CLIMATE
Climate is the average weather in a given area over a longer period of time. A description of a climate includes information on, e.g. the average temperature in different seasons, rainfall, and sunshine. Also a description of the (chance of) extremes is often included
Climate of India is affected by the factors
of latitude, altitude, distance from the seas,
monsoon wind, relief features and jet stream.
Latitude
Latitudinally, India lies between 8°4'N
and 37°6'N latitudes. The Tropic of cancer
divides the country into two equal halves. The area located to the south of Tropic of cancer
experiences high temperature and no severe
cold season throughout the year whereas, the
areas to the north of this parallel enjoys sub-tropical climate.
Altitude
When the altitude increases, The
temperature decreases. Temperature decreases
at the rate of 6.50C for every 1000 metres of
ascent. It is called normal lapse rate.
Hence, places in the mountains are cooler
than the places on the plains. Ooty and several
other hill stations of south India and of the
Himalayan ranges like Mussourie, Shimla etc.,
are much cooler than the places located on the
Great Plains.
Distance from the Sea
A large area of India, especially the
peninsular region, is not very far from the sea and
this entire area has a clear maritime influence on climate. This part of the country does not have a
very clearly marked winter and the temperature
is equable almost throughout the year. Areas
of central and north India experience much
seasonal variation in temperature due to the
absence of influence of seas. Here, summers
are hot and winters are cold. The annual
temperature at Kochin does not exceed 30°C
as its location is on the coast while it is as
high as 40°C at Delhi, since it is located in
the interior part. Air near the coast has more
moisture and greater potential to produce
precipitation. Due to this fact, the amount
of rainfall at Kolkata located near the coast is
119 cm and it decreases to just 24 cm at Bikaner
which is located in the interior part.
Monsoon Wind
The most dominant factor which affects the
climate of India is the monsoon winds. These
are seasonal reversal winds and India remains in
the influence of these winds for a considerable
part of a year. Though, the sun’s rays are vertical
over the central part of India during the midJune, the summer season ends in India by the
end of May. It is because the onset of southwest
monsoon brings down the temperature of the
entire India and causes moderate to heavy
rainfall in many parts of the country. Similarly,
the climate of southeast India is also influenced
by northeast monsoon.
Relief
Relief of India has a great bearing on
major elements of climate such as temperature,
atmospheric pressure, direction of winds and
the amount of rainfall. The Himalayas acts as
a barrier to the freezing cold wind blows from
central Asia and keep the Indian subcontinent warm. As such the north India experiences
tropical climate even during winter. During
southwest monsoon, areas on the western slope
of the Western Ghats receive heavy rainfall.
On the contrary, vast areas of Maharashtra,
Karnataka, Telangana, Andhra Pradesh and
Tamil nadu lie in rain shadow or leeward side
of the Western Ghats receive very little rainfall.
During this season, Mangalore, located on
the coast gets the rainfall of about 280 cm
whereas the Bengaluru located on the leeward
side receives only about 50 cm rainfall.
Jet Streams
Jet streams are the fast moving winds
blowing in a narrow zone in the upper
atmosphere. According to the Jet stream
theory, the onset of southwest monsoon is
driven by the shift of the sub tropical westerly
jet from the plains of India towards the
Tibetan plateau. The easterly jet streams cause
tropical depressions both during southwest
monsoon and retreating monsoon.
MONSOON
The word ‘monsoon’
has been derived from the
Arabic word ‘Mausim’ which
means ‘season’. Originally,
the word ‘monsoon’ was used
by Arab navigators several
centuries ago, to describe a
system of seasonal reversal of winds along the
shores of the Indian Ocean, especially over the
Arabian Sea. It blows from the south-west to
north-east during summer and from the northeast to south-west during winter.
Meteorologists have developed a number
of concepts about the origin of monsoons.
According to the Dynamic concept, Monsoon
wind originates due to the seasonal migration
of planetary winds and pressure belts following
the position of the sun. During summer solstice,
the sun’s rays fall vertically over the Tropic of
cancer. Therefore, all the pressure and wind
belts of the globe shift northwards. At this time, Inter -Tropical Convergence Zone (ITCZ)
also moves northward, and a major part of
Indian landmass comes under the influence of
southeast trade winds. While crossing equator
this wind gets deflected and takes the direction
of southwest and becomes south-west monsoon.
During the winter season, the pressure and
wind belts shift southward, thereby establishing
the north-east monsoon (trade winds) over this
region. Such systematic change in the direction
of planetary winds is known as monsoon.
Seasons
The meteorologists recognize the four distinct seasons in India. They are;
1. Winter season (January - February).
2. Summer season (March - May).
3. Southwest monsoon or Rainy season (June - September).
4. Northeast monsoon season (October - December).
1. Winter season
During this period, the vertical rays of the
sun falls over tropic of capricorn which is far away
from India. Hence, India receives the slanting
sun’s rays which results in low temperature.
The cold weather season is characterized by
clear skies, fine weather, light northerly winds,
low humidity and large day time variations of
temperature. During this season a high pressure
develops over north India and a north-westerly
wind blows down the Indus and Ganges valleys.
In south India, the general direction of wind is
from east to west.
The rain during this season generally occurs
over the Western Himalayas, Tamil nadu and
Kerala. Western disturbances and associated
trough in westerlies are main rain bearing
system in northern part of the country. The jet
stream plays a dominant role in bringing these
disturbances to India. Western disturbances
cause rainfall in Punjab, Haryana and Himachal
Pradesh, and snowfall in the hills of Jammu and Kashmir. This rainfall is very useful for the
cultivation of winter wheat.
2. Summer season
During this season, the vertical rays of the
sun falls over the peninsular India. Hence, there
is a steady increase in temperature from south
to north. It is practically hot and dry in the
entire country in the initial part of this season.
Weather over the land areas of the country is
influenced by thunderstorms associated with
rain and sometimes with hail mostly in the
middle and later part.
During this season, temperature starts
increasing all over the country and by April,
the interior parts of south India record mean
daily temperatures of 30°C–35°C. Central
Indian land mass becomes hot with day-time
maximum temperature reaching about 40°C
at many locations.
Because of the atmospheric pressure
conditions, the winds blow from southwest to
northeast direction in Arabian Sea and Bay of
Bengal. They bring pre monsoon showers to the
west coast during the month of May. There are
few thunder showers called “Mango Showers”
which helps in quick ripening of mangoes along
the coast of Kerala and Karnataka.
“Norwesters” or “Kalbaisakhis” are the
local storms with thunder that blow from north
western part and rain lasting for short durations.
It occurs over the eastern and north eastern
parts over Bihar, West Bengal and Assam during
April and May. They approach the stations from
the northwesterly direction.
3. Southwest monsoon or Rainy
Season
The southwest monsoon is the most
significant feature of the Indian climate. The
onset of the southwest monsoon takes place
normally over the southern tip of the country
by the first week of June, advances along the
Konkan coast in early June and covers the
whole country by 15th July. The monsoon is
influenced by global phenomenon like ElNino.
Prior to the onset of the southwest
monsoon, the temperature in north India
reaches upto 46°C. The sudden approach of
monsoon wind over south India with lightning
and thunder is termed as the ‘break’ or ‘burst
of monsoon’. The monsoon wind strikes against
the southern tip of Indian land mass and gets
divided into two branches. One branch starts
from Arabian sea and the other from Bay of
Bengal.
The Arabian sea branch of southwest
monsoon gives heavy rainfall to the west coast of
India as it is located in the windward side of the
Western Ghats. The other part which advances
towards north is obstructed by Himalayan
Mountains and results in heavy rainfall in north.
As Aravalli Mountain is located parallel to the
wind direction, Rajasthan and western part do
not get much rainfall from this branch.
The Bay of Bengal branch moves towards
northeast India and Myanmar. This wind is
trapped by a chain of mountains namely Garo,
Khasi and Jaintia are mainly responsible for the
heaviest rainfall caused at Mawsynram located
in Meghalaya. Later on, this wind travel towards west which results in decrease in rainfall from
east to west. Over all about 75% of Indian
rainfall is received from this monsoon.
4. Northeast monsoon season
The southwest monsoon begins to retreat
from north India by the end of September due
to the southward shifting pressure belts. The
southwest monsoon wind returns from Indian
landmass and blows towards Bay of Bengal. The
coriolis force deflects this wind and makes it
to blow from northeast. Hence, it is known as
Northeast monsoon or Post-monsoon season.
The season is associated with the establishment
of the north-easterly wind system over the
Indian subcontinent. Andhra Pradesh, Tamil
nadu, Kerala and south interior Karnataka
receive good amount of rainfall accounting for
35% of their annual total. Many parts of Tamil
nadu and some parts of Andhra Pradesh and
Karnataka receive rainfall during this season
due to the storms forming in the Bay of Bengal.
Large scale losses to life and property occur due
to heavy rainfall, strong winds and storm surge
in the coastal regions. The day time temperatures
start falling sharply all over the country.
Distribution of rainfall
The average annual rainfall of India is
118 cm. However, spatial distribution of rainfall
in the country is highly uneven.
The Western coast, Assam, South Meghalaya,
Tripura, Nagaland and Arunachal Pradesh are
the heavy rainfall areas which get more than
200 cm rainfall. The whole of Rajasthan, Punjab,
Haryana, Western and Southwestern parts of
Uttar Pradesh, Western Madhya Pradesh, the
entire Deccan Trap or Plateau region east of
Western Ghats except for a narrow strip along
Tamil nadu coast receive a low rainfall of less
than 100 cm. The rest of the areas receive a
rainfall ranging between 100 and 200 cm.
NOTE:
- Equable climate is also called
as the British climate, Which is
neither too hot nor too cold.
- Weather refers to the state of
atmosphere of a place at a given
point of time.
Climate is the accumulation
of daily and seasonal weather events
of a given location over a period of
30-35 years.
- Mawsynram, the place which
receives highest rainfall (1141
cm) in the world. It is located in
Meghalaya.
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