monsoon - october - december

Summer Season in India

March to June.
High temperature and low humidity are the chief characteristics.
Sometimes referred to as pre-monsoon period.

Temperature in Summer Season

High sun’s insolation due to apparent movement of sun between the equator and the Tropic of Cancer.
The southern parts of the country are distinctly warmer in March and April whereas in June, north India has higher temperatures.
In March, the highest temperatures occur in the southern parts (40-45°C).
In April the highest temperature of about 45°C is recorded in the northern parts of Madhya Pradesh.
In May the highest temperature shifts to Rajasthan where temperatures as high as 48°C may be recorded.
In June the maximum temperature is in Punjab and Haryana.
The highest temperatures recorded are 50.5°C at Alwar on 10th May, 1956 and 50.6°C at Ganganagar on 14th June, 1935.
The highest temperatures are recorded just before the onset of the southwest monsoons (late May).
The diurnal range of temperature is also very high. It may be as high as 18°C in some parts.
The maximum summer temperatures are comparatively lower in the costal and southern peninsular regions due to moderating effect of the sea.
The temperatures along the west coast are comparatively lower than those prevailing on the east coast due to the prevailing westerly winds.
There is large contrast between land and sea temperatures.
Northern and central parts of India experience heat waves in this season.

[A heat wave is an abnormally high temperature experienced by a regions. Temperature increase of the order of 6° to 7°C above normal is termed as ‘moderate’ and 8°C and more as ‘severe’ heat wave]

Most of the heat waves develop over Rajasthan, Punjab and Haryana (location far away from the sea). From here they spread over Uttar Pradesh and Bihar.
The strong north westerly winds (caused due to strong divergence in north-west India) with a long land journey over hot regions check the onward march of the sea breeze over eastern coastal belt and create heat wave conditions over Odisha and Andhra Pradesh.
The heat waves strike by the end of April and their maximum occurrence is in May. They last till the onset of southwest monsoon.
The normal duration of heat waves is 4 to 5 days. However, heat waves are rare over the peninsula south of 13°N latitude due to maritime conditions prevailing there.

Pressure in Summer Season

The atmospheric pressure is low all over the country due to high temperature.
But strong dynamically induced divergence over north-west India prevents the onset of south-west monsoons.

Winds in Summer Season

There is a marked change in the direction and speed of the winds from winter.
The winds are by and large light and variable.

Loo

Loo winds originate over Iranian, Baloch and Thar deserts.
In May and June, high temperature in northwest India builds steep pressure gradient.
Hot, dust laden and strong wind known as loo blows.
Loo normally starts blowing by 9.00 A.M., increases gradually and reaches maximum intensity in the afternoon.
It blows with an average speed of 30-40 km per hour and persists for days.

Andhis

The strong dust storms resulting from the convective phenomena are locally known as andhis (blinding storms). They move like a solid wall of dust and sand.
The wind velocity often reaches 50-60 kmph and the visibility is reduced to a few metres.
Such dust storms are common in Rajasthan, Haryana, Punjab, Jammu region, Delhi, Uttar Pradesh, Bihar and Madhya Pradesh.
They are short lived. The squall and showers which follow these storms bring down the temperature sharply temporarily.

Frontal Thunderstorms in Summer Season

The strong convectional movements related to the westerly jet stream lead to thunderstorms in eastern and north-eastern part of the country.
They normally originate over Chota Nagpur plateau and are carried eastwards by westerly winds.
The areas with highest incidence of thunderstorms are Assam, Arunachal Pradesh, Nagaland, Mizoram, Manipur, Tripura, Meghalaya, West Bengal and the adjoining areas of Odisha and Jharkhand.

Norwesters and Thunderstorms in Summer Season

In West Bengal and the adjoining areas of Jharkhand, Odisha and Assam, the direction of squalls is mainly from the northwest, and they are called norwesters.
They are often very violent with squall speeds of 60 to 80 km per hour.
Hailstones sometimes accompany showers and occasionally attain the size of a golf ball.
They cause heavy damage to standing crops, trees, buildings, livestock and even lead to loss of human lives.
However, they are, sometimes, useful for tea, jute and rice cultivation. In Assam, these storms are known as ‘Barodoli Chheerha’.
The period of maximum occurrence of these storms is the month of Vaisakh (mid-March to mid-April) and hence, they are locally known as Kalabaisakhis, the black storms or a mass of dark clouds of Vaiasakha.

Convectional Thunderstorms in Summer Season

In the south the thunderstorms occur in Kerala (Mango Showers) and adjoining parts of Karnataka (Blossom Showers) and Tamil Nadu, particularly during evenings and nights.

Western Disturbances in Summer Season

Their frequency and intensity gradually decrease with advancement of summer.
Approximately 4, 3 and 2 western disturbances visit north-west India in March, April and May respectively.
They cause snowfall in higher reaches of the Himalayas.

Tropical Cyclones in Summer Season

Tropical cyclones originate in the Bay of Bengal and Arabian Sea.
A few cyclones are formed in the Bay of Bengal in the month of March but they do not affect the mainland of India.
Their frequency rises steeply in April and the number of cyclones originating in May is more than double than those originating in April.
About three-fourths of the tropical cyclones are born in the Bay of Bengal and the rest originate in the Arabian Sea.
Most of the depressions in April originate to the south of 10°N while those originating in May are born to the north of this latitude.
Most of the storms of this season initially move west or north-west but later they recurve northeast and strike Bangladesh and the Arakan Coast of Myanmar.
Very few hit Indian coast while some dissipate over the sea itself.
The whole of the east coast of India, the coastal areas of Bangladesh and Arakan Coast of Myanmar are liable to be hit by tropical storms in May.
Many of them are quite severe and cause heavy damage to life and property.
In the Arabian Sea, major storms are formed in May between 7° and 12° N latitudes.
Most of them move away from the Indian coast in a north-westerly direction and dissipate in the sea.
Few originate close to the Indian coast. They move towards the north-east and hit somewhere along the west coast of India.

Precipitation in Summer Season

This season is not totally rainless (only one per cent of the annual rainfall).
In the northeastern parts of the country, dust storms bring little rainfall.
The precipitation in Kashmir is mainly in the form of snow caused by western disturbances.
The norwesters bring some rainfall in Assam, West Bengal and Odisha. The intensity of rainfall is high.
The rainfall brought by the norwesters is known as the spring storm showers.
This small amount of rainfall is very useful for the cultivation of tea, jute and rice and is known as tea showers in Assam.
Coastal areas of Kerala and Karnataka receive rainfall from thunderstorms.
Such showers are called mango showers in Tamil Nadu and Andhra Pradesh because they are very beneficial to mango crop.
In Karnataka they are called cherry blossoms due to their effect on the coffee plantations.

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