THE INDIAN MONSOON
1.
The monsoons are experienced in
the tropical area roughly between 20° N and 20° S.
2.
To understand the mechanism of
the monsoons, the following facts are important.
·
The differential heating and
cooling of land and water creates low pressure on the landmass of India while
the seas around experience comparatively high pressure.
·
(b) The shift of the position
of Inter Tropical Convergence Zone (ITCZ) in summer, over the Ganga plain (this
is the equatorial trough normally positioned about 5°N of the equator – also
known as the monsoon trough during the monsoon season).
·
(c) The presence of the
high-pressure area, east of Madagascar, approximately at 20°S over the Indian
Ocean. The intensity and position of this high-pressure area affects the Indian
Monsoon.
·
(d) The Tibetan plateau gets
intensely heated during summer, which results in strong vertical air currents
and the formation of high pressure over the plateau at about 9 km above sea
level.
·
(e) The movement of the
westerly jet stream to the north of the Himalayas and the presence of the
tropical easterly jet stream over the Indian peninsula during summer.
Inter
Tropical Convergence Zone
1.
The Inter Tropical Convergence
Zone (ITCZ,) is a broad trough of low pressure in equatorial latitudes.
2.
This is where the northeast and
the southeast trade winds converge.
3.
This convergence zone lies more
or less parallel to the equator but moves north or south with the apparent
movement of the sun.
4.
Changes in the pressure conditions
over the southern oceans also affect the monsoons.
Southern
Oscillation or SO
1.
Normally when the tropical
eastern South Pacific Ocean experiences high pressure, the tropical eastern
Indian Ocean experiences low pressure.
2.
But in certain years, there is a
reversal in the pressure conditions and the eastern Pacific has lower pressure
in comparison to the eastern Indian Ocean.
3.
If the pressure differences
were negative, it would mean below average and late monsoons.
El Nino
1.
A feature connected with the SO
is the El Nino, a warm ocean current that flows past the Peruvian Coast
2.
In place of the cold Peruvian
current, every 2 to 5 years.
ENSO (El
Nino Southern Oscillations)
·
The changes in pressure
conditions are connected to the El Nino. Hence, the phenomenon is referred to
as ENSO
El
Nino
1.
This is a name given to the
periodic development of a warm ocean current along the coast of Peru
2.
It occurs when easterly trade winds
in the tropical pacific relax- even reverse- to allow a vast pool of warm water
piled up in of central and South America, leading to higher- than- normal sea-
surface temperatures across the equatorial Pacific.
3.
As a temporary replacement of
the cold Peruvian current
4.
El Nino’ is a Spanish word
meaning ‘the child’,
5.
Refers to the baby Christ, as
this current starts flowing during Christmas
6.
As the ocean releases its heat and
moisture to the atmosphere, intense thunderstorms once cooped up over the
western pacific spread along the equator as well.
7.
The presence of the El Nino
leads to an increase in sea-surface temperatures and weakening of the trade
winds in the region
8.
The cumulative effect of this
activity changes large scale circulation patterns at higher latitudes, altering
storm tracks that change the typical distribution of rain and snowfall, as well
seasonal temperatures.
THE
ONSET OF THE MONSOON AND WITHDRAWAL
1.
The Monsoon, unlike the trades,
are not steady winds but are pulsating in nature, affected by different
atmospheric conditions encountered by it
2.
The duration of the monsoon is
between 100-120 days from early June to mid-September.
‘Burst
‘of the monsoon,
·
Around the time of its arrival,
the normal rainfall increases suddenly and continues constantly for several
days. This is known as the ‘burst ‘of the monsoon
3.
The monsoon arrives at the
southern tip of the Indian peninsula generally by the first week of June.
4.
Subsequently, it divides into
two – the Arabian Sea branch and the Bay of Bengal branch.
5.
The Arabian Sea branch reaches
Mumbai about ten days later on approximately the 10 Th of June.
6.
The Bay of Bengal branch also
advances rapidly and arrives in Assam in the first week of June.
7.
The lofty mountains causes the
monsoon winds to deflect towards the west
8.
Over the Ganga plains
9.
By mid-June the Arabian Sea
branch of the monsoon arrives over Saurashtra-Kuchchh and the central part of
the country.
10.
The Arabian Sea and the Bay of
Bengal branches of the monsoon merge over the north-western part of the Ganga
plains.
11.
Delhi generally receives the
monsoon showers from the Bay of Bengal branch by the end of June (tentative
date is 29 Th of June).
12.
By the first week of July,
western Uttar Pradesh, Punjab, Haryana and eastern Rajasthan experience the
monsoon.
13.
By mid-July, the monsoon
reaches Himachal Pradesh and the rest of the country
14.
The withdrawal of the monsoon
begins in north-western states of India by early September.
15.
By mid-October, it withdraws
completely from the northern half of the peninsula.
16.
The withdrawal from the
southern half of the peninsula is fairly rapid
17.
By early December, the monsoon
has withdrawn from the rest of the country.
18.
The islands receive the very
first monsoon showers, progressively from south to north,
19.
From the first week of April to
the first week of May.
20.
The withdrawal, takes place
progressively from north to south from the first week of December to the first
week of January.
THE SEASONS
1.
The monsoon type of climate is
characterised by a distinct seasonal pattern.
2.
The weather conditions greatly
change from one season to the other.
3.
Any seasons are experienced in
your place? Four main seasons can be identified in India –
·
The cold weather season,
·
The hot weather season
·
The advancing monsoon
·
The retreating monsoon
The Cold Weather Season (winter)
1.
The cold weather season begins
from mid-November in northern India and stays till February.
2.
December and January are the
coldest months in the northern part of India.
3.
The northeast trade winds
prevail over the country.
4.
They blow from land to sea and
hence, for most part of the country, it is a dry season.
5.
Some amount of rainfall occurs
on the Tamil Nadu coast from these winds as, here they blow from sea to land.
6.
In the northern part of the
country, a feeble high-pressure region develops, with light winds moving
outwards from this area.
7.
Influenced by the relief, these
winds blow through the Ganga valley from the west and the northwest.
8.
The weather is normally marked
by clear sky, low temperatures and low humidity and feeble, variable winds.
9.
A characteristic feature of the
cold weather season over the northern plains is the inflow of cyclonic disturbances
from the west and the northwest.
10.
These low-pressure systems,
originate over the Mediterranean Sea and western Asia and move into India, Along
with the westerly flow.
‘Mahawat
They cause the much-needed winter rains over the plains and snowfall
in the mountains. Although the total amount of winter rainfall locally known as
‘mahawat ’is small, they are of immense importance for the cultivation of ‘rabi
’crops
The Hot
Weather Season (summer)
1.
Due to the apparent northward
movement of the sun, the global heat belt shifts northward.
2.
In March, the highest
temperature is about 38° Celsius, recorded on the Deccan plateau.
3.
In April, temperatures in
Gujarat and Madhya Pradesh are around 42° Celsius.
4.
In May, temperature of 45°
Celsius is common in the north-western parts of the country
5.
In peninsular India,
temperatures remain lower due to the moderating influence of the oceans.
‘Loo’
These are strong, gusty, hot, dry winds blowing during the day over
the north and north-western India.
1.
Sometimes they even continue
until late in the evening
2.
Direct exposure to these winds
may even prove to be fatal
‘Kaal Baisakhi’
In West Bengal, these storms
are known as the ‘Kaal Baisakhi’ calamity for the month of Baisakh
Advancing
Monsoon (The Rainy Season)
1.
By early June, the low-pressure
condition over the northern plains intensifies.
2.
It attracts, the trade winds of
the southern hemisphere.
3.
These south-east trade winds
originate over the warm subtropical areas of the southern oceans. They cross
the equator
4.
Blow in a south-westerly
direction entering the Indian peninsula as the south-west monsoon.
5.
As these winds blow over warm
oceans, they bring abundant moisture to the subcontinent.
6.
These winds are strong and blow
at an average velocity of 30 km per hour.
7.
Early in the season, the
windward side of the Western Ghats receives very heavy rainfall, more than 250
cm
8.
The Deccan Plateau and parts of
Madhya Pradesh also receive some amount of rain in spite of lying in the rain
shadow area.
9.
The maximum rainfall of this
season is received in the north-eastern part of the country.
10.
Mawsynram in the southern
ranges of the Khasi Hills receives the highest average rainfall in the world.
11.
Rainfall in the Ganga valley
decreases from the east to the west.
12.
Rajasthan and parts of Gujarat
get scanty rainfall.
13.
Phenomenon associated with the
monsoon is its tendency to have ‘breaks’ in rainfall.
14.
It has wet and dry spells.
15.
The monsoon rains take place
only for a few days at a time.
16.
These breaks in monsoon are
related to the movement of the monsoon trough.
17.
The trough and its axis keep on
moving northward or southward
18.
When the axis of the monsoon
trough lies over the plains, rainfall is good in these parts
19.
Whenever the axis shifts closer
to the Himalayas, there are longer dry spells in the plains,
20.
Widespread rain occur in the
mountainous catchment areas of the Himalayan Rivers.
21.
These heavy rain bring in their
wake, devastating floods causing damage to life and property in the plains.
22.
The frequency and intensity of
tropical depressions too, determine the amount and duration of monsoon rains.
23.
These depressions form at the
head of the Bay of Bengal and cross over to the mainland.
24.
The depressions follow the axis
of the “monsoon trough of low pressure”.
25.
The monsoon is known for its
uncertainties.
26.
The alternation of dry and wet
spells vary in intensity, frequency and duration.
Retreating
Monsoon (The Transition Season)
1.
During October-November, with
the apparent movement of the sun towards the south,
2.
The monsoon trough or the
low-pressure trough over the northern plains becomes weaker.
3.
Gradually replaced by a
high-pressure system.
4.
The south-west monsoon winds
weaken and start withdrawing gradually
5.
By the beginning of October,
the monsoon withdraws from the Northern Plains.
6.
The months of October-November
form a period of transition from hot rainy season to dry winter conditions.
7.
Mawsynram, the wettest place on
the earth is also reputed for its stalagmite and stalactite caves.
Cyclonic
depressions
1.
The low-pressure conditions,
over north-western India
2.
Get transferred to the Bay of
Bengal by early November.
3.
This shift is associated with
the occurrence of cyclonic depressions,
4.
Which originate over the
Andaman Sea.
5.
These cyclones generally cross
the eastern coasts of India cause heavy and widespread rain.
6.
These tropical cyclones are
often very destructive.
7.
Deltas of the Godavari, the
Krishna and the Kaveri are frequently struck by cyclones, which cause great
damage to life and property.
DISTRIBUTION
OFRAINFALL
1.
The western coast and
north-eastern India receive over about 400 cm of rainfall annually.
2.
It is less than 60 cm in
western Rajasthan and adjoining parts of Gujarat, Haryana and Punjab.
3.
Rainfall is equally low in the
interior of the Deccan plateau, and east of the Sahyadris
4.
A third area of low
precipitation is around Leh in Jammu and Kashmir
5.
The rest of the country
receives moderate rainfall.
6.
Snowfall is restricted to the
Himalayan region.
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