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CBSE Notes 2011-2012

Climate

Climate:
 
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:
 
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. 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.
 
The climate of India is described as the ‘monsoon’ type.
 
• The word monsoon is derived from the Arabic word ‘mausim’ which
literally means season.
• ‘Monsoon’ refers to the seasonal reversal in the wind direction during a year.
 
This type of climate is found mainly in the south and the Southeast Asia. Despite an
overall unity in the general pattern, there are perceptible regional variations in
climatic conditions within the country. In summer, the mercury occasionally touches
50°C in some parts of the Rajasthan desert, whereas it may be around 20°C in
Pahalgam in Jammu and Kashmir. On a winter night, temperature at Drass in Jammu
and Kashmir may be as low as minus 45°C. Tiruvananthapuram, on the other hand,
may have a temperature of 20°C. In certain places.
 
There are variations not only in the form and types of precipitation but also in its
amount and the seasonal distribution. While precipitation is mostly in the form of
snowfall in the upper parts of Himalayas, it rains over the rest of the country. The
annual precipitation varies from over 400 cm in Meghalaya to less than 10 cm in
Ladakh and western Rajasthan. Most parts of the country receive rainfall from June
to September. But some parts like the Tamil Nadu coast get most of its rain during
October and November. In general, coastal areas experience less contrasts in
temperature conditions. Seasonal contrasts are more in the interior of the country.
There is decrease in rainfall generally from east to west in the Northern Plains. These
variations have given rise to variety in lives of people – in terms of the food they eat,
the clothes they wear and also the kind of houses they live in.
 
CLIMATIC CONTROLS
 
There are six major controls of the climate of any place. They are:
 
Latitude
Altitude
Pressure and wind system
Distance from the sea
 
 
 
Ocean currents and
Relief features
 
 
FACTORS AFFECTING INDIA’S CLIMATE
 
Latitude
 
The Tropic of Cancer passes through the middle of the country from the Rann of
Kuchchh in the west to Mizoram in the east. Almost half of the country, lying south
of the Tropic of Cancer, belongs to the tropical area. All the remaining area, north of
the Tropic, lies in the sub-tropics. Therefore, India’s climate has characteristics of
tropical as well as subtropical climates.
 
Altitude
 
India has mountains to the north, which have an average height of about 6,000
metres. India also has a vast coastal area where the maximum elevation is about 30
metres. The Himalayas prevent the cold winds from Central Asia from entering the
subcontinent. It is because of these mountains that this subcontinent experiences
comparatively milder winters as compared to central Asia.
 
Pressure and Winds
 
The climate and associated weather conditions in India are governed by the following
atmospheric conditions:
 
• Pressure and surface winds;
• Upper air circulation; and
• Western cyclonic disturbances and tropical cyclones.
 
India lies in the region of north easterly winds. These winds originate from the
subtropical high-pressure belt of the northern hemisphere. They blow south, get
deflected to the right due to the Coriolis force, and move on towards the equatorial
low-pressure area. Generally, these winds carry very little moisture as they originate
and blow over land. Therefore, they bring little or no rain. Hence, India should have
been an arid land, but, it is not so.
 
The pressure and wind conditions over India are unique. During winter, there is a
high-pressure area north of the Himalayas. Cold dry winds blow from this region to
the low-pressure areas over the oceans to the south. In summer, a low-pressure
area develops over interior Asia as well as over northwestern India. This causes a
 
 
 
complete reversal of the direction of winds during summer. Air moves from the high-
pressure area over the southern Indian Ocean, in a south-easterly direction, crosses
the equator, and turns right towards the low-pressure areas over the Indian
subcontinent. These are known
as the Southwest Monsoon winds. These winds blow over the warm oceans, gather
moisture and bring widespread rainfall over the mainland of India. The upper air
circulation in this region is dominated by a westerly flow. An important component of
this flow is the jet stream. These jet streams are located approximately over 27°-
30° north latitude, therefore, they are known as subtropical westerly jet streams.
Over India, these jet streams blow south of the Himalayas, all through the year
except in summer. The western cyclonic disturbances experienced in the north and
north-western parts of the country are brought in by this westerly flow. In summer,
the subtropical westerly jet stream moves north of the Himalayas with the
apparent movement of the sun. An easterly jet stream, called the tropical easterly
jet stream blows over peninsular India, approximately over 14°N during the summer
months.
 
THE INDIAN MONSOON
 
The climate of India is strongly influenced by monsoon winds. The sailors who came
to India in historic times were one of the first to have noticed the phenomenon of the
monsoon. They benefited from the reversal of the wind system as they came by
sailing ships at the mercy of winds. The Arabs, who had also come to India as
traders named this seasonal reversal of the wind system ‘monsoon’. The monsoons
are experienced in the tropical area roughly between 20° N and 20° S. To
understand the mechanism of the monsoons, the following facts are important.
 
(a) 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 monsoontrough
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.
 
 
 
Apart from this, it has also been noticed that changes in the pressure conditions over
the southern oceans also affect the monsoons. Normally when the tropical eastern
south Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean
experiences low pressure. 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. This periodic change in pressure conditions is known as the Southern
Oscillation or SO. The difference in pressure over Tahiti (Pacific Ocean,
18°S/149°W) and Darwin in northern Australia (Indian Ocean, 12°30’S/131°E) is
computed to predict the intensity of the monsoons. If the pressure differences were
negative, it would mean below average and late monsoons. A feature connected with
the SO is the El Nino, a warm ocean current that flows past the Peruvian Coast, in
place of the cold Peruvian current, every 2 to 5 years. The changes in pressure
conditions are connected to the El Nino. Hence, the phenomenon is referred to as
ENSO (El Nino Southern Oscillations).
 
THE ONSET OF THE MONSOON AND WITHDRAWAL
 
The Monsoon, unlike the trades, are not steady winds but are pulsating in nature,
affected by different atmospheric conditions encountered
by it, on its way over the warm tropical seas. The duration of the monsoon is
between 100- 120 days from early June to mid-September. 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, and can be distinguished from the pre-monsoon
showers. The monsoon arrives at the southern tip of the Indian peninsula generally
by the first week of June. Subsequently, it divides into two – the Arabian Sea branch
and the Bay of Bengal branch. The Arabian Sea branch reaches Mumbai about ten
days later on approximately the 10th of June. This is a fairly rapid advance. The Bay
of Bengal branch also advances rapidly and arrives in Assam in the first week of
June. The lofty mountains causes the monsoon winds to deflect towards the west
over the Ganga plains. By mid-June the Arabian Sea branch of the monsoon arrives
over Saurashtra-Kuchchh and the central part of the country. The Arabian Sea and
the Bay of Bengal branches of the monsoon merge over the northwestern part of the
Ganga plains. Delhi generally receives the monsoon showers from the Bay of Bengal
branch by the end of June (tentative date is 29th of June). By the first week of July,
western Uttar Pradesh, Punjab, Haryana and eastern Rajasthan experience the
monsoon. By mid-July, the monsoon reaches Himachal Pradesh and the rest of the
country.
Withdrawal or the retreat of the monsoon is a more gradual process . The withdrawal
of the monsoon begins in northwestern states of India by early September. By mid-
October, it withdraws completely from the northern half of the peninsula. The
withdrawal from the southern half of the peninsula is fairly rapid. By early December,
the monsoon has withdrawn from the rest of the country. The islands receive the
very first monsoon showers, progressively from south to north, from the first week of
April to the first week of May. The withdrawal, takes place progressively from north
to south from the first week of December to the first week of January. By this time
the rest of the country is already under the influence of the winter monsoon.
 
THE SEASONS
 
 
 
The monsoon type of climate is characterized by a distinct seasonal pattern. The
weather conditions greatly change from one season to the other. These changes are
particularly noticeable in the interior parts of the country. The coastal areas do not
experience much variation in temperature though there is variation in rainfall
pattern. Four main seasons can be identified in India – the cold weather season, the
hot weather season, the advancing monsoon and the retreating monsoon with some
regional variations.
 
The Cold Weather Season (Winter)
 
The cold weather season begins from mid- November in northern India and stays till
February. December and January are the coldest months in the northern part of
India. The temperature decreases from south to the north. The average temperature
of Chennai, on the eastern coast, is between 24° - 25° Celsius, while in the northern
plains, it ranges between 10° - 15° Celsius. Days are warm and nights are cold.
Frost is common in the north and the higher slopes of the Himalayas experience
snowfall. During this season, the northeast trade winds prevail over the country.
They blow from land to sea and hence, for most part of the country, it is a dry
season. Some amount of rainfall occurs on the Tamil Nadu coast from these winds
as, here they blow from sea to land. In the northern part of the country, a feeble
high-pressure region develops, with light winds moving outwards from this area.
Influenced by the relief, these winds blow through the Ganga valley from the west
and the northwest. The weather is normally marked by clear sky, low temperatures
and low humidity and feeble, variable winds. A characteristic feature of the cold
weather season over the northern plains is the inflow of cyclonic disturbances from
the west and the northwest. These low-pressure systems, originate over the
Mediterranean Sea and western Asia and move into India, along with the westerly
flow. 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 peninsular region does not have a welldefined
cold season. There is hardly any
noticeable seasonal change in temperature pattern during winters due to the
moderating influence of the sea.
 
The Hot Weather Season (Summer)
 
Due to the apparent northward movement of the sun, the global heat belt shifts
northward. As such, from March to May, it is hot weather season in India. The
influence of the shifting of the heat belt can be seen clearly from temperature
recordings taken during March-May at different latitudes. In March, the highest
temperature is about 38° Celsius, recorded on the Deccan plateau. In April,
temperatures in Gujarat and Madhya Pradesh are around 42° Celsius. In May,
temperature of 45° Celsius is common in the northwestern parts of the country. In
peninsular India, temperatures remain lower due to the moderating influence of the
oceans. The summer months experience rising temperature and falling air pressure
in the northern part of the country. Towards the end of May, an elongated low-
pressure area develops in the region extending from the Thar Desert in the
northwest to Patna and Chotanagpur plateau in the east and southeast. Circulation of
 
 
 
air begins to set in around this trough. A striking feature of the hot weather season
is the ‘loo’. These are strong, gusty, hot, dry winds blowing during the day over the
north and northwestern India. Sometimes they even continue until late in the
evening. Direct exposure to these winds may even prove to be fatal. Dust storms are
very common during the month of May in northern India. These storms bring
temporary relief as they lower the temperature and may bring light rain and cool
breeze. This is also the season for localised thunderstorms, associated with violent
winds, torrential downpours, often accompanied by hail. In West Bengal, these
storms are known as the ‘Kaal Baisakhi’ calamity for the month of Baisakh. Towards
the close of the summer season, pre-monsoon showers are common especially, in
Kerala and Karnataka. They help in the early ripening of mangoes, and are often
referred to as ‘mango showers’.
 
Advancing Monsoon (The Rainy Season)
 
By early June, the low-pressure condition over the northern plains intensifies. It
attracts, the trade winds of the southern hemisphere. These south-east trade winds
originate over the warm subtropical areas of the southern oceans. They cross the
equator and blow in a southwesterly southwesterly direction entering the Indian
peninsula as the south-west monsoon. As these winds blow over warm oceans, they
bring abundant moisture to the subcontinent. These winds are strong and blow at an
average velocity of 30 km per hour. With the exception of the extreme north-west,
the monsoon winds cover the country in about a month. The inflow of the south-west
monsoon into India brings about a total change in the weather. Early in the season,
the windward side of the Western Ghats receives very heavy rainfall, more than 250
cm. The Deccan Plateau and parts of Madhya Pradesh also receive some amount of
rain in spite of lying in the rain shadow area. The maximum rainfall of this season is
received in the north-eastern
part of the country. Mawsynram in the southern ranges of the Khasi Hills receives
the highest average rainfall in the world. Rainfall in the Ganga valley decreases from
the east to the west. Rajasthan and parts of Gujarat get scanty rainfall. Another
phenomenon associated with the monsoon is its tendency to have ‘breaks’ in rainfall.
Thus, it has wet and dry spells. In other words, the monsoon rains take place only
for a few days at a time. They are interspersed with rainless intervals. These breaks
in monsoon are related to the movement of the monsoon trough. For various
reasons, the trough and its axis keep on moving northward or southward, which
determines the spatial distribution of rainfall. When the axis of the monsoon trough
lies over the plains, rainfall is good in these parts. On the other hand, whenever the
axis shifts closer to the Himalayas, there are longer dry spells in the plains, and
widespread rain occur in the mountainous catchment areas of the
Himalayan rivers. These heavy rain bring in their wake, devastating floods causing
damage to life and property in the plains. The frequency and intensity of tropical
depressions too, determine the amount and duration of monsoon rains. These
depressions form at the head of the Bay of Bengal and cross over to the mainland.
The depressions follow the axis of the “monsoon trough of low pressure”. The
monsoon is known for its uncertainties. The alternation of dry and wet spells vary in
intensity, frequency and duration. While it causes heavy floods one part, it may be
 
 
responsible for droughts in the other. It is often irregular in its arrival and its retreat.
Hence, it sometimes disturbs the farming schedule of millions of farmers all over the
country.
 
Retreating Monsoon
(The Transition Season)
 
During October-November, with the apparent movement of the sun towards the
south, the monsoon trough or the low-pressure trough over the northern plains
becomes weaker. This is gradually replaced by a high-pressure system. The south-
west monsoon winds weaken and start withdrawing gradually. By the beginning of
October, the monsoon withdraws from the Northern Plains. The months of October-
November form a period of transition from hot rainy season to dry winter conditions.
The retreat of the monsoon is marked by clear skies and rise in temperature. While
day temperatures are high, nights are cool and pleasant. The land is still moist.
Owing to the conditions of high temperature and humidity, the weather becomes
rather oppressive during the day. This is commonly known as ‘October heat’. In the
second half of October, the mercury begins to fall rapidly in northern India. The low-
pressure conditions, over northwestern India, get transferred to the Bay of Bengal by
early November. This shift is associated with the occurrence of cyclonic depressions,
which originate over the Andaman Sea. These cyclones generally cross the eastern
coasts of India cause heavy and widespread rain. These tropical cyclones are often
very destructive. The thickly populated deltas of the Godavari, the Krishna and the
Kaveri are frequently struck by cyclones, which cause great damage to life and
property. Sometimes, these cyclones arrive at the coasts of Orissa, West Bengal and
Bangladesh. The bulk of the rainfall of the Coromandel Coast is
derived from depressions and cyclones.
 
DISTRIBUTION OF RAINFALL
 
The western coast and northeastern India receive over about 400 cm of rainfall
annualy. However, it is less than 60 cm in western Rajasthan and adjoining parts of
Gujarat, Haryana and Punjab. Rainfall is equally low in the interior of the Deccan
plateau, and east of the Sahyadris. Why do these regions receive low rainfall? A third
area of low precipitation is around Leh in Jammu and Kashmir. The rest of the
country receives moderate rainfall. Snowfall is restricted to the Himalayan region.
Owing to the nature of monsoons, the annual rainfall is highly variable from year to
year. Variability is high in the regions of low rainfall such as parts of Rajasthan,
Gujarat and the leeward side of the Western Ghats. As such, while areas of high
rainfall are liable to be affected by floods, areas of low rainfall are
drought-prone.
 
MONSOON AS A UNIFYING BOND
 
Himalayas protect the subcontinent from extremely cold winds from central Asia.
This enables northern India to have uniformly higher temperatures when compared
to other areas on the same latitudes. Similarly, the peninsular plateau, under the
influence of the sea from three sides, has moderate temperatures. Despite such
moderating influences, there are great variations in the temperature conditions.
 
 
Nevertheless, the unifying influence of the monsoon on the Indian subcontinent is
quite perceptible. The seasonal alteration of the wind systems and the associated
weather conditions provide a rhythmic cycle of seasons. Even the uncertainties of
rain and uneven distribution are very much typical of the monsoons. The Indian
landscape, its animal and plant life, its entire agricultural calendar and the life of the
people, including their festivities, revolve around this phenomenon. Year after year,
people of India from north to south and from east to west, eagerly await the arrival
of the monsoon. These monsoon winds bind the whole country by providing water to
set the
agricultural activities in motion. The river valleys which carry this water also unite as
a single river valley unit.
 
Coriolis force: An apparent force caused by the earth’s rotation. The Coriolis force
is responsible for deflecting winds towards the right in the northern hemisphere and
towards the left in the southern hemisphere. This is also known as ‘Ferrel’s Law’.
 
Jet stream: These are a narrow belt of high altitude (above 12,000 m) westerly
winds in the troposphere. Their speed varies from about 110 km/h in summer to
about 184 km/h in winter. A number of separate jet streams have been identified.
The most constant are the mid-latitude and the sub tropical jet stream.
 
Western Cyclonic Disturbances: The western cyclonic disturbances are weather
phenomena of the winter months brought in by the westerly flow from the
Mediterranean region. They usually influence the weather of the north and north-
western regions of India. Tropical cyclones occur during the monsoon as well as in
October - November, and are part of the easterly flow. These disturbances affect the
coastal regions of the country.
 
Inter Tropical Convergence Zone: The Inter Tropical Convergence Zone (ITCZ,) is
a broad trough of low pressure in equatorial latitudes. This is where the northeast
and the southeast trade winds converge. This convergence zone lies more or less
parallel to the equator but moves north or south with the apparent movement of the
sun.
 
El Nino: This is a name given to the periodic development of a warm ocean current
along the coast of Peru as a temporary replacement of the cold Peruvian current. ‘El
Nino’ is a Spanish word meaning ‘the child’, and refers to the baby Christ, as this
current starts flowing during Christmas. The presence of the El Nino leads to an
increase
in sea-surface temperatures and weakening of the trade winds in the region.