CBSE
Notes Class 9 Geography Chapter 4 – 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 refers to the state of the atmosphere
over an area at any point in time.
The elements of weather and climate are the same, i.e.
temperature, atmospheric pressure, wind, humidity and precipitation. On the
basis of the monthly atmospheric conditions, the year is divided into seasons
such as:
- Winter
- Summer
- Rainy
Seasons
Climatic Controls
The climate of any place depends on the following factors:
1) Latitude
The amount of solar energy received varies according to
latitude due to the curvature of the earth. That’s why air temperature
generally decreases from the equator towards the poles.
2) Altitude
The atmosphere becomes less dense, and temperature decreases
when we go to a higher altitude from the Earth’s surface. This is the reason
why hills are cooler during summer.
The pressure and wind system of any area depend on the
latitude and altitude of the place. It influences the temperature and rainfall
pattern.
The sea exerts a moderating influence on the climate. As the
distance from the sea increases, its moderating influence decreases and people
experience extreme weather conditions. This condition is known as continentality, i.e.
very hot during summers and very cold during winters.
Ocean currents, along with onshore winds, affect the climate
of the coastal areas. For example, any coastal area with warm or cold currents
flowing past it will be warmed or cooled if the winds are onshore.
Relief plays a major role in determining the climate of a
place. High mountains act as barriers to cold or hot winds. They may also cause
precipitation if they are high enough and lie in the path of rain-bearing
winds. Precipitation is any form of moisture which falls to the earth. It
includes rain, snow, hail and sleet.
Factors Affecting India’s Climate
Latitude
The Tropic of Cancer passes through the middle of the
country from the Rann of Kachchh in the west to Mizoram in the east. India’s
climate has characteristics of tropical as well as subtropical climates.
Altitude
India has mountains to the north and also has a vast coastal
area where the maximum elevation is about 30 metres. Owing to the mountains,
the subcontinent experiences comparatively milder winters as compared to
central Asia.
Pressure and winds
The climate and weather conditions in India are governed by
the following atmospheric conditions:
- Pressure
and surface winds
- Upper
air circulation
- Western
cyclonic disturbances and tropical cyclones
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 territories of India.
Jet streams are fast-flowing, narrow, meandering
air currents in the atmosphere.
The Indian Monsoon
The climate of India is strongly influenced by monsoon
winds. The seasonal reversal of the
wind system is known as “monsoon”. The monsoons are
experienced in the tropical area roughly between 20° N and 20° S. Go through
the following facts to understand the mechanism of the monsoons in a better
way:
- The differential
heating and cooling of land and water create a low pressure on
the landmass of India while the seas around experience comparatively high
pressure.
- The shift
of the position of the Inter-Tropical Convergence Zone (ITCZ) in
summer over the Ganga plain is also known as the monsoon trough during the
monsoon season.
- The
presence of the high-pressure area east of Madagascar,
approximately 20°S over the Indian Ocean, affects the Indian Monsoon.
- The Tibetan
plateau gets intensely heated during summer, which results in
strong vertical air currents and the formation of low pressure over the
plateau at about 9 km above sea level.
- 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 also impact the Indian
Monsoon.
The changes in the pressure conditions over the southern
oceans also affect the monsoons. The irregularly periodic variation in winds
and sea surface temperatures over the tropical eastern Pacific Ocean that
affects the climate of the tropics and subtropics is known as Southern
Oscillation or SO.
The Onset of the Monsoon and Withdrawal
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, which is
known as the “burst” of the monsoon.
Arrival of the Monsoon in Different Parts of India
The monsoon arrives at the southern tip of the Indian
peninsula generally by the 1st week of June. Subsequently, it proceeds into two
– the Arabian Sea branch and the Bay of Bengal branch.
- The
Arabian Sea branch reaches Mumbai on approximately the 10th of June.
- The
Bay of Bengal branch arrives in Assam in the 1st week of June.
- 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
receives the monsoon showers from the Bay of Bengal branch by the end 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 of the Monsoon
The withdrawal of the monsoon begins in the northwestern
states of India by early September. By mid-October, it withdraws completely
from the northern half of the peninsula. From north to south, the withdrawal of
monsoon takes place from the first week of December to the first week of
January. By early December, the monsoon has withdrawn from the rest of the
country.
The Seasons
4 main seasons can be identified in India:
- The
cold weather season (Winter)
- The
hot weather season (Summer)
- The
advancing monsoon (Rainy Season)
- The
retreating monsoon with some regional variations (Transition Season)
Let’s discuss each of them in detail:
The Cold Weather Season (Winter)
Winter begins in 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 the south to the north.
- Days
are warm and nights are cold.
- The
weather is marked by clear sky, low temperatures and low humidity and
feeble, variable winds.
- This
season is extremely important for the cultivation of ‘rabi’ crops.
The Hot Weather Season (Summer)
From March to May, a hot weather season is observed in
India. The summer months experience rising temperatures and falling air
pressure in the northern part of the country. A striking feature of the hot
weather season is the ‘loo’. Loo is the strong, gusty, hot, dry winds
blowing during the day over north and northwestern India.
Towards the close of the summer season, pre-monsoon showers
come, which help in the early ripening of mangoes and are often referred to as
‘mango showers’.
Advancing Monsoon (The Rainy Season)
South-east trade winds originate over the warm subtropical
areas of the southern oceans. They cross the equator and blow in a
southwesterly direction entering the Indian peninsula as the southwest monsoon.
The monsoon winds cover the country for about a month. Mawsynram in the
southern ranges of the Khasi Hills receives the highest average rainfall in the
world.
A phenomenon associated with the monsoon is its tendency to
have ‘breaks’ in rainfall. These breaks in monsoon are related to the movement
of the monsoon trough. The trough and its axis keep on moving northward or
southward and determine the spatial distribution of rainfall. The
frequency and intensity of tropical depressions also
determine the amount and duration of monsoon rains. The monsoon is known for
its uncertainties. The alternation of dry and wet spells varies in intensity,
frequency and duration.
Retreating/Post Monsoons (The Transition Season)
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 a rise in temperature. While day temperatures are
high, nights are cool and pleasant. 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.
Distribution of Rainfall
- Parts
of the western coast and northeastern India receive over about 400 cm of
rainfall annually.
- Rainfall
is less than 60 cm in western Rajasthan and adjoining parts of Gujarat,
Haryana and Punjab.
- Rainfall
is low in the interior of the Deccan plateau and east of the Sahyadris.
- Snowfall
is restricted to the Himalayan region.
- The
annual rainfall is highly variable from year to year.
Monsoon as a Unifying Bond
The unifying influence of the monsoon on the Indian
subcontinent is quite noticeable. The seasonal alteration of the wind systems
and the associated weather conditions provide a rhythmic cycle of seasons. The
uncertainties of rain and uneven distribution are very much typical of the
monsoons. Year after year, the 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 agricultural activities in
motion.
