IAS Prelims (GS) Preparation 2018
Day # 24 (April 16, 2017)
Topics of the day: Geography 9th NCERT chapter 4 key points
Chapter 4
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.
The climate of India is described as the ‘monsoon’ type. In Asia, this type of climate is found mainly in the south and the southeast.
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.
There are six major controls of the climate of any place. They are: latitude, altitude, pressure and wind system, distance from the sea (continentality), ocean currents and relief features.
The sea exerts a moderating influence on climate: As the distance from the sea increases, its moderating influence decreases and the people experience extreme weather conditions. This condition is known as continentality (i.e. very hot
during summers and very cold during winters).
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.
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.
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.
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.
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 distrurbances affect the coastal regions of the country.
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.
The cold weather season
It begins from midNovember 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.
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.
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 muchneeded 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
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.
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.
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’.
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 south westerly direction entering the Indian peninsula as the south-west monsoon. As these winds blow over warm oceans, they bring abundant moisture to the subcontinent.
Mawsynram in the southern ranges of the Khasi Hills receives the highest average rainfall in the world.
Mawsynram, the wettest place on the earth is also reputed for its stalagmite and stalactite caves.
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.
Retreating/Post Monsoons (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’.