Gist of NCERT
Climate refers to the sum total of weather conditions and
variations over a large area for a long period of time (more than thirty years).
Weather refers to the state of the atmosphere over an area at any point of time.
The elements of weather and climate are the same, i.e. temperature, atmospheric
pressure, wind, humidity and precipitation. Youmay have observed that the
weather conditions fluctuate very often even within a day. But there is some
common pattern over a few weeks or months, i.e. days are cool or hot,windy or
calm, cloudy or bright, and wet or dry. On the basis of the generalized monthly
atmospheric conditions, the year is divided into seasons such as winter, summer
or rainy seasons. During the summer season the desert area of Rajasthan
witnesses 50º temperature whereas Pahalgamsector of Jammu and Kashmir has 20ºC
temperature. During winter nights Dras sector of Jammu and Kashmirwitnesses–
45ºC temperature where as Thiruvananth puram has 20ºC. Rainfall also varies in
terms of quantity and distribution in the regions of Himalaya rainfall is in the
from of snowy balls where as in the rest of part of India it is a general rain.
Again annual rainfall varies from 400ºC in the Meghalya to 10ºc in Ladakh and
West Rajasthan. In the coastal area the variation of rainfall is less. Whereas
in the inner part of country the seasonal variation is more. Accordingly the
Indians show their unity in diversity in terms of food, clothing, housing and
Factors determining the climate of India
India’s climate is controlled by a number of factors which can be broadly
divided into two groups- (a) factors related to location and relief, and (b)
factors related to air pressure and winds.
(a) Factors related to Location and Relief Latitude:
You know that the Tropic of Cancer passes through the central
part of India in east- west direction. This, northern part of the India lies in
sub-tropical and temperate zone and the part lying south of the Tropic of Cancer
falls in the tropical zone. The tropical zone being nearer to the equator,
experiences high temperatures throughout the year with small daily and
annual range. Area north of the Tropic of Cancer being away from the equator,
experiences extreme climate with high daily and annual range of temperature.
The Himalayan Mountains: The towering mountain chain provides an
invincible shield to protect the subcontinent from the cold northern winds. The
Himalayas also trap the monsoon winds, forcing them to shed their moisture
within the subcontinent.
Distribution of Land and Water: India is flanked by
the India Ocean on three sides in the south and girdled by a high and continuous
mountainwall in the north. As compared to the landmass, water heats up or cools
down slowly. This differential heating of land and sea creates different air
pressure zones in different seasons in and around the Indian subcontinent.
Difference in air pressure causes reversal in the direction of monsoon winds.
Distance from the Sea: With a long coastline, large
coastal areas have an equable climate. Areas in the interior of India are far
away from the moderating influence of the sea. Such areas have extremes of
climate. That is why, the people of Mumbai and the Konkan coast have hardly any
idea of extremes of temperature and the seasonal rhythm of weather. On the other
hand, the seasonal contrasts in weather at places in the interior of the country
such as Delhi, Kanpur and Amritsar affect the entire sphere of life.
Altitude : Temperature decreaseswith height. Due to
thin air, places in the mountains are cooler than places on the plains. For
example, Agra and Darjeeling are located on the same latitude, but
temperature of January in Agra is 16ºC whereas it is only 4ºC in Darjeeling.
Relief: The physiography or relief of India also
affects the temperature, air pressure, direction and speed of wind and the
amount and distribution of rainfall. The windward sides of Western Ghats and
Assam receive high rainfall during June- September whereas the southern plateau
remains dry to its leeward situation along theWestern Ghats.
(b) Factors Related to Air Pressure and Wind
To understand the differences in local climates of India, we need to
understand the mechanism of the following three factors:
- Distribution of air pressure and winds on the surface of the earth.
- Upper air circulation caused by factors controlling global weather and
the inflow of different air masses and jet streams.
- Inflow of western cyclones generally known as disturbances during the
winter season and tropical depressions during the south-west monsoon period
into India, creating weather conditions favourable to rainfall.
The mechanism of these three factors can be understood with reference to
winter and summer seasons of the year separately.
Mechanism of Weather in the Winter Season
Surface pressure and winds: In winter months, the
weather conditions over India are generally influenced by the distribution of
pressure in Central and Western Asia. A high pressure centre in the region lying
to the north of the Himalayas during winter. This centre of high pressure gives
rise to the flow of air at the low level from the north towards the Indian
subcontinent, south of the mountain range. The surface winds blowing out of the
high pressure centre over Central Asia reach India in the form of a dry
continental air mass. These continental winds come in contact with trade winds
over northwestern India. The position of this contact zone is not, however,
stable. Occasionally, it may shift its position as far east as the middle Ganga
valley with the result that whole of northwestern and northern India up to the
middle Ganga valley comes under the influence of dry northwestern winds.
Jet Stream and Upper Air Circulation: The pattern of air
circulation discussed above is witnessed only at the lower level of the
atmosphere near the surface of the earth. Higher up in the lower troposphere,
about three km above the surface of the earth, a different pattern of air
circulation is observed. The variations in the atmospheric pressure closer to
the surface of the earth have no role to play in the making of upper air
circulation. All of Western and Central Asia remains under the influence
ofwesterly winds along the altitude of 9- 13 km from west to east. These winds
blow across the Asian continent at latitudes north of the Himalayas roughly
parallel to the Tibetan highlands. These are known as jet streams. Tibetan
highlands act as a barrier in the path of these jet streams. As a result, jet
streams get bifurcated. On of its branches blows to the north of the Tibetan
highlands, while the southern branch blows in an eastward direction, south of
the Himalayas. It has its mean position at 25ºN in February at 200-300 mb level.
It is believed that this southern branch of the jet stream exercise an important
influence on the winter weather in India.
Western Cyclonic Disturbance and Tropical Cyclones:
The western cyclone disturbances which enter the Indian subcontinent from the
west and the northwest during the winter months originate over the Mediterranean
Sea and are brought into India by the westerly jet stream. An increase in the
prevailing night temperature generally indicates an advance in the arrival of
these cyclones disturbances.
Tropical cyclones originate over the Bay of Bengal and the
Indian Ocean. These tropical cyclones have very high wind velocity and heavy
rainfall and hit the Tamil Nadu, Andhra Pradesh and Orissa coast. Most of these
cyclones are very destructive due to high wind velocity and torrential rain that
Inter Tropical Convergence Zone (ITCZ) The Inter Tropical Convergence Zone (ITCZ)
is a low pressure zone located at the equator where trade winds converge,
and so, it is a zone where air tends to ascend. In July, the ITCZ is located
around 20ºN latitudes (over the Gangetic plain), sometimes called
themonsoon trough. This monsoon trough encourages the development of thermal
low over north and northwest India. Due to the shift of ITCZ, the trade
winds of the southern hemisphere cross the equator between 40ºE and 60ºE
longitudes and start blowing from southwest to northeast due to the Coriolis
force. It becomes southwest monsoon. In winter, the ITCZ moves southward,
and so the reversal of winds from northeast to south and southwest, takes
place. They are called northeast monsoons.
Mechanism of Weather in the Summer Season
Surface Pressure and Winds: As the summer sets in and the sun
shifts northwards, the wind circulation over the subcontinent undergoes a
complete reversal at both, the lower as well as the upper levels. By themiddle
of July, the low pressure belt nearer the surface (termed as Inter Tropical
Convergence Zone (ITCZ) shifts northwards, roughly parallel to the Himalayas
between 20ºN and 25ºN. By this time, the westerly jet stream withdraws from the
Indian region. In fact, meteorologists have found an interrelationship between
the northward shift of the equatorial trough (ITCZ) and the withdrawal of the
westerly jet stream from over the North Indian Plain. It is generally believed
that there is a cause and effect relationship between the two. The ITCZ being a
zone of low pressure attracts inflow of winds from different directions. The
maritime tropical air mass (mT) from the southern hemisphere, after crossing the
equator, rushes to the low pressure area in the general southwesterly direction.
It is this moist air current which is popularly known as the southwest monsoon.
Jet Streams and Upper Air Circulation: The pattern of
pressure and winds asmentioned above is formed only at the level of the
troposphere. An easterly jet stream flows over the southern part of the
Peninsula in June, and has a maximum speed of 90 km per hour. In August, it is
confined to 15ºN latitude, and in September up to 22ºN latitudes. The easterlies
normally do not extend to the north of 30ºN latitude in the upper atmosphere.
Easterly Jet Stream and Tropical Cyclones: The
easterly jet streamsteers the tropical depressions into India. These depressions
play a significant role in the distribution of monsoon rainfall over the Indian
subcontinent. The tracks of these depressions are the areas of highest rainfall
in India. The frequency at which these depressions visit India, their direction
and intensity, all go a long way in determining the rainfall pattern during the
southwest monsoon period.
The Nature of Indian Monsoon
Monsoon is a familiar, though a little known climatic
phenomenon. Despite the observations spread over centuries, the monsoon
continues to puzzle the scientists. Many attempts have been made to discover the
exact nature and causation of monsoon, but so far, no single theory has been
able to explain the monsoon fully. A real breakthrough has come recently when it
was studied at the global rather than at regional level.
Systematic studies of the causes of rainfall in the South Asian region help
to understand the causes and salient features of the monsoon, particularly some
of its important aspects, such as:
- The onset of the monsoon.
- Rain-bearing systems (e.g. tropical cyclones) and the relationship
between their frequency and distribution ofmonsoon rainfall.
- Break in the monsoon.
Onset of the Monsoon
Towards the end of the nineteenth century, it was believed
that the differential heating of land and sea during the summer months is the
mechanism which sets the stage for the monsoon winds of drift towards the
subcontinent. During April and May when the sun shines vertically over the
Tropic of Cancer, the large landmass in the north of Indian Ocean gets intensely
heated. This causes the formation of an intense low pressure in the northwestern
part of the subcontinent. Since the pressure in the Indian Ocean in the south of
the landmass is high as water gets heated slowly, the low attracts the southeast
trades across the Equator. These conditions help in the northward shift in the
position of the ITCZ. The southwest monsoon may thus, be seen as a continuation
of the southeast trades deflected towards the Indian subcontinent after crossing
the Equator. These winds cross the Equator between 40ºE and 60ºE longitudes.
The shift in the position of the ITCZ is also related to the
phenomenon of the withdrawal of the westerly jet streamfromits position over the
north Indian plain, south of the Himalayas. The easterly jet stream sets in
along 15ºN latitude only after the western jet stream has withdrawn itself from
the region. This easterly jet stream is held responsible for the burst of the
monsoon in India. Entry of Monsoon into India: The southwest monsoon sets in
over the Kerala coast by 1st June and moves swiftly to reach Mumbai and Kolkata
between 10th and 13th June. By mid- July, southwest monsoon engulfs the entire
Rain-bearing Systems and Rainfall Distribution
There seem to be two rain-bearing systems in India. First
originate in the Bay of Bengal causing rainfall over the plains of north India.
Second is the Arabian Sea current of the southwestmonsoon which brings rain to
the west coast of India. Much of the rainfall along the Western Ghats is
orographic as the moist air is obstructed and forced to rise along the Ghats.
The intensity of rainfall over the west coast of India is, however, related to
- The offshore meteorological conditions.
- The position of the equatorial jet stream along the eastern coast of
The frequency of the tropical depressions originating fromthe
Bay of Bengal varies fromyear to year. Their paths over India are mainly
determined by the position of ITCZ which is generally termed as the monsoon
trough. As the axis of the monsoon trough oscillates, there are fluctuations in
the track and direction of these depressions, and the intensity and the amount
of rainfall vary from year to year. The rain which comes in spells, displays a
declining trend from west to east over the west coast, and from the southeast
towards the northwest over the North Indian Plain and the northern part of the
EI-Nino and the Indian Monsoon
EI-Nino is a complex weather system that appears once
every three to seven years bringing drought, floods and other weather
extremes to different parts of the world. The system involves oceanic and
atmospheric phenomena with the appearance of warm currents off the coast of
peru in the Eastern Pacific and affects weather in many places including
India. EINino is merely an extension of the warm equatorial current which
gets replaced temporarily by cold Peruvian current or Humbolt current
(locate these currents in your atlas). This current increases the
temperature of water on the Peruvian coast by 10ºC. This results in:
- The distortion of equatorial atmospheric circulation;
- Irregularities in the evaporation of sea water;
- Reduction in the amount of planktons which further reduces the
number of fish in the sea.
The word EI-Nino means ‘Child Christ’ because this current appears around
Christmas in December. December is a summer month in Peru (Southern
EI-Nino is used in India for forecasting long range monsoon rainfall. In
1990-91, there was a wild EI-Nino even and the onset of southwest monsoon
was delayed over most parts of the country ranging from five to twelve days.
Break in the Monsoon
During the south-west, monsoon period after having rains for
a few days, it rain fails to occur for one or more weeks, it is known as break
in the monsoon. These dry spells are quite common during the rainy season. These
breaks in the different regions are due to different reasons:
- In northern India rains are likely to fail if the rain-bearing storms
are not very frequent along the monsoon trough or the ITCZ over this region.
- Over the west coast the dry spells are associated with dayswhen winds
blow parallel to the coast.
The Rhythm of Seasons
The climatic conditions of India can best be described in terms of an annual
cycle of seasons. The meteorologists recognize the following four seasons:
- The cold weather season
- The hot weather season
- The southwest monsoon season
- The retreating monsoon season.
Some Famous Local Storms of Hot Weather Season
Mango Shower: Towards the end of summer. There are
pre-monsoon showers which are a common phenomena in Kerala and coastal
areas of Karnataka. Locally, they are known as mango showers since they
help in the early ripening of mangoes.
Blossom Shower: With this shower, coffee
flowers blossom in Kerala and nearby areas.
Nor Westers: These are dreaded evening thunderstorms
in Bengal and Assam. Their notorious nature can be understood from the
local nomenclature of ‘Kalbaisakhi’, a calamity of the month of Baisakh.
These showers are useful for tea, Jute and rice cultivation. In Assam,
these storms are known as “Bordoiseela”.
Loo: Hot, dry and oppressing winds blowing in the
Northern plains from Punjab to Bihar with higher intensity between Delhi
The Cold Weather Season
Temperature: Usually, the cold weather season sets in by mid-November
in northern India. December and January are the coldest months in the northern
plain. The mean daily temperature remains below 21ºC over most parts of northern
India. The night temperature may be quite low, sometimes going below freezing
point in Punjab and Rajasthan.
There are three main reasons for the excessive cold in north India during
- States like Punjab, Haryana and Rajasthan being far away from the
moderating influenced of sea experience continental climate.
- The snowfall in the nearby Himalayan ranges creates cold wave situation;
- Around February, the cold winds coming from the Caspian Sea and
Turkmenistan bring cold wave along with frost and fog over the northwestern
parts of India.
The Peninsular region of India, however, does not have any
well-defined cold weather season. There is hardly any seasonal change in the
distribution pattern of the temperature in coastal areas because of moderating
influence of the sea and the proximity to equator. For example, the mean maximum
temperature for January at Thiruvanantapuram is as high as 31ºC, and for June,
it is 29.5ºC. Temperatures at the hills of Western Ghats remain comparatively
low. Pressure andWinds: By the end of December (22nd December), the sun shines
vertically over the Tropic of Capricorn in the southern hemisphere. The weather
in this season is characterized by feeble high pressure conditions over the
northern plain. In south India, the air pressure is slightly lower. The isobars
of 1019 mb and 1013 mb pass through northwest India and far south, respectively.
As a result, winds start blowing from northwestern high
pressure zone to the low air pressure zone over the Indian Ocean in the south.
Due to low pressure gradient, the light winds with a low
velocity of about 3-5 km per hour begin to blow outwards.By and large, the
topography of the region influences the wind direction. They are westerly or
northwesterly down the Ganga Valley. They become northerly in the
Ganga-Brahamputra delta. Free from the influence of topography, they are clearly
northeasterly over the Bay of Bengal. During the winters, the weather in India
is pleasant. The pleasant weather conditions, however, at intervals, get
disturbed by shallow cyclonic depressions originating over the east Medirranean
Sea and travelling eastwards across West Asia, Iran, Afghanistan and Pakistan
before the reach the northwestern parts of India. On their way, the moisture
content gets augmented from the Caspian Sea in the north and the Persian Gulf in
Role of Westerly Jet Stream
Rainfall: Winter monsoons do not cause rainfall as they move from land to the
sea. It is because firstly, they have little humidity; and secondly, due to anti
cyclonic circulation on land, the possibility of rainfall from them reduces. So,
most parts of India do not have rainfall in the winter season. However,
there are some exceptions to it:
In northwestern India, some weak temperate cyclones from
the Mediterranean sea cause rainfall in Punjab, Haryana, Delhi and western
Uttar Pradesh. Although the amount is meager, it is highly beneficial for
rabi crops. The precipitation is in the form of snowfall in the lower
Himalayas. It is this snow that sustains the flow of water in the Himalayan
Rivers during the summer months. The precipitation goes on decreasing
fromwest to east in the plains and fromnorth to south in the mountains. The
average winter rainfall in Delhi is around 53 mm. In Punjab and Bihar,
rainfall remains between 25 mm and 18 mm respectively.
Central parts of India and northern parts of southern
Peninsula also get winter rainfall occasionally.
- Arunachal Pradesh and Assam in the northeastern parts of India also have
rains between 25 mm and 50 mm during these winter months.
- During October and November, northeast monsoon while crossing over the
Bay of Bengal, pick up moisture and causes torrential rainfall over the
Tamil Nadu coast, southern Andhra Pradesh, southeast Karnataka and southeast
The Hot Weather Season
Temperature: With the apparent northward movement of the sun
towards the Tropic of Cancer in March, temperatures start rising in north India.
April, May and June are the months of summer in north India. In most parts of
India, temperatures recorded are between 30º-32ºC. In March, the highest day
temperature of about 38ºC occurs in the Deccan Plateau while in April,
temperature ranging between 38ºC and 43ºC are found in Gujarat andMadhya
Pradesh. InMay, the heat belt moves further north, and in the north-western part
of India, temperatures around 48ºC are not uncommon.
The hot weather season in south India is mild and not so
intense as found in north India. The Peninsular situation of south India with
moderating effect of the oceans keeps the temperatures lower than that
prevailing in north India. So, temperatures remain between 26ºC and 32ºC. Due to
altitude, the temperatures in the hills ofWesternGhats remain below 25ºC. In the
coastal regions, the north-south extent of isotherms parallel to the coast
confirms that temperature does not decrease from north to south rather it
increases fromthe coast to the interior. The mean daily minimum temperature
during the summer months also remains quite high and rarely goes below 26ºC.
Pressure and Winds: The summer months are a period of
excessive heat and falling air pressure in the northern half of the country.
Because of the heating of the subcontinent, the ITCZ moves northwards occupying
a position centred at 25ºN in July. Roughly, this elongated low pressure monsoon
trough extends over the Thar desert in the north-west to Patna and Chotanagpur
plateau in the east-southeast. The location of the ITCZ attracts a surface
circulation of the winds which are southwesterly on the west coast as well as
along the coast ofWest Bengal and Bangladesh. They are easterly or southeasterly
over north Bengal and Bihar. It has been discussed earlier that these currents
of southwesterly monsoon are in reality ‘displaced’ equatorial westerlies. The
influx of these winds by mid-June brings about a change in the weather towards
the rainy season.
In the heart of the ITCZ in the northwest, the dry and hot
winds known as ‘Loo’, blowin the afternoon, and very often, they continue to
well intomidnight. Dust storms in the evening are very common duringMay in
Punjab, Haryana, Eastern Rajasthan and Uttar Pradesh. These temporary storms
bring a welcome respite from the oppressing heat since they bring with them
light rains and a pleasant cool breeze. Occasionally, The moisture-laden winds
are attracted towards the periphery of the trough. A sudden contact between dry
and moist air masses gives rise to local storms of great intensity. These local
storms are associated with violent winds, torrential rains and even hailstorms.