GEOGRAPHY (IX)-LESSON-4 CLIMATE (LESSON NOTES)

                                             GEOGRAPHY (IX)-LESSON-4

CLIMATE

(LESSON NOTES)

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v INTRODUCTION

  Ø There are three basic elements to know about the natural environment of any area as landforms, drainage and climate.

  Ø In this lesson we will know about the climate of India.

v CLIMATE AND WEATHER

WEATHER	CLIMATE
The atmospheric condition of any area in a point of time is referred as weather.	Climate refers to the sum total of weather conditions and variations over a large area.
It is measured as point of time.	It is measured over a long period of time (30 years average)
Example- Sunny day, cloudy day, calm day	Example-moderate climate, extreme climate,

v ELEMENTS OF CLIMATE AND WEATHER

  Ø The elements of climate and weather are same as: -

§  Temperature

§  Humidity

§  Air pressure

§  Cloudiness or sunshine

§  Precipitation (Rainfall or snowfall)

§  Wind

  Ø As we know that the weather conditions fluctuate very often within a day. Based on the generalized atmospheric conditions, the year is divided into seasons such as winter, summer and rainy seasons.

  Ø The world is divided into a number of climatic regions. In Asia, India and other South and South-Eastern countries have monsoon type of climate.

v MONSOON

  Ø 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.

v REGIONAL CLIMATIC VARIATION IN INDIA

  Ø Although there is an overall unity in the general climatic pattern in India, there are some perceptible regional variations. We will see the variation by taking two elements: -

  Ø TEMPERATURE

§  The temperature in the winter in North-Western mountainous regions can go down to – 45° C (at Drass in Jammu and Kashmir), while it is 22°C in Thiruvananthapuram in Kerala.

§  Similarly, it can go up to 50°C in summer in some parts of Western Rajasthan and 20°C in Shillong.

§  In many areas, there is a wide variation between day and night temperatures. In the Thar Desert, the day temperature may rise up to 50°C and drop down to near 15°C the same night. On the other hand, there is hardly any difference in day and night temperatures in the Andaman and Nicobar Islands or in Kerala.

  Ø PRECIPITATION

§  There is a wide variation observed in its amount and seasonal distribution. The annual precipitation varies from over 400 cm in Meghalaya to less than 10 cm in Ladakh and Western Rajasthan.

§  Precipitation in the form of snowfall occurs only in upper parts of Himalayas, the rest of the country receives rainfall.

§  Most parts of the country receive rainfall from June to September, but the Tamil Nadu coast gets most of its rain during October and November.

  Ø CONCLUSION

§  By these variations we can see that Coastal regions experience mild temperature compare to the interior regions.

§  The rainfall also decreases from East to West.

§  Such differences help to create a variety in lives of people— the food they eat, the clothes they wear, the kind of houses they used for living and so on.

v CLIMATIC CONTROLS

  Ø Permanent factors which govern the general nature of the climate of any location on the earth are called factors of Climatic Controls. There are six major controls of the climate of any place.

§  Latitude: - Due to the curvature of the earth, latitude changes the amount of solar energy received. As a result, temperature decreases from the equator towards the poles.

§  Altitude: -It refers to the height above mean sea level. With increase of height from the earth surface, the temperature decreases and air becomes less dense. Therefore, hilly regions are cooler in summer.

§  Pressure and Wind System: - It depends on the latitude and altitude of a place. Thus, it influences the temperature and rainfall pattern of the area.

§  Continentality or Distance from the Sea: -The sea exerts a moderating influence on the climate. As the distance from the sea increases, the weather conditions become more extreme (high temperature and rainfall variation between seasons).

§  Ocean Currents :- Along with onshore winds, the ocean currents (warm or cold) affect the climate of coastal areas. For example, cold onshore currents bring coolness in coastal areas.

§  Relief Features: -High mountains stop cold or hot winds from reaching a location. It can also cause rain or snow if the place is on the windward side of the mountains. The leeward side of the mountains are remains relatively dry.

v FACTORS AFFECTING INDIA’S CLIMATIC

  Ø LATITUDE

§  The Tropic of Cancer (23°30” N) divides the country into the tropical zone and the sub-tropical zone. The line runs from the Rann of Kuchchh (West) to Mizoram (East). So, India’s climate has characteristics of tropical as well as sub-tropical climates.

  Ø ALTITUDE

§  Mountains- in the North of India have an average elevation of about 6000 m, whereas on the coastal areas as well as islands, maximum elevation is about 30 m. The Indian sub-continent experiences milder winters as compared to Central Asia because of the Himalayas which prevent the cold winds from entering the sub-continent.

  Ø PRESSURE AND WINDS

§  The following atmospheric conditions govern the climate and associated weather conditions in India

·      Pressure and surface winds

·      Upper air circulation

·      Western cyclonic disturbances and tropical cyclones

  Ø Pressure and Surface Winds

§  Winds always move from a high-pressure area to a low-pressure area.

§  During winter, there is a high-pressure area in the north India due to low temperature.  So, Cold dry winds blow from land to ocean.

§  In summer, a low-pressure area develops over interior India due to high temperature. As a result, the low-pressure system attracts the southeast trade winds of the southern hemisphere.

§  On crossing the equator, this trade winds-due to the Coriolis force-turn right towards the low-pressure areas over the Indian subcontinent. So, these known as a south-wester monsoon wind.

§  Coriolis force an apparent force that as a result of the Earth’s rotation, deflects moving objects like air currents to the right in the Northern Hemisphere and to the left in the Southern hemisphere. This is known as Ferrel’s law.

§  These winds blow over the warm oceans, gather moisture and bring widespread rainfall over the mainland of India.

  Ø UPPER AIR CIRCULATION

§  The upper air circulation of the region (Indian subcontinent) is dominated by a westerly flow which is governed by 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 subtropical jet stream.

§  Due to their location over 27°-30° N latitude, these jet streams are known as sub-tropical westerly jet streams.

§  They blow South of the Himalayas, throughout the year except in summer.

  Ø WESTERN CYCLONIC DISTURBANCES

§  The Western cyclonic disturbances are weather phenomena of the winter months brought in by the westerly flow from the Mediterranean region.

§  The westerly flow brings the Western cyclonic disturbances in the North and North-Western India. It brings the rainfall in Punjab, Haryana, Rajasthan, Uttar Pradesh.

§  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.

v THE INDIAN MONSOON

  Ø Monsoon winds strongly influence climate of India. The monsoons are experienced in the tropical area roughly between 20° N and 20° S.

v MECHANISM OF MONSOON

  Ø The following facts are important to understand the mechanism of the monsoons

§  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.

§  The Inter-Tropical Convergence Zone (ITCZ) in summer season shifts its position over the Ganga plain. This is the equatorial trough normally positioned about 5°N of the equator. It 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). The intensity and position of this high-pressure area affect the Indian monsoon.

§  The Tibetan plateau gets intensely heated during summer, which results in strong vertical air currents and the formation of low pressure, affect the Indian monsoon.

 

§ 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.

§  Southern Oscillation: -Apart from the given facts, it has been noticed that changes in the pressure conditions over the Southern Pacific and Indian oceans also affect the monsoons. Normally, when Eastern South Pacific Ocean (Tahiti) experiences high pressure, the tropical Eastern Indian Ocean (Darwin)  experiences low pressure. But in past a few 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 (SO).

§  EL NINO: -The EL Nino phenomenon is a feature connected with the Southern Oscillation. El Nino’ is a Spanish word meaning ‘the child’, and refers to the baby Christ, In this, a warm ocean current flows past the Peruvian Coast, in place of the cold Peruvian current. It occurs at the interval of 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).

v THE ONSET OF MONSOON

  Ø Monsoon generally reaches the Southern tip of the peninsula during the first week of June.

  Ø After striking the Southern tip, it branches into two parts- the Arabian Sea branch and the Bay of Bengal branch; both branches move rapidly.

  Ø The Arabian Sea branch advances North along the Western Ghats, reaching Mumbai by about 10th of June and soon covers the Saurashtra-Kuchchh and central most part of the Deccan Plateau also.

  Ø The Bay of Bengal branch reaches Assam in the first week of June and gets deflected towards the West by the mountain ranges, thus giving rainfall to the Ganga plains.

  Ø Both the branches again merge over the North-Western part of the Ganga plains. Delhi receives rainfall from Bay of Bengal branch by the end of June (tentative date is 29th June) and by the first-week of July, monsoon covers Western Uttar Pradesh, Punjab, Haryana and Eastern Rajasthan.

v WITHDRAWAL OF THE MONSOON

  Ø Withdrawal or the retreat of the monsoon is a more gradual process.

  Ø The process begins by early September in North-Western states. By mid-October, it withdraws completely from the Northern half of the peninsula.

  Ø 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.

v IMPORTANT FEATURES OF MONSOON

The important features of monsoon are as follows

  Ø The Indian monsoon is known for its uncertainties.

  Ø The duration of the monsoon is between 100-120 days from early June to mid-September. In this time the normal rainfall increases suddenly, this is known as the ‘burst’ of the monsoon.

  Ø There is an alteration of dry and wet spells which vary in intensity, frequency and duration. It called as break in monsoon.

  Ø Its irregular arrival and retreat (sometimes due to the effect of EL Nino), causing disruption of farming schedules and causing droughts in certain areas of the country.

v THE SEASONS

  Ø As we know that the weather conditions fluctuate very often within a day. Based on the generalised atmospheric conditions, the year is divided into seasons such as winter, summer and advancing monsoon and retreating monsoon.

    1.    THE COLD WEATHER SEASON (WINTER)

§  The cold weather season begins from mid-November and stays till February in Northern parts of India with December and January as the coldest months.

§  The temperature decreases from South to North.

§  Clear sky, low temperature and humidity, and feeble, variable winds are the characteristics of the weather. Frost and snow fall also fall in the high mountainous regions of Himalayas.

§  Most of the country remains dry as wind blow from land towards sea. The only rain occurs in Tamil Nadu and Southern Andhra Pradesh due to these winds picking up moisture from the Bay of Bengal.

§  In winter rainfall also occur in Punjab, Haryana, Rajasthan, western up because of western cyclonic disturbance occur in Mediterranean Sea. That is locally known as Mahawat. It is useful for cultivation of the Rabi crops.

    2.    THE HOT WEATHER SEASON(SUMMER)

§  The hot weather season starts with the apparent movement of the sun towards the North, from March and lasts up to the end of May.

§   The temperature of the Northern part of India goes up and the atmospheric pressure comes down.

§  The summer months experience rising temperature and falling air pressure.

§  A hot gusty and dry wind, locally known as Loo, blows during this season over the North and North-Western India.

§  Localised thunderstorms also occur during summer, which may have high speed winds and even precipitate hail. Such thunderstorms are called Kaal Baisakhi in West Bengal.

§  Near the end of summer, there may be pre-monsoon showers. These are called Mango Showers in Kerala and Karnataka, as they help in the early ripening of the mango fruit.

    3.    ADVANCING MONSOON (THE RAINY SEASON)

§  By mid June, low-pressure area intensifies over the Northern plains and attracts the trade winds. These trade winds originate over the warm tropical ocean in the Southern hemisphere.

§  After crossing equator, these blow in the South-West direction entering peninsula as South-West monsoon.

§  Maximum rainfall due to these winds occurs in North-Eastern India (mainly Meghalaya (Mawsynram-Highest) and Assam) and the windward side of the Western Ghats (Thiruvananthapuram to Mumbai).

§  In spite lying in the rain shadow area, the Deccan Plateau and parts of Madhya Pradesh also receive some amount of rainfall.

§  In the Northern plains precipitation decreases from East to West,

§  Tropical Depression, which form at the head of the Bay of Bengal is another phenomenon, which determines amount and Suration of the monsoon.

    4.    RETREATING MONSOON SEASON (THE TRANSITION)

§  By the beginning of October, the monsoon withdraws from the Northern plains. The months of October-November form a transition period from hot rainy to dry winter conditions

§  The sun starts shifting towards the South during October-November. The period is marked by clear skies and rise in temperature.

§  Due to the temperature still remaining high and humidity not reducing, the heat is oppressive. This phenomenon is also called October heat.

§  During this time, the low-pressure trough over the Northern plains weakens and is replaced gradually by a high-pressure system.

§  Cyclonic depression which occurs in Bay of Bengal also other phenomena of the retreating monsoon. Often, they cause a lot of destruction at coasts of Odisha, West Bengal and Bangladesh.

v DISTRIBUTION OF RAINFALL

  Ø Annually, parts of Western coast and North-Eastern India receive over about 400 cm of rainfall.

  Ø However, it is less than 60 cm in Western Rajasthan, some parts of Gujarat, Haryana, rain-shadow area of western ghat, Leh in Jammu and Kashmir.

  Ø The rest of the country receives moderate rainfall. Snowfall, is restricted to the Himalayan region.

  Ø 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.

  Ø Due to this, the areas of high rainfall are liable to be affected by floods whereas, areas of low rainfall are drought-prone.

v MONSOON AS A UNIFYING BOND

  Ø There is a great diversity of climatic condition in India due to different latitude and relief. But these diversities are subdued by the monsoon. It provides a rhythmic cycle of seasons.

  Ø The vegetation, animal life and agricultural activities are all revolving around the effects of the monsoon.

  Ø The life of the people, their celebration of festivals and other activities all are geared to the monsoon as India is still primarily an agricultural nation.

  Ø The monsoon provides the water to set agricultural activities in motion and hence, the arrival of the monsoon is awaited eagerly.

  Ø The river valleys which carry this water also unite as single river valley unit.