Climate

Type of climate in India

Through the vast North-South extent of India, stretching from 8°4’ N and 37°6’ N, a large
variety of climate prevails with many unique climatic contrasts. However, the special
factors which unify the climate of India are:
- the fact of monsoons – SW Monsoon and NE Monsoon winds, alternating seasons
and a reversal of the monsoon winds; and
- the greater part of the Indian subcontinent being present within the tropics.
Hence, the climate of India can be described as Tropical Monsoon type.

Factors affecting the climate of India

Latitude.
The Indian subcontinent is divided climatically into two parts by the Tropic of Cancer.
Northern portion Southern portion
It lies in the temperate/sub-tropical It lies in the tropical zone, between
zone, between 8°4’ N and 23°30’ N. 23°30’ N and 37°6’ N.
The warm subtropical climate here gives Lying close to the equator, it remains
it cold winter season and hot summer relatively warmer throughout the year
season. and does not experience a clear-cut
winter season.
It does not have the midday Sun almost It has the midday Sun almost vertically
vertically overhead during any part of the overhead at least twice every year.
year.

Influence of the mountains.

- The Himalayas:
• The great Himalayan range forms a climatic barrier and obstructs the bitterly
cold Siberian winds from entering into the Indian subcontinent. Thus, India
does not experience severe cold during winter, and is prevented from turning
into a cold desert.
• The Himalayas also intercept the rain-bearing SW monsoon winds, forcing
them to shed their moisture, resulting in heavy rainfall across the Indo-
Gangetic plains. Otherwise, India would have been a dry region. Meanwhile,
the rest of Central Asia is in the rain-shadow of the Himalayas.
 - The Western Ghats: These ranges obstruct the SW monsoon winds blowing from
the Arabian sea. As a result, the Deccan Plateau gets very little rain as it is in the
rain-shadow of the Western Ghats.
- The Aravalli ranges:
• The Aravalli ranges run parallel to the direction of the SW monsoon winds and
are very low.
• As such, when these winds pass through Rajasthan, the Aravalli ranges do not
provide any obstruction to them, and so they do not shed their moisture. This
accounts for Rajasthan’s dry climate.
- The hills of Assam:
• The hills of Assam force the moisture-laden Bay of Bengal branch of the SW
monsoon winds to shed their moisture in the windward side of the hills before
proceeding further northwards.
• As a result, the south of Assam receives heavy rainfall, while the north of Assam
is comparatively dry.

Altitude. In general, there is a decrease in the temperature by 1°C for every 166m
ascent (normal lapse rate). Thus, the mountains are cooler than the plains.

Distance from the Sea.

- Areas located in the interior of India, far away from the seas, experience extreme
or continental climate.
- Areas located close to the seas or coastal areas experience equable, maritime or
moderate climate. The differential rate of heating and cooling of land and sea
causes land and sea breezes, which are responsible for moderating the climate
here.

Western Disturbances (Temperate Cyclones).

- Due to the shifting of the pressure belts during winter, an inflow of low-pressure
systems occurs, which originate over the eastern Mediterranean Sea, cross Iran,
Afghanistan, and Pakistan and enter India. These are called western disturbances
or temperate cyclones.
- These low-pressure depressions cause winter rainfall in NW India – Punjab,
Haryana, Uttarakhand and western U.P. As they proceed eastwards, they cause
cold wave and snowfall around the western Himalayan belt.
- They are generally active between December and February, and while the amount
of rainfall is small, it is of considerable importance for the rabi crops.

Tropical Cyclones. Due to the local variations of temperature and humidity at the
head of the Bay of Bengal, low-pressure systems are originated. These are called Tropical
cyclones.
- They cross the eastern coast of the southern peninsula and cause heavy,
widespread, cyclonic and convectional rainfall on the coasts of Tamil Nadu and
 parts of Odisha. The bulk of the rainfall of the Coromandel Coast is derived from
depressions and cyclones.
- Their crossing over into the mainland may result in the Bay of Bengal branch of
the SW Monsoon winds getting drawn into the cyclonic depression resulting in dry
spells in the SE parts of the Indo-Gangetic plains.
- The frequency and intensity of these depressions are responsible for the overall
strength or weakness of the Monsoon.
- They cause tremendous loss to life and property.

Jet Streams. Jet streams are fast-flowing, narrow, meandering air currents produced
by two air masses of contrasting temperature at a height of 10-12 km above the mid-
latitude, near the altitude of the tropopause.
- The two jet streams usually used to forecast weather are the subtropical jet streams
or westerly jet located near 30° latitudes and the polar jet streams or easterly jet
located near 60° latitudes.
- The westerly jet is responsible for the formation of Western Disturbances, while
the easterly jet is responsible for the Tropical Cyclones.

Effect of the El-Niño. El-Niño is a warm ocean current which flows off the coast of
Peru in South America, during the month of December.
- It occurs once in every 3-4 years, and its effect lasts for about 18-24 months.
- It increases the surface temperature of the sea abnormally and affects the
movement of monsoon winds in the Indian Ocean, resulting in a weak or delayed
monsoon.
- This, in turn, causes drought-like conditions, poor productivity and failure of crops
in Peninsular India. The effect of these conditions is far-reaching, culminating in
famine, poverty, inflation, debts and even farmer suicides.
Seasons in India
The Hot and Dry Summer Season (March-May).
Temperature • Due to the apparent migration of the Sun towards the northern
conditions hemisphere, temperature starts rising in the northern hemisphere,
with the Sun shining directly overhead the Tropic of Cancer during
Summer Solstice.
• As such, most parts of India experience temperatures ranging from
30°C to 32°C. A steady increase in temperature is seen from south
to north.
• In the NW, temperatures around 48°C are not uncommon. NW
India comprising mainly of Rajasthan has special features like
sandy soil, direct insolation and lack of cloud cover, which result in
such conditions.
• There is some respite from the heat in the south, especially in the
coastal regions, because of the moderating influence of the seas,
while the north, being farther away, experiences intense heat and
large diurnal variation in temperatures.
• The plateau and hilly regions are also relatively cooler, because of
their elevation.
Pressure • The high temperature conditions in the subcontinent gives rise to
conditions the low-pressure trough extending from the Thar Desert and
Chotanagpur Plateau.
• The surrounding seas, being cooler, develop high-pressure
conditions. The Deccan Plateau, owing to its altitude, is cooler and
so also experiences slightly high-pressure conditions.
• By the end of May, the high-pressure conditions intensify near
Cape Comorin, extending towards the Arabian sea.
• By June, the low-pressure trough becomes more intense and moves
northward with its centre over the Indus lowlands and Baluchistan.
The influx of these winds by mid-June brings about a change in the
weather towards the rainy season.
Local winds The low-pressure conditions that develop over the Northern plains
gives rise to local storms, sometimes accompanied by light spell of rain.
• Kalbaisakhi (West Bengal), Bardoli Chheerha (Assam) or
Norwesters (Punjab and U.P.) –
o They are thunderstorms accompanied with strong, hot and
wet winds and heavy rainfall which originate over the
Chotanagpur Plateau in April/May.
o The light rainfall caused by them is quite beneficial for tea in
Assam and jute and rice in West Bengal.
• Loo –
o It is a strong, hot, dry and dusty wind blowing during the
daytime in the months of May/June.
 o It is very common in Punjab, Haryana, U.P. (aandhi), Bihar,
Delhi, etc.
o Also called sand storms, the fierce heat accompanied with
temperatures upto 45°C and 50°C is hot enough to cause
fatal heat-strokes.
• Mango showers (as they are helpful for early ripening of mango) or
Cherry Blossoms (as they are helpful for coffee plantation in
Karnataka) –
o They occur in Kerala and Karnataka during April/May.
o They bring a brief spell of rainfall which is important for
mango, tea and coffee plants.

The South-West Monsoon Season (June-September).

Mechanism of • By the end of May, the differential heating of the land and sea
the Monsoon brings about a seasonal low pressure on the large landmass in the
north and high pressure on the surrounding water bodies.
• This low-pressure trough in the NW part of the Indian
subcontinent attracts the SE Trade winds which prevail in the
southern hemisphere.
• The SE Trade winds blow towards the north-west of the earth.
However, due to the Coriolis effect and by Ferrel’s law, the winds
after crossing the equator get deflected towards the right, and
blow as SW Monsoon winds in India.
Burst and • The onset of monsoon is marked by violet thunder and lightning
Break and heavy rainfall in the first week of June. This is termed as the
Burst of the Monsoon. The first state to receive the monsoon
showers and also the last to see its withdrawal is Kerala.
• During the months of July and August, there is a tendency of
failure of the SW Monsoon and spells of dry weather in the rainy
season. This is called the Break of Monsoon. This pulsating
nature of the Monsoon is related to the tropical cyclones
originating at the head of Bay of Bengal.

Branches Due to the tapering topography of the peninsular India, the SW

Monsoon winds forks into two branches.
• The Arabian Sea Branch of the SW Monsoon –
o This branch further divides into three branches –
▪ One branch goes up the slopes of Western Ghats,
condenses and brings very heavy rainfall on its windward
side of the Sahyadris and the Western Coastal Plain. Then,
these winds descend, get heated up and thus become less
 humid, causing little rainfall in the rain-shadow of the
Western Ghats (the Deccan plateau region).
▪ The second branch strikes the coast north of Mumbai,
moves further along the Narmada and Tapi river valleys
and causes fair amount of rainfall in Central India. Then,
it enters the northern plains and mingles with the Bay of
Bengal branch.
▪ The third branch strikes the Saurashtra Peninsula and the
Kutch. After that, it passes over west Rajasthan and along
the Aravallis, causing scanty (<25cm) rainfall. Then, it
sheds its moisture after the Bay of Bengal branch and
cause rainfall at the foothills of the Himalayas.
o The total volume of the Arabian sea branch is three times
more powerful than the Bay of Bengal branch. This is because
–
▪ It is much larger than the Bay of Bengal branch.
▪ The whole of it is used up by the entire country, whereas
only a small part of the Bay of Bengal branch enters India
through the SE part; its major part blows over Myanmar
and Thailand.
o The Arabian Sea branch takes 4 to 6 weeks to move from
Kanyakumari to the Shiwaliks.
• The Bay of Bengal Branch of the SW Monsoon –
o The main target of this branch is a low-pressure area in north
Myanmar and a part of SE Bangladesh.
▪ However, owing to the Arakan Hills along the coast of
Myanmar and the Garo-Khasi-Jaintia hills in the NE, a
large part of this branch is deflected towards the Indian
subcontinent.
▪ They, therefore, enter West Bengal and Bangladesh from
south and SE instead of the SW.
o One part of this branch is intercepted by the Garo-Khasi-
Jaintia hills.
▪ A funnel shaped valley at the edge of the Meghalaya
plateau traps the rain-bearing winds.
▪ As such, Mawsynram, a station 16km west of Cherrapunji
in Meghalaya and on the windward side of the Khasi hills,
receives the highest average annual rainfall in the world.
▪ Shillong, on the other hand, lying in the rain-shadow, gets
little rainfall.
o The remaining portion of this branch deflects into the Ganga
plain towards west and NW, along the Himalayas, attracted
by the low-pressure area in Punjab and Rajasthan.
 ▪ The rainfall progressively decreases as the branch reaches
the eastern Himalayas, blows westwards into the Ganga
valley and proceeds through it.
▪ Bikaner, lying in the rain-shadow of the Aravali ranges,
receives no rainfall.
o The current finally merges with the Arabian sea branch and
sheds the remaining moisture in the foothills of the
Himalayas.
Characteristics • The monsoonal rainfall in India is largely orographic (relief) in
nature, as per the effects of windward and leeward sides of the
mountains.
• The monsoons are erratic, unpredictable, irregular, unevenly
distributed and torrential. As such, sometimes there are floods;
and sometimes droughts. Floods and droughts even occur at the
same time at different places in India.
• The bulk (~70%) of the rainfall occurs during the SW Monsoon
season, and only over three months, except the Tamil Nadu coast.
The rest of the year is mostly dry.
• India is an agricultural land and the monsoons are of great
importance to the majorly agrarian economy of the country.
Delays and long breaks in the monsoons are very dangerous for
agricultural crops.

The Retreating SW Monsoon Season (October-November).

Mechanism • By 1st September, with the apparent southward migration of the
sun, the temperature in the Northern plain begins to decrease as
the sun’s rays no longer fall directly at the Tropic of Cancer.
• Consequently, the low-pressure trough weakens, is gradually
replaced by high-pressure, and is no longer able to attract the
monsoon winds towards the land.
• During Autumn Equinox, when the Sun shines directly over the
equator, the high-pressure begins to build over the mainland,
and subsequently, with low-pressure over the sea, the SW
Monsoon starts withdrawing in stages from India.
Stages of • By the beginning of September, it withdraws from Rajasthan.
Withdrawal • By the end of September, it withdraws from Punjab and U.P.
• By mid-October, it withdraws from Central India.
• By the end of November, it withdraws from South India.
Characteristics • The withdrawal of the Monsoon is a much slower process than its
onset. It is just the reversal of the process – as the SW Monsoon
had travelled towards NE, now it retreats towards the SW.
 • The retreating monsoon season is marked by cyclonic
depressions originating at the head of the Bay of Bengal, caused
due to the shifting of the pressure belts. [See Factors affecting
the climate of India > Tropical Cyclones.]
• The retreating monsoons are generally dry, except on the
Coromandel coastal areas. The rainfall caused by them is as heavy
as the summer monsoons in some places. Meanwhile, the interior
parts of the Deccan plateau region remain dry because they lie in
the rain-shadow of the Eastern Ghats.
• This season is a transition period between the rainy season and
cold dry season with clear, cloudless sky.
• The month of October, i.e., the withdrawal of the SW Monsoon,
is marked by clear cloudless sky, high temperature and high
humidity. This sultry and oppressive weather is referred to as the
October Heat.

The Cold Weather Season / Season of the North-East Monsoon

(December-February).
Temperature • In the North –
conditions o The northern part of the country has a mean temperature
below 20°C and especially much lower in H.P., J & K, Punjab
and Haryana.
o January is the coldest month when the temperature falls as
low as 2.5°C.
o The night temperature falls below freezing point in many
places. It would have been much colder if the great Himalayan
ranges did not shield the subcontinent from the cold Siberian
winds.
o The northern plains are located far away from the moderating
influence of the sea, and as such experience a continental
climate. Thus, the temperature recorded here is very low.
o The excessive cold is also attributed to the cold wave brought
by two sources –
▪ In February, the cold winds from the Caspian Sea and the
Turkmenistan bring cold wave over the NW parts.
▪ Many places in the northernmost part of the Himalayas
experience heavy snowfall, which results in cold waves
enveloping the surrounding areas of Punjab, Haryana, etc.
• In the South –
o Peninsular India remains relatively warmer and does not
have a distinctly defined cold weather season.
o The isotherm of 20°C runs in E-W direction, roughly parallel
to the Tropic of Cancer. The result is that in the south of this
isotherm, i.e., in the Peninsular region, the temperatures
always remain above 21°C and in extreme south above 25°C.
 o The coastal areas, especially, hardly experience any seasonal
change in the distribution pattern and range of temperature
due to the moderating influence of the sea and proximity to
the equator. E.g.: January temperature at Trivandrum is 31°C.
Pressure • Because of the apparent migration of the Sun towards the south,
conditions shining directly over the Tropic of Capricorn, the subcontinent
receives only oblique rays of the Sun.
• As a result, a feeble high pressure builds up on the land with low
pressure centred on the water bodies surrounding the peninsula.
Winds • As the pressure belts are swapped – high pressure on land and
low pressure on the seas – off-shore winds, as part of the NE
Trade winds, start blowing towards the low-pressure centred in
the Bay of Bengal.
• The Deccan plateau obstructs these winds to blow directly
towards the Arabian Sea. Hence, they are forced to blow from NE
to SW down the Ganga plain towards the Bay of Bengal.
Rainfall • For the most parts of the country, especially the Ganga valley, the
off-shore winds bring no rainfall and hence it is dry season.
• However, as they proceed after crossing the Bay of Bengal, they
gather enough moisture.
• As they strike the Coromandel coast, Tamil Nadu and the
southern tip of A.P. receive fairly heavy rainfall, accompanied by
stormy winds. Since these winds blow from NE to SW, they are
called the NE Monsoons.
• In the NW, the sky is generally clear. However, the spell of fine
cloudless weather is often broken by light rainfall which occurs
due to the inflow of western depressions. [See Factors affecting
the climate of India > Western Disturbances (Temperate
Cyclones)].
Characteristics • The cold weather season is characterized by clear sky, fine and
pleasant weather, low temperature and low humidity, high range
of temperature and slow, off-shore northern winds.
• Unlike the SW Monsoon, the onset of the NW Monsoon is not so
clearly defined.