Climate and Rainfall Patterns in India | Geography for UPSC CSE PDF Download

Introduction

Northern India during September and October experiences high mean maximum temperatures. June is the hottest month in India. With the onset of monsoon rains, temperatures begin to drop, with a reduction of about 6°C in Northern India. After the rainy season, temperatures start to rise again. In September and October, the Southwest (S.W.) monsoons begin to retreat, leading to dry spells. Southern India faces high temperatures as the sun is directly overhead, while Northern India experiences a second peak in temperature. The average maximum temperature during September and October in Delhi exceeds 33°C.

Cherrapunji, one of the rainiest places globally, receives an average annual rainfall of 1080 centimeters (cms). Situated on the southern slopes of the Khasi Hills in Meghalaya at an altitude of 1500 meters above sea level and surrounded by hills on three sides, its unique geography traps the Bay of Bengal branch of the monsoon winds. These winds rise repeatedly, resulting in heavy rainfall. In 1861, a record rainfall of 2262 cms was documented here. The Tamil Nadu coastal region primarily receives its rainfall in winter, located on the eastern coastal plain of the Coromandel coast. While this region experiences rain in both winter and summer, the majority occurs in winter.

• Summer: Tamil Nadu remains dry during the summer months because it is located in the rain shadow area of the Western Ghats.
• Winter: The Northeast (N.E.) monsoons gather moisture as they traverse the Bay of Bengal, bringing rainfall to Tamil Nadu.
• Retreating Monsoons: The retreating monsoons, which are on-shore winds, affect the Tamil Nadu coastal plain. These winds are influenced by the Eastern Ghats, which forces them to bring moderate rainfall to the region.

Western Disturbances

Western disturbances are low-pressure systems that originate from West Asia and the Mediterranean region. These systems move eastward across Iran and Pakistan before reaching India during the winter months. The westerly jet stream plays a crucial role in guiding these disturbances into India. These disturbances are most active in winter across Northern India, with an average of four to five such systems arriving each month. They bring rainfall to the regions of Jammu & Kashmir, Punjab, Himachal Pradesh, Haryana, and Rajasthan, and snowfall to the northwestern Himalayas. The rainfall from these disturbances typically decreases from west to east and is essential for the rabi crops, particularly wheat. The average rainfall from these disturbances ranges from 20 mm to 50 mm.

Hottest Regions in India

The highest temperatures in India are recorded in the western parts of Rajasthan, with Barmer being the hottest place where summer temperatures can reach 50°C. Several factors contribute to this extreme heat:

• Inland Location: Barmer's distance from the Indian Ocean leads to a continental effect, raising summer temperatures.
• Hot Winds: The Loo, a hot and dusty wind, further elevates temperatures in the region.
• Sandy Soils and Low Humidity: These factors also play a role in the high temperatures experienced in Barmer.

Summer Monsoon Rainfall: Key Features

• Seasonal Rainfall: The majority of rainfall occurs during the summer months.
• Unpredictable Rainfall: The amount of rainfall can vary significantly from one area to another.
• Coastal Effect:Rainfall decreases as one moves inland from the coast. For instance:
  • Calcutta: 119 cms
  • Patna: 105 cms
  • Allahabad: 100 cms
  • Delhi: 65 cms
• Breaks in Rainfall: The Southwest monsoon is characterized by dry spells; rainfall does not occur continuously.
• Influence of Weather Systems: The intensity and amount of rainfall are enhanced by weather phenomena such as the jet stream and depressions.
• Rain Shadow Areas: Regions like Tamil Nadu and the Deccan Plateau experience dry conditions due to the rain shadow effect.

Monsoon Burst

• Monsoon winds blow from the southwest over the west coast, carrying a large amount of water vapor.
• These winds arrive quickly, usually in the first week of June.
• This sudden and intense rainfall is referred to as a "Monsoon Burst," characterized by heavy downpours often accompanied by thunder and lightning.
• The experience of this rain can be likened to a balloon filled with water bursting.

Loo

• ‘Loo’ is a hot, dry wind that blows during the day in northern India.
• This wind raises temperatures to extremely dry levels, between 40°C to 50°C.
• These hot winds can be unbearable and may cause heat waves that can lead to fatalities.

Coldest Parts of India

• The trans-Himalayan region, including Jammu and Kashmir and Himachal Pradesh, is the coldest part of India.
• Temperatures can drop to –40°C in places like Dras or Kargil.
• These areas are at high altitudes and experience snowfall in winter, with temperatures often below freezing.

Intertropical Convergence Zone

• The Intertropical Convergence Zone (ITCZ) is a low-pressure area located near the equator, which shifts in response to the sun's position.
• During summer, the ITCZ moves north to approximately 25°N, which helps attract the Southwest Monsoons.
• In winter, the ITCZ shifts back south.
• This phenomenon contributes to the uneven distribution of rainfall in India, where western regions are more prone to drought, while eastern regions are more likely to experience floods.
• Rainfall distribution across India is highly variable:
  • Western regions: receive less than 50 cms of rain, making them drought-prone, such as Rajasthan.
  • Eastern regions: receive around 200 cms of rain, making them flood-prone, like the North Eastern states.

Monsoons' Influence on Indian Weather

• Monsoons play a crucial and unifying role in shaping India's weather patterns.
• The country primarily experiences a monsoon type climate, which varies seasonally but is consistent across different regions, including the Thar Desert, Assam, and Kerala.
• This climate creates a cohesive meteorological system that impacts agriculture throughout India.
• The Himalayas also contribute significantly to the distinct characteristics of the Indian monsoon.
• There are regional differences in how monsoons affect weather; for example, depressions bring rainfall to the Northwest, while retreating monsoons influence the east coast.

Distribution of Annual Rainfall in India

• The average annual rainfall in India is 110 cms.
• Rainfall varies greatly across the country due to its diverse topography.
• India can be divided into different rainfall regions:
  • Heavy rainfall areas: More than 200 cms of rain, found in the western coast, Western Ghats, sub-Himalayas, and northeastern India.
  • Moderate rainfall areas: 100-200 cms of rain, including West Bengal, Odisha, eastern U.P., Madhya Pradesh, and Tamil Nadu coastal plains.
  • Low rainfall areas: 50-100 cms of rain, including western U.P., Haryana, Punjab, Gujarat, and eastern Rajasthan.
  • Scanty rainfall areas: Less than 50 cms of rain, such as Ladakh, southwestern Punjab, southern Haryana, western Rajasthan, Kutch, and the Thar Desert.

Impact of Relief on Rainfall in India

• Geographic Features. The distribution of rainfall in India is significantly influenced by geographic features, particularly the presence of mountains.
• Relief Rainfall. This type of rainfall, known as relief rainfall in India, occurs when onshore winds bring moisture-laden air that is forced to rise over mountains, leading to heavy rainfall on the windward slopes.
• Khasi-Jaintia Hills. In the mountain regions of the Khasi-Jaintia hills, annual rainfall exceeds 1000 centimeters due to the orographic effect.
• Ganges Valley. Rainfall in the Ganges Valley decreases as one moves up the valley, illustrating the impact of relief on rainfall distribution.
• Windward vs. Rain Shadow Areas. Onshore winds cause heavy rainfall on windward slopes of mountains, while the rain shadow areas, located on the leeward side, remain dry. For example, the Garo-Khasi hills receive over 1000 centimeters of rainfall annually, but this drops to 200 centimeters on the Shillong plateau and Brahmaputra Valley, which are in the rain shadow.
• Malabar Coast and Western Ghats. The Malabar coast and Western Ghats receive more than 300 centimeters of rainfall due to their favorable geographic position, while the Deccan plateau lies in the rain shadow of the Western Ghats and receives only 60 centimeters of rainfall.
• Rajasthan and the Aravalli System. In Rajasthan, the Aravalli system runs parallel to the direction of the southwest monsoons, preventing the winds from rising and leaving the region mostly dry.

Cyclones

• Definition. A cyclone is a low-pressure area where winds spiral in at speeds exceeding 64 km per hour.
• Formation of Low Pressure Area. A low-pressure area is created when stagnant moist air warms up and rises, being replaced by air from all directions. The Earth's rotation causes these incoming air currents to spiral rather than converge directly at the center.
• Energy Source. The cyclone gains energy from the latent heat released during the condensation of moisture. In tropical regions, weak vertical wind currents allow this heat to escape, continuously strengthening the cyclone as long as it remains over warm water.

Anti-Cyclone Measures

• Risk to Coastal Cities. Coastal cities are more vulnerable to cyclones due to rapid population growth.
• Long-Term Measures (10 to 20 years):
  • Constructing resilient houses that can withstand cyclones is more cost-effective than rebuilding after a disaster.
  • Urban planning should prioritize keeping vulnerable areas as green spaces to reduce risk.
  • Conducting scientific research to understand and mitigate cyclone impacts is crucial.
• Medium-Term Measures.
  • Enhancing communication networks and warning systems to provide timely alerts.
  • Setting up emergency centres and cyclone shelters for affected populations.
• Short-Term Measures.
  • Evacuating vulnerable areas promptly when a cyclone alert is issued.
  • Implementing public health and sanitation precautions during cyclones.
• Warning Systems. Weather satellites like INSAT can provide a 48-hour warning for cyclones, enabling authorities to alert the public in advance.

Understanding the Indian Monsoon

The Indian monsoon is a complex weather phenomenon influenced by various atmospheric factors. Its mechanism involves several key elements:

• Shifting of the Inter-Tropical Convergence Zone (I.T.C.). The I.T.C. is a region near the equator where trade winds from the Northern and Southern Hemispheres converge. Its northward shift plays a crucial role in initiating the monsoon.
• Northward Movement of the Westerly Jet Stream. The Westerly Jet Stream is a fast-flowing air current in the upper atmosphere. When it moves northward, it influences weather patterns in India, contributing to the onset of the monsoon.
• Replacement by the Easterly Jet Stream. The Easterly Jet Stream, which flows from east to west, also impacts the monsoon by altering wind patterns and moisture distribution.
• Upper Air Circulation over Tibet. The atmospheric conditions over Tibet, including temperature and pressure variations, affect the upper air circulation, which in turn influences the monsoon.

MONEX. To enhance the understanding of the monsoon, the World Meteorological Organisation conducted a monsoon experiment called MONEX over the Arabian Sea and the Bay of Bengal. This research aimed to unravel the complexities of the monsoon system. However, due to the difficulties in recording and measuring upper air observations over vast sea areas, the complete functioning of the monsoon is still not fully understood.

El Niño Effect

El Niño is a climate phenomenon characterized by the periodic warming of sea surface temperatures in the central and eastern Pacific Ocean. It typically occurs around Christmas time and has a significant impact on global weather patterns, including the Indian monsoon. El Niño affects the Southern Oscillation, which is the seesaw movement of weather conditions between the Pacific and Indian Oceans.

The Southern Oscillation Index (SOI) is a crucial measure used to predict the strength of the monsoon. By monitoring the SOI, meteorologists can determine whether the upcoming monsoon season is likely to be weak or strong. For instance, a positive SOI often indicates a strong monsoon, while a negative SOI may suggest a weaker monsoon.