Climate Notes For Class 9 Geography

Climate is the sum total of weather conditions (including variations) over a large area for a long period of time (more than thirty years).

Weather refers to the state of the atmosphere at a place and time with reference to the following elements:

  • Temperature Humidity
  • Air pressure
  • Cloudiness or sunshine
  • Precipitation (Rainfall or snowfall)
  • Wind

The weather conditions fluctuate very often within a day. Based on the generalized monthly atmospheric conditions, the year is divided into seasons such as winter, summer, and rainy seasons. The world is divided into a number of climatic regions. In Asia, India and other South and South-Eastern countries have monsoon-type of climates.

The Climate of India is described as the ‘monsoon’.

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.

  • Regional Climatic Variation In India.
  • Although there is an overall unity in the general climatic pattern in India, there are some perceptible regional variations.

Temperature 

In Summer, the mercury touches 50ºC in some areas of the Rajasthan desert, whereas it may be around 20ºC in Jammu and Kashmir. 

On a winter night, the temperature at Drass in Jammu and Kashmir may be as low as minus 45ºC. On the other hand, Thiruvananthapuram’s temperature is 22ºC.

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.

Precipitation

  • There are variations not only in the form and types of precipitation but also 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.
  • Tamil Nadu coast gets heavy rainfall during October and November.
  • Generally, Northern Plains from east to west decrease in rainfall.

Climatic Controls

There are six major controls of the climate of any place.

Latitude: The amount of solar energy received varies according to latitude due to the rotation of the earth. That’s why air temperature generally decreases from the equator towards the poles.

Altitudes: The atmosphere becomes less dense and temperature decreases when we go to the higher altitude from the earth’s surface. This is the reason why hills are cooler during summers.

Pressure and Wind: The pressure and wind system of any area depends on the latitude and altitude of the place. It influences the temperature and rainfall pattern.

Distance From the Sea: 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: 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: 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 that falls to the earth. It includes rain, snow, hail, and sleet.

Factors Affecting India’s Climatic:

  1. Latitude: The Tropic of Cancer (23°3 C N) divides the country into the tropical zone (South of this line) and the sub-tropical zone (North of this line). The line runs from the Rann of Kuchchh (West) to Mizoram (East). All the remaining area, North of the Tropic, lies in the sub-tropics. So, India’s climate has characteristics of tropical as well as sub-tropical climates.
  2. 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, subcontinent experiences comparatively milder winters as compared to central Asia.

  3. Pressure and Winds

    The following atmospheric conditions governed the climate and associated weather conditions in India:

    • 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, cross 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.

India lies in the region of North-Easterly surface winds. These winds originate during winter from the sub-tropical high-pressure belt of the Northern hemisphere.

Jet Stream: The upper air circulation in the North of the Himaliyan region (Indian subcontinent) is dominated by a westerly flow governed by a 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.

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.

Note: 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.

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. This law states that a wind in any direction tends to deflect towards right (West to East) in the Northern Hemisphere and to the left in the Southern Hemisphere with a force that is directly proportional to the mass of the wind in question, its velocity, the sine of the latitude and the angular velocity of the Earth’s rotation.

The Monsoon

The climate of India is strongly influenced by monsoon winds. The Arabs, who had also come to India as traders named this seasonal reversal of the wind system is known as the “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.

  • 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 Inter-Tropical Convergence Zone (ITCZ) in the 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 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.

 The Inter Tropical Convergence Zone (ITCZ)

  • 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.

Mechanism of Monsoons:

Apart from the given facts, it has 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 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).

 

SO (Southern Oscillation)

  • 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.

ENSO (EI Nino Southern Oscillations)

  1. A feature connected with the SO is the El Nino phenomenon in which a warm ocean current that flows past the Peruvian Coast, in place of the cold Peruvian Current, every 2 to 5 years.
  2. The changes in pressure conditions over south eastern Pacific Ocean and East Indian Ocean are connected to the El Nino.
  3. Hence, the phenomenon is referred to as ENSO (El Nino Southern Oscillations). 

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.
  • ‘EI Nino’ is a Spanish word meaning ‘the child’ and refers to the baby Chirst, 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.

 

 Prediction of the Intensity of Monsoon in India:

  • 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.

Charactersitics of Monsoon:
1. 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.
2. The duration of the monsoon is between 100-120 days from early June to mid-September.
3. 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.
4. 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.
5. The monsoon is known for its uncertainties.
6. The alternation of dry and wet spells varies in intensity, frequency and duration. While it causes heavy floods in one part, it may be responsible for droughts in the other.
7. It is often irregular in its arrival and its retreat.

Note: 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.

The Onset and Withdrawal of the Monsoon:

  • The monsoon arrives at the southern tip of the Indian peninsula generally by the first week of June.
  • Subsequently, it proceeds 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 cause 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-Kachchh 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.

 

Important Features of Monsoon

The important features of monsoon are as follows:

  • The monsoon is also known for its uncertainties.
  • There is an alteration between dry and wet spells which vary in intensity, frequency, and duration.
  • While it causes heavy floods in one part, it may be responsible for drought in other parts.
  • Its irregular arrival and retreat (sometimes due to the effect of EL Nino), cause disruption .of farming schedules and cause droughts in certain areas of the country.
The Seasons
 
  • The distinct, seasonal pattern is an important characteristic of the monsoon type of climate.
  • The weather conditions in India greatly change from one season to another.
  • 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.

There are basically four seasons identified in India:

Cold Weather Season:

DURATION: It extends from December to February.
TEMPERATURE: Vertical sun rays shift towards the southern hemisphere. North India experiences intense cold whereas this season is not well defined in south India. Days are warm and nights are cold. Frost is common in the north and the higher slopes of the
Himalayas experience snowfall.
WIND: the northeast trade winds prevail over the country. They blow from land to sea and hence, for most parts of the country, it is a dry season. North East trade wind Light wind blow makes this season pleasant in south India. Occasional tropical cyclone visit
eastern coast in this season.
RAINFALL: Northwestern part receives rainfall due to western disturbances whereas Tamil Nadu coast receive rainfall by North East trade winds which blow here from sea to land.

Hot Weather Season:

DURATION: It extends from March to May.
TEMPERATURE:

  • The summer months experience rising temperatures and falling air pressure in the northern part of the country.
  • Temperature rises gradually from south to north.
  • Highest Temperature experiences in Karnataka in March, Madhya Pradesh in April and Rajasthan in May due to apparent movement of the sun.

AIR PRESSURE: Intense heating in North West India creates a low-pressure regions.


WIND AND STORMS:

  • ‘Loo’ -These are strong, gusty, hot, dry winds blowing during the day over north and northwestern India.
  • Kaal Baisakhi- This relates to localized thunderstorms, associated with violent winds, torrential downpours, often accompanied by hail. The name is used in West Bengal because it occurs in the month of Baisakh.

RAINFALL:

  • Mango shower-These are pre-monsoon showers common especially in Kerala and Karnataka. They help in the early ripening of mangoes.

Advancing Monsoon (Rainy Season)

  • It extends from June to September.
  • Intense heating in North West India creates a low-pressure region.
  • Low pressure attracts the wind from the surrounding region.
  • After having rains for a few days sometimes monsoon fails to occur for one or more weeks is known as a break in the monsoon.

Mechanism of Monsoon

  • By early June, the low-pressure condition over the northern plains intensifies.
  • It attracts the trade winds of the southern hemisphere.
  • These southeast 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.
  • 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 northwest, the monsoon winds cover the country in about a month.

Rainfall in the Western Ghats and Deccan Plateau
The monsoon winds cover the country in about a month. A total change in weather is brought up in India by the inflow of the South-West monsoon in India. The windward side of the Western Ghats receives very heavy rainfall, more than 250 cm in the early season. Inspite lying in the rain shadow area, the Deccan Plateau and parts of Madhya Pradesh also receive some amount of rainfall.

Retreating/Post Monsoon Season (The Transition Season)

  • It extends from October to November.
  • Vertical sun rays start shifting towards the Southern hemisphere.
  • Low pressure region shift from northern parts of India towards the south.
  • Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive. This is commonly known as the ‘October heat’.
  • Tropical cyclones frequently struck the eastern coast of India.

Distribution of Rainfall

  • Maximum rainfall receiving area: The windward side of the Western Ghats receives very heavy rainfall, more than 250 cm.
  • The maximum rainfall of this season is received in the northeastern part of the country.
  • Mawsynram in the southern ranges of the Khasi Hills receives the highest average rainfall in the world.
  • Low rainfall receiving area: Rainfall is low in the Deccan plateau, east Sahyadris, Leh, Rajasthan, and Gujarat.
  • Moderate rainfall receiving area: The rest of the part of India receives moderate rainfall.
  • Rainfall in the Ganga valley decreases from the east to the west.

Effects of monsoon rainfall in India:

  • Indian agriculture is largely dependent upon the water from the monsoon rains
  • Late, low or excessive rains have a negative impact on crops.
  • Due to the uneven distribution of rainfall across the country, there are some regions That Are Drought Prone And Some That Are Flood affected.
  • The monsoon provides India with a distinct climatic pattern.
  • It sometimes disturbs the farming schedule of millions of farmers all over the country.
  • Hence, in spite of the presence of great regional variations, it has a unifying influence on the country and its people.

Monsoons act as a unifying bond

  • Most parts of India receive rainfall from these winds.
  • The life of the people, including festivals and other economic and social activities revolve around the monsoon.
  • India is agricultural land. Monsoons are vital in agricultural production.
  • The Indian landscape, its animal and plant life, its entire agricultural calendar and the life of the people revolve around this phenomenon.
  • These monsoon winds bind the whole country by providing water to set agricultural activities in motion.
  • The river valleys which carry this water also unite as a single river valley unit.
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