NCERT Summary: Climate - 1 | IBPS PO Prelims & Mains Preparation - Bank Exams PDF Download

Introduction 

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.

Weather and Climate

Think of weather as what's happening outside on a particular day—whether it's hot, cold, rainy, or windy. Climate, on the other hand, is the average weather conditions over a longer period.

Elements of Weather 

These are the things that make up our weather and climate:

• Temperature: How hot or cold it is.
• Atmospheric Pressure: The weight of the air around us.
• Wind: How fast the air is moving.
• Humidity: How much moisture is in the air.
• Precipitation: This includes rain, snow, and other forms of water falling from the sky.

Seasons

Over a year, we notice patterns in weather, and we divide the year into seasons:

• Summer: Hot temperatures, like 50ºC in the Rajasthan desert.
• Winter: Cold temperatures, with places like Dras in Jammu and Kashmir reaching as low as -45ºC.
• Rainy Seasons: Some areas get a lot of rain, like in the Himalayas where it falls as snow, while other places might have a general rainy season.

Variety Across India

India is diverse, and the weather shows it:

• Temperature Differences: Rajasthan can be scorching at 50ºC in summer, while Jammu and Kashmir can be cool at 20ºC.
• Rainfall: Meghalaya gets a lot of rain (400ºC annually), while Ladakh and West Rajasthan get much less (10ºC annually).
• Coastal vs. Inland: Coastal areas have more consistent rainfall, while inland areas experience more seasonal changes.

Unity in Diversity

Indians show unity despite these differences:

• Food, Clothing, Housing: People adapt their food, clothing, and homes based on the weather where they live.
• Culture: Different regions have unique cultures, yet there is a sense of unity in being Indian.

In simple terms, weather changes day to day, but we can see patterns over weeks and months, leading to the different seasons we experience. And despite the diverse weather, Indians come together in their own ways.

Factors determining the Climate of India 

India’s climate is controlled by a number of factors which can be broadly divided into two groups-

• factors related to location and relief.
• factors related to air pressure and winds.

(a) Factors Related to Location and Relief

• Latitude: Imagine India is like a sandwich. The middle part is hotter because it's closer to the middle of the Earth (equator). The top part is not as hot because it's farther from the middle. So, the weather in the north is extreme, changing a lot, while the south has more consistent high temperatures.
• Himalayan Mountains: Think of the Himalayas as a giant wall in the north of India. This wall protects India from really cold winds and makes the rains (monsoons) stay inside the country.
• Distribution of Land and Water: India is surrounded by the ocean on three sides in the south and has big mountains in the north. The ocean doesn't change its temperature quickly, but the land does. This causes different pressure in the air, making the winds change direction and bring rain in different seasons.
• Distance from the Sea: Places near the ocean have more stable and mild weather, like Mumbai. But places in the middle of India, far from the ocean, have more extreme weather with hot summers and cold winters, like Delhi.
• Altitude: Imagine going up a mountain. The higher you go, the cooler it gets. So, places in the mountains are cooler than places on flat land. For example, Agra is warmer than Darjeeling, even though they are at the same latitude.
• Relief: The shape of the land affects the weather. Places facing the wind get more rain, like the western side of the mountains. But places on the other side get less rain, like the southern plateau.

So, India's weather is like a mix of its location, the shape of the land, and how close it is to the ocean.

(b) Factors Related to Air Pressure and Wind

1. Air Pressure and Winds:

• Imagine the Earth having different pockets of air pressure, like high and low-pressure areas.
• Winds are like the air moving from one place to another. They move because of differences in air pressure.
• During winter and summer, these pressure and wind patterns change, affecting the local climates.

2. Upper Air Circulation and Global Weather:

• Picture the sky having layers above us. There are high-altitude winds and air masses moving around globally.
• These upper air movements are influenced by things happening around the world.
• They play a role in how air moves in and out of different regions, impacting local weather in India during winter and summer.

3. Western Cyclones and Tropical Depressions:

• Think of cyclones as big swirling storms in the ocean, and depressions as similar, but less intense.
• In winter, there are disturbances from the west that bring changes in weather conditions.
• During the summer monsoon, tropical depressions influence rainfall patterns in India.

So, in simple terms, the air pressure and winds around us, upper air movements globally, and the influence of cyclones and depressions all work together to create the different weather conditions we experience in India during winter and summer.

Surface pressure and winds

1. High Pressure in Central and Western Asia:
   • Think of a big high-pressure area like a mountain of air over Central and Western Asia.
   • This high-pressure zone north of the Himalayas affects the weather in India during winter.
2. North Winds Towards India:
   • Picture air flowing like a river from the north towards India, south of the Himalayas.
   • This air is dry and brings a cool continental influence.
3. Interaction with Trade Winds:
   • Now, imagine this dry air meeting the regular trade winds in northwestern India.
   • Sometimes, this meeting point shifts. When it does, the dry air covers a larger area, reaching up to the middle Ganga valley.

In simpler terms, during winter, a giant air mound over Central Asia pushes dry air southward. This dry air meets the usual winds in northwestern India, impacting the weather. Sometimes, this meeting point shifts east, affecting a larger part of northwestern and northern India.

Jet Stream and Upper Air Circulation

1. Big Air Mountain in Central Asia:
   • Imagine a huge pile of air, like a mountain, over Central and Western Asia.
   • This "air mountain" affects India's winter weather.
2. Cool, Dry Wind from the North:
   • Think of air moving like a river from the north towards India, below the Himalayas.
   • This air is dry and brings a cool feeling to the region.
3. Meeting Regular Winds in Northwest India:
   • Now, see this dry air meeting the usual winds in northwest India.
   • Sometimes, this meeting point moves. When it does, the dry air covers more areas, even reaching the middle Ganga valley.

In simple terms, during winter, a massive pile of air over Central Asia pushes dry and cool air towards India. This dry air mixes with the regular winds in northwest India, and sometimes it covers a larger part of the region.

Western Cyclonic Disturbance and Tropical Cyclones

1. Winter Cyclones:
   • Imagine disturbances like storms coming into India from the west and northwest during winter.
   • These disturbances start over the Mediterranean Sea and are carried by a fast-flowing air current called the westerly jet stream.
   • Warmer nights can signal that these cyclones are on their way.
2. Tropical Cyclones:
   • Now, picture big storms forming over the warm waters of the Bay of Bengal and the Indian Ocean.
   • These storms are called tropical cyclones and are powerful, with strong winds and heavy rainfall.
   • They often hit the coasts of Tamil Nadu, Andhra Pradesh, and Orissa.
   • Due to their intense winds and heavy rain, these cyclones can be very destructive.

In simple terms, during winter, we get disturbances from the west, while tropical cyclones in the Bay of Bengal and the Indian Ocean bring heavy rain and strong winds to certain coastal areas.

Question for NCERT Summary: Climate - 1

Try yourself:

What is the main factor that determines the climate of a place in India?

• A.

  Latitude
• B.

  Altitude
• C.

  Distance from the sea
• D.

  Relief

Explanation

- India's climate is mainly determined by its location and relief factors.
- The latitude of a place plays a significant role in its climate.
- Places closer to the equator, like the middle part of India, experience hotter temperatures compared to places farther away.
- The north of India has extreme weather conditions due to its proximity to the middle of the Earth.
- The south of India, being farther from the equator, has more consistent high temperatures.
- Therefore, the main factor determining the climate of a place in India is its latitude.

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Inter Tropical Convergence Zone (ITCZ)

• Think of ITCZ as a low-pressure area at the equator where trade winds meet, causing air to rise.
• Two factorsinfluence it:
  • A) Idealized winds created by pressure and the Coriolis force.
  • B) Actual wind patterns due to how land is spread across the Earth.

A) Idealized winds generated by pressure gradient and Coriolis Force. B) Actual wind patterns owing to land mass distribution.

1. July - Southwest Monsoon:
   • In July, ITCZ is around 20ºN (over the Gangetic plain), known as the monsoon trough.
   • This trough helps create a thermal low over north and northwest India.
   • The ITCZ shift prompts trade winds from the southern hemisphere to cross the equator (40ºE to 60ºE) and blow from southwest to northeast due to the Coriolis force, creating the southwest monsoon.
2. Winter - Northeast Monsoons:
   • In winter, ITCZ moves southward.
   • This shift causes winds to reverse, blowing from northeast to south and southwest.
   • These winds are called northeast monsoons.

In simple terms, the ITCZ is a key player in the monsoon seasons. In July, it encourages the southwest monsoon, while in winter, its southward movement brings the northeast monsoons.

Mechanism of Weather in The Summer Season

Surface Pressure and Wind

• As the summer sets in and the sun shifts northwards, the wind circulation over the subcontinent undergoes a complete reversal at both, the lower as well as the upper levels. By the middle of July, the low pressure belt nearer the surface (termed as Inter Tropical Convergence Zone (ITCZ) shifts northwards, roughly parallel to the Himalayas between 20ºN and 25ºN. By this time, the westerly jet stream withdraws from the Indian region.
  
  
• In fact, meteorologists have found an interrelationship between the northward shift of the equatorial trough (ITCZ) and the withdrawal of the westerly jet stream from over the North Indian Plain. It is generally believed that there is a cause and effect relationship between the two. The ITCZ being a zone of low pressure attracts inflow of winds from different directions. The maritime tropical air mass (mT) from the southern hemisphere, after crossing the equator, rushes to the low pressure area in the general southwesterly direction. It is this moist air current which is popularly known as the southwest monsoon.

• The pattern of pressure and winds as mentioned above is formed only at the level of the troposphere. An easterly jet stream flows over the southern part of the Peninsula in June, and has a maximum speed of 90 km per hour.
• In August, it is confined to 15ºN latitude, and in September up to 22ºN latitudes. The easterlies normally do not extend to the north of 30ºN latitude in the upper atmosphere.

• The easterly jet stream steers the tropical depressions into India. These depressions play a significant role in the distribution of monsoon rainfall over the Indian subcontinent.
• The tracks of these depressions are the areas of highest rainfall in India. The frequency at which these depressions visit India, their direction and intensity, all go a long way in determining the rainfall pattern during the southwest monsoon period.

The Nature of Indian Monsoon

Understanding the Monsoon:

• The monsoon is a familiar but somewhat mysterious weather pattern.
• Scientists have observed it for centuries, but it still puzzles them.
• Despite many attempts, no single theory fully explains the monsoon.

Recent Breakthrough:

• Recently, scientists looked at the monsoon on a global scale instead of just regionally.
• This broader perspective provided a breakthrough in understanding.

Studying Causes in South Asia:

• Scientists systematically studied what causes rainfall in South Asia.
• This helped understand key aspectsof the monsoon, like:
  • The start of the monsoon.
  • Rain-bringing systems, for example, tropical cyclones, and how they relate to monsoon rainfall.
  • Breaks in the monsoon.

In simpler terms, scientists are working to unravel the mysteries of the monsoon. They've made progress by looking at it globally and studying specific causes in the South Asian region, helping to understand when it starts, how rain-bringing systems affect it, and why there are breaks in the monsoon.

Onset of the Monsoon

Summer Heating and Monsoon Winds:

• Back in the late 1800s, people thought that the difference in heating between land and sea in summer was the reason for the monsoon winds towards the subcontinent.

April and May - Intense Heating:

• In April and May, when the sun is directly over the Tropic of Cancer, the northern part of the Indian Ocean gets very hot.
• This intense heating creates a powerful low-pressure area in the northwest of the subcontinent.

High Pressure in the Indian Ocean:

• In the south of the landmass, the Indian Ocean has high pressure because water heats up more slowly than land.
• This difference attracts southeast trade winds from across the Equator.

Formation of Monsoon Winds:

• The low-pressure area in the north and the high-pressure area in the south cause the southeast trade winds to shift northward.
• This shift in the position of the Intertropical Convergence Zone (ITCZ) helps form the southwest monsoon winds.
• These winds cross the Equator between longitudes 40ºE and 60ºE.

Role of Jet Streams:

• The movement of the ITCZ is connected to the withdrawal of the westerly jet stream from the north Indian plain, south of the Himalayas.
• Once the western jet stream withdraws, the easterly jet stream takes its place along 15ºN latitude and contributes to the burst of the monsoon in India.

Entry of Monsoon into India:

• The southwest monsoon usually begins over the Kerala coast by June 1st and quickly moves to Mumbai and Kolkata between June 10th and 13th.
• By mid-July, the southwest monsoon covers the entire subcontinent.

In simpler terms, the monsoon winds are set in motion by the intense heating of the land and sea in summer. This creates a low-pressure area, drawing in winds that eventually become the southwest monsoon, reaching different parts of India by June and July.

Rain-bearing Systems and Rainfall Distribution

Two Rain-bearing Systems in India:

• Bay of Bengal System: One source of rainfall originates in the Bay of Bengal, bringing rain to the plains of northern India.
• Arabian Sea System (Southwest Monsoon): The second system is the Arabian Sea current of the southwest monsoon, which brings rain to the west coast of India.

Rainfall along the Western Ghats: The Western Ghats experience a lot of rainfall, mainly due to the obstruction of moist air, forcing it to rise along the Ghats. This type of rainfall is called orographic.

Factors Affecting West Coast Rainfall Intensity: The intensity of rainfall on the west coast is influenced by two main factors: 

(i) Offshore meteorological conditions. 

(ii) The position of the equatorial jet stream along the eastern coast of Africa.

Variability in Rainfall Frequency:

• Tropical depressions from the Bay of Bengal contribute to rainfall, and their frequency changes from year to year.
• The path of these depressions over India is linked to the position of the Intertropical Convergence Zone (ITCZ), known as the monsoon trough.
• Fluctuations in the monsoon trough's axis lead to variations in the track and direction of depressions, affecting the amount and intensity of rainfall.

Rainfall Patterns:

• Rainfall in India comes in spells and follows a declining trend from west to east over the west coast.
• Over the North Indian Plain and the northern part of the Peninsula, the trend is from southeast towards the northwest.

In simpler terms, India experiences rainfall from two main systems - one from the Bay of Bengal and the other from the Arabian Sea. The Western Ghats get a lot of rain due to obstructed moist air. The intensity of west coast rainfall depends on offshore conditions and the equatorial jet stream. The variability in rainfall is influenced by the frequency and paths of tropical depressions, which, in turn, are connected to the position of the monsoon trough.

EI-Nino and the Indian Monsoon

El Niño is a complex weather system that occurs every three to seven years, causing various weather extremes globally, such as droughts and floods.