CLIMATE

CLIMATE

Meaning of Climate

The term “climate” refers to the long-term atmospheric conditions that prevail over a specific region. Unlike weather, which can change daily or even hourly, climate is based on the average of weather data collected over an extended period—typically 30 years. This allows for a more stable understanding of the environmental trends of a region.

Climate vs. Weather

A critical distinction exists between climate and weather. While both relate to atmospheric conditions, they vary in terms of time scale and impact:

• Climate: Long-term phenomena, which include average temperatures, precipitation, and wind patterns.
• Weather: Short-term phenomena that describe the conditions we experience on a day-to-day basis, such as rain or sunshine.

CLIMATE IN RAJASTHAN

Rajasthan, being one of the largest states in India, predominantly experiences a sub-tropical climate. This means the region sees extreme weather patterns, such as scorching summers, cold winters, and very little rainfall in some parts.

Direct and Slanting Rays of the Sun

The sun’s position significantly influences the temperature distribution across Rajasthan. Depending on the angle of the sun’s rays, some regions experience direct sunlight, while others receive slanted rays.

• Direct Rays: Areas like Banswara experience the direct rays of the sun, leading to intense heat, particularly in summer. The direct exposure contributes to high temperatures that are typical of desert or semi-arid regions.
• Slanting Rays: In contrast, Ganganagar, located in the northern part of Rajasthan, receives slanted rays of the sun. This results in less intense heat compared to areas receiving direct rays. However, Ganganagar still experiences significant temperature fluctuations, characteristic of Rajasthan’s desert climate. 

Comparison of Direct and Slanting Sun Rays in Rajasthan

Region	Type of Sun Rays	Impact on Climate
Banswara	Direct	Extremely hot summers
Ganganagar	Slanting	Moderate but fluctuating temperatures

This table illustrates how different parts of Rajasthan experience varying intensities of sunlight, influencing the overall climate.

CLIMATE CLASSIFICATION

Climate classification helps to categorize the state of Rajasthan into different climatic zones based on various parameters such as temperature, rainfall, vegetation, and other climatic factors. Rajasthan’s climate is largely shaped by its geographical location, with parts of the state falling under tropical and sub-tropical zones.

Rajasthan’s climate can be further classified based on factors such as temperature, humidity, and seasonal rainfall. The state is divided into three broad climatic zones:

1. Arid Region:
   • Covers the western part of Rajasthan, including cities like Jaisalmer and Barmer.
   • Characterized by extremely hot summers, mild winters, and minimal rainfall.
2. Semi-Arid Region:
   • Found in central Rajasthan, including cities like Jaipur and Jodhpur.
   • Receives slightly more rainfall than the arid regions but still faces extreme heat.

 

1. Humid Region:
   • Predominantly in southern Rajasthan, near the Aravalli Range.
   • Cities like Udaipur experience a relatively more moderate climate, with a fair amount of rainfall, especially during the monsoon season.

 

1. General Classification

According to general climate classification, Rajasthan’s climate is divided into five parts based on rainfall. These classifications range from arid to very humid climatic conditions, which are critical in determining the vegetation, agricultural potential, and living conditions across the state.

The diagram shows Rajasthan’s division into arid, semi-arid, and humid climatic zones.

Climatic Zones of Rajasthan (based on Rainfall)

• Arid Climate: Characterized by extremely low rainfall (below 250 mm annually). This region, covering western Rajasthan (Jaisalmer, Barmer), consists mostly of deserts.
• Semi-Arid Climate: Receives moderate rainfall (250-500 mm annually) and includes areas such as Jodhpur and Bikaner.
• Sub-Humid Climate: Covers regions like Jaipur and Ajmer, with rainfall ranging from 500 to 800 mm annually.
• Humid Climate: Found in regions like Udaipur where annual rainfall ranges from 800 to 1200 mm.
• Very Humid Climate: Southern Rajasthan, especially parts of the Aravalli Range, experiences heavy rainfall, often exceeding 1200 mm. Banswara is an example.

Climate zones of Rajasthan based on rainfall and their physical division. The table below illustrates the relationship between climate types, rainfall levels, and the corresponding physical divisions in Rajasthan:

Climate	Rainfall (in cm)	Physical Division
Arid Climate	0 – 20	North-Western Desert
Semi-Arid Climate	20 – 40	North-Western Desert
Sub-Humid Climate	40 – 60	Aravalli
Humid Climate	60 – 80	Eastern Plain
Very Humid Climate	80 – 120	Hadoti

 

2. Individual Classification

In addition to the general classification, climate scientists and geographers often rely on specific climate classification systems that factor in temperature, evaporation, vegetation, and other climatic variables. The three major classification systems are:

Climate Classification Systems in Rajasthan

Classification System	Focus	Types in Rajasthan
Koppen	Temperature & Vegetation	4
Trewartha	Rainfall	4
Thornthwaite	Temperature, Evaporation, Rainfall	4

• Koppen Classification System

The Koppen Climate Classification is one of the most widely used systems for categorizing climates across the world. In Rajasthan, this system divides the climate into four primary categories based on vegetation, temperature, and rainfall.

• Bwhw: Represents the hot desert climate, where extreme temperatures and minimal vegetation are dominant features. This covers much of western Rajasthan.
• Bshw: Semi-arid climates fall under this category, characterized by slightly more vegetation and moderate rainfall. This includes parts of central Rajasthan.
• Cwg: Found in the eastern plains of Rajasthan, this climate type supports more significant vegetation due to higher rainfall.
• Aw: The tropical wet and dry climate, predominantly found in southern Rajasthan, especially in regions like Banswara and Dungarpur, where there is substantial seasonal rainfall.

Map of Koppen Climate Zones in Rajasthan

Map showing the division of Rajasthan into different climatic zones based on the Koppen Climate Classification. The regions are color-coded to represent different climate zones: red (Bwhw – hot desert), orange (Bshw – semi-arid), yellow (Cwg – humid), and blue (Aw – tropical wet and dry).

Textual Explanation for the Map:

• Red Zone (Bwhw): Hot desert climate dominates the Thar Desert and covers western Rajasthan, including Jaisalmer and Barmer. This zone is characterized by minimal rainfall and extreme temperature variations.
• Orange Zone (Bshw): The semi-arid climate zone extends into Jodhpur and parts of Bikaner. It experiences slightly more rainfall than the desert but is still prone to arid conditions.
• Yellow Zone (Cwg): This zone represents the humid subtropical areas of eastern Rajasthan, which includes Jaipur and surrounding regions. It receives more rainfall, allowing for more agricultural activity.
• Blue Zone (Aw): The tropical wet and dry climate, found in southern Rajasthan, including Banswara, experiences distinct wet and dry seasons, supporting a different type of flora and fauna compared to other parts of the state.

Koppen Climate Classification in Rajasthan

The Koppen Climate Classification divides Rajasthan into several climate zones based on factors such as temperature, vegetation, and rainfall. Each zone is associated with specific regions, types of vegetation, and unique geographical features. Let’s explore the primary classifications as applied to Rajasthan.

1. AW (Tropical Humid / Very Humid Climate)

• Climate: Tropical Humid or Very Humid Climate
• Vegetation: This region predominantly has Savanna-type vegetation, characterized by grasslands with scattered trees, typical of tropical climates with a marked wet and dry season.
• Geographical Extension:
  • The Vagad region, which includes Banswara, Dungarpur, and Pratapgarh, falls under this climate zone.
  • The Hadoti region, including Southern Kota, Baran, Jhalawar, and Bundi, also experiences this climate.
  • Additionally, Mount Abu, a famous hill station, exhibits a tropical humid climate.
• Special Characteristics:
  • The vegetation density is at its maximum in this region due to the high amount of rainfall. This supports more extensive agricultural activities and biodiversity compared to other regions of Rajasthan.

2. BWhw (Arid Desert Type Climate)

• Climate: Arid Desert Climate, characterized by extremely hot temperatures and low rainfall.
• Vegetation: Vegetation in this zone is adapted to arid conditions, with Xerophytes (plants that can survive with little water) and thorny bushes dominating the landscape.
• Geographical Extension:
  • The Thar Desert region, encompassing Jaisalmer, Bikaner, and Churu, primarily falls under this category.
• Special Characteristics:
  • This region experiences minimum rainfall and supports thorny vegetation, which is well-adapted to the harsh desert environment. The extreme aridity leads to desertification processes that challenge agriculture and settlement.

3. BSHw (Steppe or Semi-Arid Climate)

• Climate: Steppe-type or Semi-Arid Climate, characterized by moderate rainfall and larger grassland areas.
• Vegetation: This zone is dominated by Steppe vegetation, which includes small grasslands with sparse trees or shrubs.
• Geographical Extension:
  • The Luni Basin, Nagaur, Shekhawati, and parts of the Ghaggar Basin experience this semi-arid climate.
• Special Characteristics:
  • This is the largest climate zone in Rajasthan according to the Koppen classification, covering a vast area of semi-arid land. While this zone experiences relatively more rainfall than the desert region, it still faces significant challenges related to water scarcity and sustainable agriculture.

4. Cwg (Sub-Tropical Sub-Humid Climate)

• Climate: Sub-Tropical Sub-Humid Climate, characterized by a distinct dry season and a marked wet season with monsoonal rainfall.
• Vegetation: This region supports Dry Deciduous Vegetation, where trees shed their leaves during the dry season to conserve moisture. Typical vegetation includes species like teak, sal, and various hardwood trees.
• Geographical Extension:
  • The Aravalli Range and surrounding regions, including Alwar, Bharatpur, Karoli, Dholpur, and Ajmer, fall under this climate zone.
  • Other regions include Bhilwara, Bundi, Chittorgarh, Dausa, Tonk, Swai-Madhopur, Udaipur, and Jaipur.
• Special Characteristics:
  • Population density and agriculture production are the highest in this region due to favorable climatic conditions and better availability of water compared to the arid and semi-arid regions. This zone is crucial for Rajasthan’s agrarian economy, with crops like wheat, mustard, and pulses being widely cultivated.

 

Comparison of Climate Classification Systems in Rajasthan (Koppen vs. Trewartha)

Two major climate classification systems, Koppen and Trewartha, are often used to describe the climate zones in Rajasthan. Both systems categorize climates based on a variety of factors, such as temperature, vegetation, and rainfall, but they use slightly different parameters and categorizations. The table below highlights the relationship between these two systems, alongside the average rainfall levels in each climate zone.

Climate Classification and Rainfall in Rajasthan

Koppen Climate Type	Trewartha Climate Type	Average Annual Rainfall
Aw (Tropical Humid)	Aw (Tropical Wet)	100 cm
BWhw (Arid Desert)	BWh (Arid)	10 cm
BSHw (Steppe/Semi-Arid)	BSh (Semi-Arid)	30 cm
Cwg (Sub-Tropical Sub-Humid)	Caw (Sub-Tropical Humid)	70 cm

 

Table Explanation:

• Koppen Classification: This system divides Rajasthan’s climate into four main categories:
  • Aw: Tropical humid regions, such as parts of Hadoti and Mount Abu, receive about 100 cm of rainfall annually.
  • BWhw: The arid desert climate, typical of the Thar Desert region, receives minimal rainfall, only about 10 cm per year.
  • BSHw: The steppe or semi-arid climate covers regions like Nagaur and parts of the Shekhawati region, with moderate rainfall of 30 cm.
  • Cwg: Sub-tropical sub-humid climates, found in areas like Jaipur and Alwar, receive about 70 cm of annual rainfall, contributing to higher agricultural productivity.
• Trewartha Classification: While similar to the Koppen classification, Trewartha also categorizes Rajasthan’s climate into corresponding zones based on temperature and rainfall. The equivalent types are:
  • Aw: Same as in Koppen, representing tropical wet regions with high rainfall.
  • BWh: The arid desert zones, identical to Koppen’s BWhw, with extremely low rainfall.
  • BSh: Semi-arid regions with rainfall levels similar to Koppen’s BSHw.
  • Caw: Corresponds to Koppen’s Cwg, representing sub-tropical humid regions.

 

Rainfall Distribution:

• The tropical humid (Aw) regions in southern Rajasthan (like Banswara and Dungarpur) receive the highest annual rainfall, around 100 cm, supporting savanna-type vegetation and dense forests.
• The arid desert (BWhw) regions in the Thar Desert receive very little rainfall, approximately 10 cm annually, leading to sparse vegetation dominated by xerophytes and thorny plants.
• The semi-arid (BSHw) regions, including parts of Nagaur and Shekhawati, get moderate rainfall, around 30 cm, which supports grasslands and steppe vegetation.
• The sub-tropical sub-humid (Cwg) regions in the Aravalli Range and surrounding plains receive about 70 cm of rainfall per year, allowing for higher population density and extensive agricultural activities.
• Trewartha Climate Classification of Rajasthan

The Trewartha Climate Classification is another widely used system to categorize climate regions, focusing primarily on temperature, evaporation, and rainfall. According to this system, Rajasthan’s climate is divided into four parts. Each classification in Trewartha’s system corresponds to different levels of aridity and humidity, which are essential for understanding the state’s diverse geographical features.

Key Classifications under Trewartha System:

The Trewartha system divides Rajasthan into the following climatic categories:

• A – Very Humid (Not Found in Rajasthan)
• B – Humid
• C – Sub-Humid
• D – Arid
• E – Semi-Arid

Map of Trewartha Climate Zones in Rajasthan:

The map below illustrates the spatial distribution of these climate zones in Rajasthan. It uses codes to represent the various climate types, with the following breakdown:

• CAww: Sub-humid climate zone
• DAww: Arid climate zone
• DBww: Humid climate zone
• EB’d4: Semi-arid climate zone
  

 

1. CAww (Sub-Humid Climate)

• Description: This climate zone experiences moderate levels of rainfall and temperature. It is characterized by a longer wet season compared to arid regions, contributing to better vegetation and agricultural opportunities.
• Location: This zone covers parts of southern Rajasthan, including the Hadoti region.
• Special Features: The sub-humid climate supports dry deciduous forests and mixed agriculture, making it a crucial zone for crops like wheat, barley, and mustard.

 

2. DAww (Arid Climate)

• Description: This region faces arid conditions with extremely low rainfall and high evaporation rates. The climate is marked by hot summers and cooler winters with minimal precipitation.
• Location: The Thar Desert dominates this climate zone, covering regions like Jaisalmer and parts of Barmer and Bikaner.
• Special Features: Due to its harsh climate, this area supports xerophytic vegetation, such as thorny bushes and cacti, and faces significant challenges in agriculture and water availability.

 

3. DBww (Humid Climate)

• Description: The humid climate zone receives more rainfall than the arid and semi-arid regions. The increased moisture contributes to richer biodiversity and better agricultural yields.
• Location: Found in the northern and northwestern parts of Rajasthan, covering areas near the Aravalli range.
• Special Features: Humid climates enable better forest cover and crop cultivation, especially during the monsoon season.

 

4. EB’d4 (Semi-Arid Climate)

• Description: Semi-arid zones are transition areas between arid and sub-humid climates. They receive limited rainfall but enough to support certain types of vegetation, such as grasses and small shrubs.
• Location: This climate zone is found in western Rajasthan, extending into parts of Nagaur, Shekhawati, and the Luni basin.
• Special Features: While this area faces water scarcity, it is still suitable for pastoralism and the cultivation of drought-resistant crops like millets and pulses.

Thornthwaite Climate Classification and Seasonal Climate of Rajasthan

The Thornthwaite Climate Classification is a system that divides regions based on temperature, evaporation, and rainfall. 

It provides a detailed perspective on the climate of Rajasthan, which is influenced by geographical diversity and seasonal variations. Thornthwaite’s classification is considered more authentic for Rajasthan’s climatic assessment, as it focuses on the balance between moisture availability and demand in the region.

1. Thornthwaite Climate Zones in Rajasthan

(i) CAww – Humid Climate

• Climate: This region experiences a humid climate, with moderate rainfall and relatively lower evaporation compared to the arid regions.
• Extension: The humid climate zone extends to southern regions of Rajasthan, including Banswara, Dungarpur, Pratapgarh, Jhalawar, Baran, and South Kota.
• Special Note: This zone supports dense vegetation and is crucial for agricultural activities, including the cultivation of staple crops such as wheat and rice.

(ii) DAww – Semi-Arid Climate

• Climate: The semi-arid climate region experiences moderate to low rainfall, with higher evaporation rates. This climate supports some agriculture, though it is prone to droughts.
• Extension: This climate zone covers a large portion of Alwar, Bharatpur, Karauli, Dholpur, Ajmer, Bundi, Bhilwara, Chittorgarh, Dausa, Nagaur, and Pali.
• Special Note: The DAww zone is the largest climatic region in Rajasthan according to the Thornthwaite classification, covering a vast area that supports dry deciduous forests and mixed agriculture.

(iii) DBww – Arid and Semi-Arid Climate

• Climate: This zone represents a transition between arid and semi-arid climates, characterized by low rainfall and extreme temperatures.
• Extension: The DBww climate zone covers parts of Bikaner, Churu, Ganganagar, and Hanumangarh.
• Special Note: This zone faces significant challenges related to water scarcity, making irrigation critical for agriculture.

(iv) EAd4 – Dry Climate

• Climate: This zone is dominated by a dry desert climate, with extremely low rainfall and very high evaporation rates, making it one of the harshest climatic regions in Rajasthan.
• Extension: This climate is found in the westernmost parts of Rajasthan, including Jaisalmer, Barmer, and parts of Western Jodhpur.
• Special Note: The EAd4 zone represents the desert core of the Thar Desert, where vegetation is scarce and only drought-resistant crops can be grown.

Climate Season Classification in Rajasthan

Rajasthan experiences four major seasons throughout the year, each characterized by distinct weather patterns:

(i) Summer Season (March – June)

• The summer season is known for its extreme heat, with hot winds known as Loo sweeping across the region.
• Loo: These are hot and dry winds that blow during the peak of summer. They are known to cause a significant rise in temperature, often leading to heatwaves.
  • Cause of Loo: The phenomenon occurs due to the horizontal or advection flow of air, which carries hot, dry air across the state.
  • Special Note: The maximum effect of Loo is felt in Barmer, where temperatures can exceed 45°C during summer.
• Additional characteristics include sandstorms and Bhabhulya (local sand-based weather phenomena).

(ii) Rainy Season (June – September)

• This is the monsoon season in Rajasthan, where the state receives the majority of its annual rainfall.
• The regions along the Aravalli Range receive relatively higher rainfall during this season, whereas western Rajasthan gets limited rainfall.
• Key Features: The onset of the monsoon transforms the dry and arid landscapes into green agricultural fields in regions that receive sufficient rainfall.

(iii) Autumn Season (October – November)

• This period marks the retreating monsoon, with decreasing rainfall and a gradual rise in temperature.
• October Heat: Although rainfall decreases, temperatures can temporarily rise again, known as October Heat, before transitioning into the cooler winter months.
• Retreating Monsoon: Winds shift from southwest to northeast, leading to a drop in humidity.

(iv) Winter Season (December – February)

• The winter season brings a sharp drop in temperatures, particularly in the northern and western parts of Rajasthan.
• Mawath: Unseasonal winter rains, locally known as Mawath, can sometimes occur during this season, which is crucial for winter crops like wheat.
• Temperatures in regions like Mount Abu can fall below freezing, while Jaisalmer and Barmer still experience mild winters due to their desert climates.

WEATHER PHENOMENA AND TEMPERATURE VARIATIONS IN RAJASTHAN

Rajasthan, known for its extreme climate, particularly in the summer months, experiences various unique weather phenomena such as sandstorms and sand cyclones. Additionally, the state records significant temperature variations between different regions, influenced by geographical factors like altitude and proximity to the desert.

1. Sandstorms

• Definition: A sandstorm refers to a dusty and humid wind that blows across desert regions, particularly during the summer season. It typically arises when wind speeds are high, and there is an abundance of loose sand and dust in the atmosphere.

Causes of Sandstorms:

• Convection flow of air: This is the vertical movement of hot air rising from the ground, which lifts dust and sand particles, creating a storm.
• Vertical flow of air: A similar upward movement of air, which can pull dust from the surface and carry it over long distances.
• Cooling phenomenon: Sandstorms often contribute to a slight decrease in temperature, as the movement of dust and air blocks some of the sun’s direct heat.
• Key Note:
  • Ganganagar experiences the maximum effect of sandstorms in Rajasthan, with sandstorms occurring for as many as 27 days during the peak summer season.

2. Sand Cyclones

• Definition: A sand cyclone is a more intense phenomenon where sandy, dusty winds swirl in a cyclonic motion, caused by extreme differences in air pressure.

Causes of Sand Cyclones:

• High temperature and low air pressure: The center of the cyclone experiences intense heat and low pressure, which causes surrounding air to move inwards and rotate, lifting dust and sand in the process.
• Opposite air pressure: The air pressure on the outer edges of the cyclone is higher, causing a rotation that gives the phenomenon its cyclonic nature.
• Key Note:
  • The maximum impact of sand cyclones is experienced in Bikaner, which is located in the heart of the Thar Desert.

3. Temperature Extremes in Rajasthan

Rajasthan is known for its wide range of temperatures, particularly during the summer. The state’s climate is characterized by hot desert conditions in some regions and cooler temperatures in higher-altitude areas.

Highest Summer Temperature:

• Location: The hottest temperatures in summer are recorded in Phalodi (Jodhpur district) and Churu district. Both regions experience extreme heat due to their desert location.
• Notable Records:
  • Phalodi once recorded a temperature as high as 51°C, making it one of the hottest places in India.

Lowest Summer Temperature:

• Location: The coolest summer temperatures are recorded in Mount Abu (Sirohi district), which is a hill station in southwest Rajasthan.
• Reason:
  • The high altitude of Mount Abu results in cooler temperatures, even during the peak of summer. This is in contrast to the surrounding desert plains that experience extreme heat.

Temperature Range in Rajasthan:

• Annual Range: Churu holds the record for having the maximum range of temperature variations throughout the year. It experiences scorching hot summers and relatively cold winters.
• Daily Temperature Variations: Jaisalmer, located in the Thar Desert, exhibits significant temperature fluctuations on a daily basis, with extremely hot days and cooler nights due to the desert’s heat retention properties.

4. Monsoon in Rajasthan and India

The monsoon season is a significant climatic event that brings most of the annual rainfall to Rajasthan and the Indian subcontinent. This section delves into the origin and characteristics of the monsoon, particularly focusing on the southwest monsoon winds.

Rainy Season in Rajasthan: The Monsoon

(a) Monsoon: Definition and Origin

• The word monsoon is derived from the Arabic word “Mausim,” which means season. This refers to the seasonal winds that reverse direction depending on the time of year, bringing heavy rainfall during the summer months to much of India, including Rajasthan.
• The concept of the monsoon was first introduced by Al-Masudi, a medieval Arab geographer and traveler, who is often credited with coining the term monsoon in reference to the seasonal weather patterns of the Indian Ocean region.

 

(b) Southwest Monsoon: Key Features

• The southwest monsoon is the primary source of rainfall in Rajasthan and across the Indian subcontinent. It generally arrives in June and lasts until September.
• Mechanism: During summer, as the land heats up, a low-pressure system forms over the Indian subcontinent, particularly over the Thar Desert. This draws in moist air from the Indian Ocean, resulting in the southwest monsoon winds.
• Characteristics:
  • The monsoon winds are warm and moisture-laden, causing widespread rainfall as they move over the subcontinent.
  • The diagram provided shows how the southwest monsoon winds enter the Indian subcontinent from the Indian Ocean and move inland, affecting regions from Kerala to Rajasthan.

Southwest Monsoon and Southeast Trade Winds

The diagram illustrates the movement of the southwest monsoon winds and their interaction with the southeast trade winds:

• Southwest Monsoon: These winds, coming from the southwest direction, blow from the Indian Ocean towards the Indian landmass, bringing the much-needed monsoon rains.
• Southeast Trade Winds: These are persistent winds that blow towards the equator and play a role in the global wind system, but the monsoon significantly alters their patterns during the summer months in India.

Range of Temperature in Rajasthan

In addition to rainfall, the monsoon also influences the range of temperature in Rajasthan. The range of temperature is defined as the difference between the maximum and minimum temperatures recorded over a given time period. In Rajasthan, this range can be extreme due to the state’s desert conditions, where temperatures may vary significantly between day and night.

MONSOON IN RAJASTHAN

The monsoon is a vital climatic event in India and Rajasthan, providing the necessary rainfall for agriculture and water resources. This section covers the arrival and retreating dates of the monsoon in India and Rajasthan, as well as the different branches of the monsoon that contribute to rainfall distribution.

1. Date of Monsoon: Arrival and Retreat

The monsoon in Rajasthan arrives later and retreats earlier than the rest of India due to its geographical position and the influence of the Aravalli Range. Below are the specific details:

Arrival Date

• India (Mainland): The monsoon typically arrives in mainland India by 1st June. The Malabar Coast (Kerala) is the first region to receive monsoon showers as the moist southwest winds from the Indian Ocean hit the coastal areas.
• Rajasthan: The monsoon reaches Rajasthan around 25th June (new) or 15th June (old records). The southernmost regions of Rajasthan, such as Banswara and Dungarpur, receive the first monsoon showers due to their proximity to the Western Ghats and the Arabian Sea branch of the monsoon.

Retreating Date

• India (North): In North India, including parts of Rajasthan, the monsoon begins to retreat by 1st October. The withdrawal of the monsoon marks the end of the rainy season and the onset of cooler autumn weather.
• Rajasthan: In Rajasthan, the monsoon retreats by 30th September, making it one of the first states to see an early end to the rainy season.
• Note: Rajasthan has a distinct monsoon pattern, characterized by its late arrival and early retreat, which affects the agricultural cycle and water availability in the region.

2. Branches of the Monsoon

The Indian monsoon splits into two main branches: the Arabian Sea branch and the Bay of Bengal branch. These branches distribute rainfall across the subcontinent, with both affecting Rajasthan to varying degrees.

Arabian Sea Branch

The Arabian Sea branch of the monsoon is further divided into smaller branches that affect different parts of western and northern India.

• Western Ghat Branch: This branch brings heavy rainfall to the Western Ghats, a mountain range that runs parallel to the Arabian Sea coast. This branch doesn’t directly affect Rajasthan but contributes to monsoonal winds in the western regions.
• Chota Nagpur Branch: The Chota Nagpur Plateau in eastern India receives rainfall from this branch. Its impact on Rajasthan is minimal.
• Himachal Branch: This branch brings the first monsoon rains to Rajasthan, particularly in the northern regions. However, the Aravalli Range runs parallel to this branch, limiting the rainfall in the central and western parts of the state.

Bay of Bengal Branch

The Bay of Bengal branch is a significant source of monsoon rains, particularly for eastern and northeastern India. Its influence on Rajasthan is secondary but notable.

• Great Northern Plain Branch: This branch provides rainfall to the northern plains of India, including parts of Rajasthan’s eastern regions. The winds from this branch hit the plains and mountains before reaching Rajasthan, often causing less rainfall by the time they arrive.
• Eastern Himalaya Branch: This branch brings moisture to the northeastern parts of India and the Himalayas. It doesn’t directly affect Rajasthan but influences the overall distribution of monsoon winds across the country.

Special Notes on Monsoon Impact in Rajasthan:

• The Himachal Branch of the Arabian Sea monsoon brings initial rains to Rajasthan. However, due to the parallel orientation of the Aravalli Range, this branch brings limited rainfall to the central and western parts of the state.
• The Western Great Plain Branch (part of the Arabian Sea branch) brings much of the rainfall to Rajasthan, especially the western parts.
• Purvai (Easterlies): The winds coming from the Bay of Bengal branch are referred to as Easterlies. These winds bring more rainfall to the eastern regions of the Aravalli Range, making this region more fertile and agriculturally productive compared to the arid west.

 

Impact of Monsoon in Rajasthan: Maximum and Minimum Rainfall

The monsoon’s impact in Rajasthan varies significantly across different regions due to the state’s geographical diversity and the influence of the Aravalli Range. The state experiences a wide range of annual rainfall, from almost zero in the western desert regions to significant amounts in the southeastern parts.

1. Maximum Rainfall in Rajasthan

• Jhalawar District:
  • Rainfall: Jhalawar, located in the southeastern part of Rajasthan, receives around 100 cm of rainfall annually, making it one of the wettest districts in the state.
  • The region benefits from the southwest monsoon, which brings heavy rains during the monsoon season. Its proximity to the Vindhya and Satpura mountain ranges also influences rainfall distribution.
• Mount Abu:
  • Rainfall: The hill station of Mount Abu, located in the Sirohi district, receives the highest rainfall in Rajasthan, with an annual average of 150 cm.
  • The elevated terrain of Mount Abu attracts significant monsoon showers, and the region enjoys a more temperate climate compared to the rest of the state.

2. Minimum Rainfall in Rajasthan

• Jaisalmer District:
  • Rainfall: Jaisalmer, located in the heart of the Thar Desert, experiences an average annual rainfall of only 10 cm, making it one of the driest places in Rajasthan.
  • The district’s location, far from the influence of the monsoon currents and shielded by the Aravalli Range, contributes to its extreme arid conditions.
• Sam Village:
  • Rainfall: The village of Sam, located near Jaisalmer, receives 0 cm of annual rainfall. It is part of the extreme arid zones of the Thar Desert, where precipitation is nearly nonexistent.

Average Rainfall in Rajasthan and India

• Average Annual Rainfall in Rajasthan: The overall average annual rainfall in Rajasthan is 57.5 cm, but this average is skewed by the extreme variations in rainfall between the arid and semi-arid regions.
• Average Annual Rainfall in India: In comparison, the average annual rainfall for India as a whole is around 125 cm. This highlights the stark difference in rainfall between Rajasthan and other parts of the country, particularly in the coastal and northeastern regions, which receive much more rainfall.

 

3. Impact of Direction on Monsoon Rainfall in Rajasthan

The amount of rainfall in Rajasthan is heavily influenced by the direction of the monsoon winds and the geography of the region, particularly the presence of the Aravalli Range.

Diagram Explanation: Impact of Monsoon Direction on Rainfall Distribution

• Southwest Monsoon: The southwest monsoon winds enter Rajasthan from the southwest and bring moisture-laden air from the Arabian Sea. The southern and eastern parts of the state, including Jhalawar and Mount Abu, receive the highest rainfall as these regions are more exposed to the incoming monsoon winds.
• Aravalli Range: The Aravalli Range runs diagonally across the state, from the southwest to the northeast. It acts as a barrier to the monsoon winds, causing areas on the eastern side of the range to receive more rainfall, while regions to the west of the range, such as Jaisalmer, remain dry.
• Rainfall Decrease: As the monsoon winds move from the southwest to the northeast, rainfall gradually decreases. The western parts of the state, such as Jaisalmer and Barmer, receive minimal rainfall due to the rain shadow effect created by the Aravalli Range.
• Easterlies: The easterly winds from the Bay of Bengal also contribute to the rainfall in the eastern parts of Rajasthan, adding to the higher precipitation in regions like Alwar and Bharatpur.

 

Events During the Monsoon in Rajasthan

The monsoon season in Rajasthan is marked by several key events, which influence the amount of rainfall, temperature fluctuations, and the overall climatic conditions during the rainy season. Below is a breakdown of the significant events:

1. Burst of Monsoon

• Definition: The burst of the monsoon refers to the sudden and heavy rainfall that occurs at the beginning of the monsoon season, signaling the arrival of the monsoon rains.
• Time Period: In Rajasthan, this typically happens in July and August. The burst brings much-needed moisture to the arid regions of the state, initiating the primary rainy season.
• Importance: The burst of the monsoon is critical for agriculture as it provides the first substantial rainfall after the dry summer, essential for crop cultivation.

2. Gap of Monsoon

• Definition: The gap of the monsoon refers to a period of no rainfall that occurs after the initial burst of monsoon. This dry spell usually lasts for two to three weeks, creating a temporary lull in the rainy season.
• Time Period: This gap occurs primarily in August or September, following the initial heavy rains. The gap can have significant implications for agriculture, especially in water-scarce regions like Rajasthan.

3. Retreating of Monsoon

• Definition: The retreating monsoon is the process of the monsoon winds withdrawing from Rajasthan and the rest of India. This signals the end of the rainy season and the beginning of the transition to autumn.
• Time Period: In Rajasthan, the monsoon typically starts retreating in October-November. The process is gradual, with the rain clouds moving away towards the southern and southeastern regions of the country.

4. Kartik Heat/October Heat

• Definition: The Kartik Heat, also referred to as October Heat, occurs when there is a rise in temperature during the retreat of the monsoon. This phenomenon results in warm and dry conditions, even though the monsoon season has not fully ended.
• Time Period: The Kartik Heat is mostly experienced in October, and it marks the transition between the rainy season and the upcoming winter.

Impact of Global Events on the Indian Monsoon

Global climatic events like El Niño and La Niña significantly affect the Indian monsoon, causing variations in its arrival, intensity, and distribution. These phenomena originate in the Pacific Ocean and have far-reaching impacts on weather patterns across the world, including India.

1. El Niño

• Meaning: El Niño is a hot ocean current that disrupts normal weather patterns by altering the trade winds and increasing the sea surface temperature in the eastern and central Pacific Ocean.
• Location: This event occurs near the eastern coast of the southern Pacific Ocean, between 30° S latitude to 24° S latitude.
• Time Period: El Niño usually forms in the last week of December and lasts for several months, often peaking during the winter.
• Effect on Monsoon:
  • Late arrival of monsoon: El Niño disrupts the normal flow of monsoon winds, often delaying the onset of the monsoon season.
  • Reduced rainfall: The overall intensity and distribution of monsoon rains are weakened, which can result in droughts and dry conditions across India, particularly affecting Rajasthan.
• Note: El Niño is also referred to as Ocean Fever or the Child of Christ because it typically develops around Christmas time.

2. La Niña

• Meaning: La Niña is the cold counterpart to El Niño, marked by cooler-than-normal ocean temperatures in the eastern Pacific Ocean.
• Location: Like El Niño, La Niña occurs near the eastern coast of the southern Pacific Ocean, around the same latitudinal range as El Niño.
• Time Period: La Niña also forms in the last week of December, but it has a different impact on global weather patterns.
• Effect on Monsoon:
  • Early arrival of monsoon: La Niña is associated with a stronger and more effective monsoon, often resulting in early rains.
  • More rainfall: The monsoon season during La Niña years tends to bring above-average rainfall, which can lead to floods or excess water, but is often beneficial for agriculture in Rajasthan.
• Note: La Niña is often called the younger sister of El Niño, as it represents the opposite climatic conditions.

 

CLIMATE EVENTS DURING AUTUMN AND WINTER IN RAJASTHAN

In Rajasthan, the autumn and winter seasons bring significant changes to the climate, with key events like the retreat of the monsoon, the phenomenon of Mawath (winter rainfall), and the influence of cold winds. These events are crucial for agriculture, particularly for crops like wheat, and they shape the seasonal cycles of the region.

1. Autumn Season

(i) Retreating of Monsoon

• Definition: The retreat of the monsoon refers to the withdrawal of the monsoon winds from Rajasthan and other parts of India. This marks the end of the rainy season and the transition towards cooler weather.
• Time Period: In Rajasthan, the retreat of the monsoon typically takes place during October and November. As the moisture-laden winds leave the region, the climate starts becoming drier, and the state enters a post-monsoon phase.

(ii) Kartik Heat/October Heat

• Definition: The Kartik Heat refers to the rise in temperature during the retreat of the monsoon. This increase in heat occurs after the rains have subsided, resulting in warm and dry conditions during the month of October.
• Impact: The Kartik Heat is noticeable in many parts of Rajasthan and can be particularly uncomfortable as it prolongs the heat before the onset of winter. However, it also allows for the drying of crops harvested during the monsoon season.

2. Winter Season

(i) Mawath (Winter Rainfall)

• Definition: Mawath refers to the rainfall that occurs during the winter season in Rajasthan, typically between December and March. This rainfall is a result of Western Disturbances, which are cyclones that originate from the Mediterranean Sea and bring moisture to northwestern India, including Rajasthan.
• Importance: Mawath is extremely beneficial for the wheat crops in Rajasthan, as it provides the necessary moisture during the critical growing period. Because of its positive impact on agriculture, the rain is often called Golden Drops by local farmers.
• Regions Affected: The Mawath rainfall is most prevalent in northern and western parts of Rajasthan, including districts like Churu, Sikar, and Bikaner.

(ii) Cold Winds

• Definition: During the winter months, cold winds flow into Rajasthan from the Himalayas. These winds bring a significant drop in temperatures, particularly in the desert regions, where nights can become freezing cold.
• Direction: The cold winds typically blow from the north-east to the south-west, affecting Rajasthan from December to January. These winds are part of the general winter pattern in northern India.
• Regions Affected: The impact of cold winds is most noticeable in Churu, which experiences the maximum drop in temperature, along with Sikar and Bikaner. These regions see a sharp contrast between daytime and nighttime temperatures during winter, making it one of the coldest areas in Rajasthan.

 

Other Important Points Related to Climate Isohyets Line Definition: An Isohyet Line is a line drawn on a map to connect areas that receive equal amounts of rainfall. These lines help in understanding the distribution of rainfall across a region. Importance: Isohyets are crucial in studying and visualizing rainfall patterns in areas like Rajasthan, where rainfall distribution varies significantly between the eastern and western parts of the state. Isobar Line Definition: An Isobar Line is a line on a map that connects places with equal atmospheric pressure. This helps in understanding weather patterns and is often used in weather forecasting. Importance: Isobars are useful in identifying pressure zones, cyclonic activities, and general wind flow patterns. They are also crucial in understanding the movement of Western Disturbances that bring Mawath to Rajasthan.

 

ISOHYETS AND ISOBAR LINES IN RAJASTHAN

1. Isohyets in Rajasthan

Isohyets are lines drawn on a map that connect areas receiving equal amounts of rainfall. In Rajasthan, Isohyets are critical in understanding the distribution of rainfall, especially considering the state’s arid and semi-arid conditions. Rajasthan is divided into various regions based on the amount of annual rainfall, which is represented through Isohyets. Below are the significant Isohyets and their impact on the state’s geography:

25 cm Isohyet

• Significance: The 25 cm Isohyet divides the desert region of Rajasthan into two parts:
  • Arid Desert: Areas receiving less than 25 cm of rainfall annually.
  • Semi-Arid Desert: Regions receiving between 25 cm and 40 cm of rainfall.
• Regions Affected: The arid and semi-arid regions are primarily located in the western part of Rajasthan, including Jaisalmer, Barmer, and Bikaner. These areas face extreme dryness and require specific agricultural practices to manage scarce water resources.

40 cm Isohyet

• Significance: The 40 cm Isohyet is a critical marker that divides Rajasthan into two equal parts based on rainfall. It marks the eastern boundary of the desert.
• Regions Affected: Areas to the east of this line, including parts of Ajmer, Jaipur, and Udaipur, receive more rainfall than the western regions. This Isohyet serves as a clear indicator of the transition from desert to semi-arid and more agriculturally productive regions.

50 cm Isohyet

• Significance: The 50 cm Isohyet is aligned along the Aravalli Range, a key geographical feature in Rajasthan. This line divides the eastern plains from the western desert, where the Aravallis act as a barrier to the monsoon winds.
• Regions Affected: The regions to the east of the Aravallis, including Banswara, Dungarpur, and Bharatpur, receive higher amounts of rainfall due to the windward effect of the mountains. In contrast, the areas to the west remain dry due to the rain shadow effect.

2. Isobar Line in Rajasthan

Isobars are lines on a map that connect points of equal air pressure. They are important for understanding weather patterns, especially changes in pressure that affect wind movement and temperature. Rajasthan’s air pressure varies significantly between the summer and winter months, and Isobars help in visualizing these differences.

• T = Temperature, AP = Air Pressure
• T↑ = High temperature, T↓ = Low temperature

Isobar Lines in January (Winter)

During the winter months, air pressure is generally higher across Rajasthan due to cooler temperatures. The Isobar lines in January are as follows:

• 1018 mb
• 1019 mb

These higher-pressure levels are characteristic of cooler winter air, which is denser and more stable. As a result, Rajasthan experiences clearer skies and lower temperatures, particularly in regions like Churu and Sikar.

Isobar Lines in July (Summer)

 

In July, the air pressure in Rajasthan decreases due to the hot summer temperatures, which cause the air to rise and create low-pressure zones. The Isobar lines in July are:

• 997 mb
• 998 mb
• 999 mb
• 1000 mb

The lower pressure during summer facilitates the movement of the southwest monsoon winds into Rajasthan, bringing much-needed rainfall to parts of the state. However, the monsoon’s effect varies widely across different regions due to geographical barriers like the Aravalli Range.

Understanding Isobar Maps of Rajasthan

The maps provided show Isobar lines across Rajasthan, which connect locations with the same air pressure. These lines are crucial for understanding weather patterns, such as wind direction and speed, and how temperature differences affect the climate during different seasons in Rajasthan.

1. Isobar Map for Summer (July)

The top map shows the Isobar lines for July, during the summer season. The air pressure values shown are in millibars (mb), with common pressure values such as 997 mb, 999 mb, and 1000 mb indicated on the map.

Key Features:

• Low-Pressure System: The 997 mb isobar is found near Jaisalmer, extending across Barmer and parts of the western desert. This low-pressure zone is typical of Rajasthan’s summer season when the region experiences extremely high temperatures, causing the air to rise and creating low-pressure conditions.
• 997 mb to 1000 mb Zones: As you move towards the east, the air pressure gradually increases to 1000 mb in regions like Sirohi, Pratapgarh, and Jhalawar. The 999 mb isobar runs through Jodhpur, Nagaur, and Ajmer, indicating slightly higher pressure areas compared to the desert regions in the west.
• Impact: These low-pressure zones are important because they draw in the southwest monsoon winds, which bring rainfall to Rajasthan. The monsoon winds generally flow from areas of high pressure in the Indian Ocean to low-pressure zones over the subcontinent.

2. Isobar Map for Winter (January)

The second map shows the Isobar lines for January, during the winter season. The values displayed on this map likely indicate higher pressure compared to the summer map, consistent with colder conditions.

Key Features:

• Higher Pressure System: The 1018 mb isobar is located in the western parts of Rajasthan, including Jaisalmer, Barmer, and Bikaner. This indicates a high-pressure zone, which is typical of Rajasthan’s winter season, when the cold air is denser and heavier, resulting in increased pressure.
• 1018 mb to 1020 mb Zones: As you move eastward, the pressure remains high, with areas like Churu, Sikar, Ajmer, and Jaipur experiencing 1019 mb to 1020 mb pressure levels. The high pressure in winter contributes to clear skies and lower temperatures, especially during nighttime.
• Impact: These high-pressure conditions result in cold winds blowing from the northeast, causing a significant drop in temperature. This pattern is typical of Churu, which often experiences the lowest winter temperatures in Rajasthan.

Conclusion

• Summer (July): The low-pressure zones in the summer attract the southwest monsoon winds, bringing rainfall to Rajasthan. The air pressure decreases towards the western desert areas, where temperatures are the hottest.
• Winter (January): The high-pressure zones during the winter result in colder weather, with clear skies and dry conditions. The cold winds, combined with high pressure, lead to the typical cool and dry winter climate in Rajasthan.

These Isobar maps are critical for understanding how air pressure affects the weather patterns in Rajasthan during different seasons, influencing the monsoon, wind patterns, and temperature extremes.