Today’s class started with an important topic of climatology – “Monsoon”. Monsoons are caused by the annual variation in the temperatures.
·
Sir Edmund Halley gave a theory on the Monsoon as “the extension of land and
sea breeze on a larger scale”.
· Flohn stated that monsoon is the result of
seasonal migration of planetary winds and pressure belts.
· Jet stream theory – causes the onset and withdrawal of
monsoons.
SUB TROPICAL WESTERLY JET STREAMS(STWJS):
i.
These STWJS flow from
the Mediterranean region and splits into two branches after hitting the Pamir
knot - China and the Indian Northern plains.
ii.
In India, it causes western
disturbances which bring “winter rainfall” in states like Punjab,
Haryana and even snow fall in Himachal Pradesh.
iii.
The intensity of high pressure (Mascarene high) and low
temperature in Madagascar region affects the Indian Monsoon.
iv.
The south west monsoon winds flow from the
high to low pressure, and causes the sudden onset known as “burst of monsoon”.
v.
By this time, the westerly jet stream
withdraws from the Indian region which can be either early or delayed, known as
early monsoon and delayed monsoon respectively.
TROPICAL EASTERLY JET STREAMS/TIBETIAN EASTERLY JET
STREAMS:
i.
The Tibetan plateau (4000m altitude)
gets intensely heated during summer, which results in the formation of low
pressure and high temperature (Peshawar region) over the plateau.
ii.
The upper atmosphere
of Tibetan plateau has high pressure. The winds here join with the STWJS and
due to “Anti-cyclonic conditions” get deflected towards Northeast India –
Coriolis effect.
iii.
These are called
Tropical easterly Jet streams = east to west circulation.
iv.
This causes heavy
rainfall to India. (Orographic rainfall)
INDIAN SEASONS:
Four main seasons can be identified in India -
1) HOT WEATHER SEASON(SUMMER) - March to May
2) COLD WEATHER SEASON(WINTER) – January to February
3) SOUTHWEST MONSOON – June to September
4) NORTHEAST MONSOON – October to December
HOT WEATHER
SEASON/SUMMER: (March to May)
·
Continentality in Northern India – heat waves and extreme
high temperature.
·
Equable climate in Southern India – three sides
surrounded by water body, hence moderate climate.
Pre-monsoon showers – Local rainfall in particular regions.
a)
Mango showers – Kerala(predominantly)
(These rainfalls help in the ripening of mango)
b)
Coffee
showers and Cherry blossom – Karnataka.
c)
Norwesters – West Bengal, Jharkhand and Odisha (Kalbaisaki)
(These are basically thunderstorms)
d)
Loo – Punjab, Haryana and Delhi.
(Dry, desert winds at extremely high temperature, dust storms)
COLD WEATHER
SEASON/WINTER: (January to February)
·
Western Disturbances cause heavy rainfall in
North-Western India, which helps in the production of winter wheat.
·
These western disturbances are caused due to the
sub-tropical westerly jet streams.
·
This is a particular type of cyclone known as Temperate
Cyclone/Extra tropical cyclone.
·
The only cold desert national park in India – Ladakh
region.
SOUTH-WEST
MONSOON: (June to September)
South-west
monsoon, after formation divides into two branches:
i)
ARABIAN BRANCH
ii)
BAY OF BENGAL BRANCH
When the south west monsoon travels along the Arabian
branch –
a)
It first brings heavy rains and floods Kerala.
b)
In Maharashtra, Mumbai gets enormous rainfall
(windward side). Pune receives no rainfall – present on leeward side of Western
Ghats.
c)
After that, the monsoon reaches Aravalli hills and
finally causes rain in the Himalayas.
When the south west monsoon travels along the Bay of
Bengal branch –
a)
Since south-west monsoon moves parallel to eastern ghats
– Tamil Nadu, Andhra Pradesh and Odisha do not receive rainfall.
b)
Mawsynram - Cherrapunji in Meghalaya receives the
highest amount of rainfall due to the funnelling effect (winds
confluence at a point in the Meghalaya plateau and hits the Khasi and Garo
hills in a funnel shape)
c)
From here, the south-west monsoon travels further and
meets the Arabian sea branch in upper Gangetic plains.
d)
Sometimes it meets at “Kosi river”- and causes heavy
floods which is why it is called as “Sorrow of Bihar”
e)
Sorrow of West Bengal – Damodar River.
NORTH-EAST
MONSOON/RETREATING MONSOON: (October to December)
Ø Sun shifts
towards Tropic of Capricorn-temperature gets reversed in the Indian Peninsula.
Ø Hence winds
begin to retreat in North-east direction
Ø North east
monsoon takes moisture from the Bay of Bengal and now moves perpendicular to
the Eastern Ghats.
Ø This causes
rainfall to Tamil Nadu, Odisha and Andhra Pradesh.
Ø
Owing to the conditions of
high temperatures and humidity, the weather becomes unbearable- commonly known
as the ‘October
heat’
Indian
monsoon is the only complete monsoon mechanism.
Credits : Leo Praneetha
