CLIMATE

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Weather and Climate

• Weather refers to short-term atmospheric conditions, while climate represents long-term patterns.
• Weather changes quickly, while climate reflects averages over at least 30 years.
• Weather is local and variable, while climate describes larger regions.
• Weather forecasts focus on short-term predictions, while climate is more predictable over longer timescales.
• Weather considers immediate atmospheric interactions, while climate encompasses broader factors.
• Weather is measured in real-time, while climate is assessed using long-term data.

India’s Climate

• India has a Tropical Monsoon climate.
• The term “monsoon” comes from the Arabic word “mausim” meaning season.
• Monsoon winds blow from the sea to the land in one season and from the land to the sea in the other.
• These winds reverse their direction over the course of a year.
• The monsoon winds influence the climate of not only India but also the entire South Asia region.

Factors Affecting India’s Climate

• Latitudes and the role of tropic of Cancer 
  • Latitudes determine the temperature and heat zones of the world.
  • The Tropic of Cancer divides India into two almost equal parts. 
  • The regions south of the Tropic of Cancer fall into the torrid zone, so the temperature is warm for most of the year. 
  • Places north of the Tropic of Cancer experience distinct summer and winter. 
• Himalayas
  • They protect the Northern part of the country from cold winds originating in the Arctic Circle.
  • These mountains act as a wall. 
  • They stop the monsoon winds and helps in bringing rainfall to most part of the country from June to September.
• Waterbodies
  • The Indian Ocean, Bay of Bengal and Arabian Sea, keep the temperature moderate in coastal areas. 
  • The coastal areas of India have a ‘maritime’ climate.
  • The temperature is kept moderate by the land and sea breezes blowing over the area. 
  • Inland areas experience hot summers and cold dry winter because they are away from the sea. 
  • The water bodies also provide moisture to the monsoon winds. 
• Monsoon Winds 
  • Ferrel’s Law: Winds deflect to the right in the northern hemisphere and to the left in the southern hemisphere. Deflection force depends on wind mass, velocity, latitude sine, and Earth’s angular rotation velocity.
  • Due to this the South-east trade winds cross the equator and come to the Indian subcontinent.
  • These winds are called Southwest-monsoon winds. 
  • These winds bring rainfall with them. 
  • Some parts of the country receive less rainfall while some receive very heavy rains.
  • In winter, the temperature changes and pressure conditions are altered. 
  • Due to this, the monsoon retreats. 
  • These winds begin to blow in opposite direction. 
  • The winds are dry, so they don’t bring much rain.
  • While retreating, they pick up moisture from the Bay of Bengal.
  • Due to this, Tamil Nadu’s coastal areas and Southern Andhra Pradesh receives rainfall. 
• Other relief features
  • The western ghats intercept incoming Southwest monsoon winds.
  • The windward side of the western ghats receives heavy rainfall.
  • Vast areas of Maharashtra, Karnataka, Andhra Pradesh and Tamil Nadu fall into the leeward side of the western ghats. 
  • They receive scanty rainfall. 
  • Pune is on the leeward side of the Western Ghats, resulting in less rainfall compared to Mumbai on the windward side.
  • Cherrapunji gets more rain as it’s on the windward side of the Khasi hills, causing uplift and increased precipitation. Shillong, on the leeward side, receives less rainfall.
  • This tells us that the position of hills and mountains also determines the climate of a place.
  • The Aravalli do not intercept the monsoon winds, so Rajasthan remains very dry. 
• Altitude 
  • Places at high elevation remain cold throughout the year. 
  • As you ascend 1000m, the temperature drops by 6°C.
  • This is the region why cities in the plains like Patna and Lucknow are warmer than the cities in the hills like Shimla and Darjeeling.
• Upper Air Circulation
  • Jet stream plays an important role in circulating monsoon. 
  • It is a fast-blowing wind that blows at the height of 3 to 5 km over subtropical areas.  
  • The Himalayas act as a barrier in their way. 
  • These winds are divided into easterly and westerly jets.
  • Westerlies draw western disturbances in India. 
•  El Nino
  • El Niño is a climate pattern with warming in the equatorial Pacific Ocean.
  • It affects global weather patterns, including the Indian climate.
  • Weakened trade winds and reduced temperature contrast occur during El Niño.
  • It disrupts monsoon circulation in India, causing below-average rainfall and drought conditions.
  • El Niño leads to uneven rainfall distribution with excess in some areas and deficits in others.
  • Impacts vary among El Niño events.
  • Monitoring El Niño helps anticipate and manage effects on agriculture, water resources, and climate.
• La Nina 
  • It has opposite effects to El Niño on global weather patterns.
  • Strengthened trade winds and increased temperature contrast occur during La Niña.
  • It influences the Indian climate by enhancing the monsoon circulation.
  • La Niña brings above-average rainfall to parts of India, especially the south and east.
  • Effects on the Indian monsoon can vary depending on other factors.
  • La Niña is a cooling pattern in the equatorial Pacific Ocean.

Features of Indian Climate

• Indian Subcontinent has varied climate due to its vast size and topography.
• Indian climate shows equatorial, tropical, semi-arid, temperate or arctic features.
• Indian has tropical monsoon type climate. 
• It is dominated by Southwest monsoon during summer and Northeast monsoon in winter.
• The monsoon wind changes directions as the season changes. 
• In summer it blows moist air, in winter, the air is dry.
• Cyclone formation in Bay of Bengal
  • Warm sea surface temperatures in the Bay of Bengal provide energy for cyclone formation.
  • Low-pressure systems serve as the starting point for cyclone development.
  • Moist air rises, causing convergence and intensifying the low-pressure system. 
  • The Coriolis force influences the air flow, causing cyclonic rotation.
  • Spiral cloud bands with dense clouds, strong winds, and heavy rainfall form.
  • Cyclone development is influenced by complex atmospheric factors and closely monitored by meteorological agencies.
• Winter rainfall in Northwest India 
  • Winter rainfall in Northwest India is primarily influenced by Western Disturbances.
  • Western Disturbances are weather systems originating from the Mediterranean region.
  • They bring winter rain to northern and northwestern parts of India, including Punjab, Haryana, Uttarakhand, and Western Uttar Pradesh.
  • Winter rainfall provided by Western Disturbances is crucial for agricultural activities during this season.
• Presence of Four distinct seasons 
  • Winter Season – December to February.
  • Summer Season – March to May.
  • Rainy Season or Monsoon – June to September. 
  • Season of Retreating Monsoon – October to. November. 

The Winter Season

• Temperature:
  • Winter season in India generally spans from December to February.
  • Temperatures vary across the country, with colder conditions experienced in the northern and mountainous regions.
  • Northern India experiences cooler temperatures, with some areas experiencing freezing temperatures and even snowfall.
  • Southern and coastal regions tend to have milder winters, with temperatures remaining relatively warmer.
• Pressure:
  • During winter, there is a high-pressure system over the northern part of the Indian subcontinent.
  • The high-pressure system leads to calm and stable weather conditions with clear skies in many parts of the country.
  • The pressure gradient between the high-pressure system over land and the low-pressure areas over the surrounding oceans influences wind patterns.
• Winds:
  • Winter is characterized by the dominance of dry and cold winds blowing from the north and northeast regions.
  • The cool and dry winds from the Himalayas, known as “northerly” or “northwesterly” winds, bring colder temperatures to the northern plains.
  • These winds contribute to the formation of fog and mist in some areas, especially during the early morning hours.
• Winter Rainfall:
  • Winter is generally considered a dry season in most parts of India, with limited rainfall.
  • However, certain regions experience winter rainfall due to the influence of Western Disturbances.
  • Northwestern parts of India, such as Punjab, Haryana, Uttarakhand, and Western Uttar Pradesh, receive winter rain from these weather systems.
  • Additionally, Tamil Nadu, located in the southern part of India, experiences rainfall during the winter season due to the influence of the Northeast Monsoon.

The Summer Season

• Temperature:
  • Summer season in India generally spans from March to May or June.
  • Temperatures across the country can be quite high, particularly in the northern and central regions.
  • Northern plains and desert areas experience scorching heat with temperatures often exceeding 40°C (104°F).
  • Coastal regions and hill stations have relatively milder temperatures during summer.
• Pressure:
  • During summer, a low-pressure area forms over northern India due to intense heating.
  • This low-pressure system leads to the inflow of moisture-laden winds from the Arabian Sea and the Bay of Bengal.
• Winds:
  • The southwest monsoon winds start to establish during summer.
  • These winds, known as the southwest monsoon, bring moisture and rain to the Indian subcontinent.
  • Pre-monsoon thunderstorms, known as “Nor’westers” or “Kalbaisakhi,” are common in some parts of the country, particularly in the eastern regions.
  • Loo is a type of hot and dry wind that originates in Rajasthan during the summer, it blows over the states of Punjab, Haryana, UP and Bihar. 
• Summer Rainfall:
  • Summer is characterized by the onset of the southwest monsoon in India.
  • The southwest monsoon brings widespread and heavy rainfall to most parts of the country.
  • It begins around May or June, starting from the southwestern coast and gradually covering the entire country.
  • The monsoon rainfall is crucial for agriculture and replenishing water resources across the country.

Monsoon

• Timing:
  • The monsoon season in India typically spans from June to September.
  • It is characterized by the arrival and progression of the southwest monsoon winds.
  • Southwest Monsoon:
  • The southwest monsoon is a seasonal wind system that brings much of the rainfall to India.
  • It originates from the Arabian Sea and the Bay of Bengal, carrying moisture-laden winds to the subcontinent.
• Onset and Progression:
  • Monsoon onset: Late May/early June from southwest coast, moving northward.
  • Covers entire country in following weeks.
  • Different regions have varying timings.
• Rainfall:
  • Crucial for agriculture, significant precipitation.
  • Supports crops like rice, wheat, and sugarcane.
  • Varies yearly, impacting agriculture and water resources.
• Monsoon Dynamics:
  • Influenced by atmospheric factors like ITCZ and ENSO.
  • Brings heavy downpours and floods in some areas, while others may experience rainfall deficit.
• Monsoon Impact:
  • Vital for India’s economy, agriculture, and water management.
  • Affects daily activities, cultural practices, and festivals.
• Arabian Sea Branch of Monsoon 
  • Reaches south-west India by end of May or start of June. 
  • Kerala receives the first rains. 
  • Wind rises high when it reaches Western Ghats. 
  • The Windward side of Western Ghats receives about 200 to 250 cm of rain.
  • Leeward side only receives 60 cm of rain..
  • Causes a good  amount of rainfall, in the Chhota Nagpur Plateau. 
  • Rajasthan does not receive rains because of the Aravalli. 
• Bay of Bengal branch of Monsoon.
  • The winds reach Bay of Bengal by the end of May.
  • Andaman and Nicobar Islands receive rain. 
  • The winds then move north and gets moisture from Ganga-Brahmaputra Delta. 
  • Sikkim, Arunachal Pradesh and West Bengal Receives heavy rainfall. 
  • Cherrapunji receives more than 1250 cm of rainfall per year.
  • One branch of these winds move toward west blowing from southeast direction. 
  • This deflected branch brings rain to northern part of India and in the regions like Jharkhand and Bihar. 
  • The eastern coastal plains lies on the leeward side of the Western Ghats, receives marginal rain from the Southwest Monsoon.
  • Temperature remains low during the wet season. 

Retreating Monsoon

• Timing:
  • Generally occurs during the months of October and November.
  • It marks the transition from the monsoon to the post-monsoon period.
• Gradual Withdrawal:
  • It is a gradual process that takes place over several weeks.
  • The intensity and frequency of rainfall gradually decreases.
  • The withdrawal is characterized by a reduction in the coverage of rainy days and the amount of rainfall.
• Southward Movement:
  • Retreat begins in northern regions (e.g., Punjab, Haryana, Uttar Pradesh).
  • Progresses southward, reaching central and southern parts of India.
  • Withdrawal extends to regions like Maharashtra, Gujarat, Madhya Pradesh.
• Weakening of Monsoon Currents:
  • Progressing monsoon season reduces temperature contrast between land and ocean.
  • Gradual weakening of monsoon currents leads to reduced rainfall.
  • Weaker monsoon winds coincide with decreased atmospheric moisture.
• Influence of Weather Systems:
  • Low-pressure systems and cyclonic circulations impact monsoon retreat.
  • They can enhance or delay the withdrawal, causing timing and spatial variations.
  • Weather systems can result in intermittent bursts of rainfall during retreat.
• Variability:
  • Timing of monsoon retreat varies annually due to climate factors.
  • El Niño, La Niña, Indian Ocean Dipole, and atmospheric conditions contribute to variability.
  • Variations in retreat timing affect agriculture and water resources.
• Changes in Wind Direction:
  • Wind direction shifts from southwest to northeast during retreat.
  • Northeast monsoon follows, bringing rainfall to southern India.
  • Change influenced by northern hemisphere high-pressure systems.
• Transition to Post-Monsoon Season:
  • Retreat signifies transition to post-monsoon season in India.
  • Lower rainfall, cooler temperatures, and shifting weather patterns.
  • Influence of systems like Western Disturbances in northern India.

Characteristics of Monsoon

• Erratic nature – No fixed time of arrival or departure.
  • Sporadic -some regions get over 500 cm of rain, others get less than 50 cm per year. 
• Orographic – Amount of rainfall is influenced by location, position and alignment of hills and mountains.
• Concentration – brings rain between June and September. 
• Economic significance – Farmers are so dependent on monsoon that crop failure is related to failure or late arrivals of monsoon.
• Some regions of the country get rain due to tropical and temperate depressions. 

Rainfall Variations

Land and Sea Breeze and Monsoon

• Localized Wind Patterns: Both exhibit localized wind patterns in specific regions.
• Diurnal and Seasonal Patterns: They follow diurnal (daily) cycles and can have seasonal occurrences.
• Wind Direction: They involve changes in wind direction, either from sea to land (land and sea breeze) or a shift in prevailing wind direction (monsoon).
• Influence on Climate: They impact local climate, affecting temperature, humidity, and precipitation patterns
• Importance for Rainfall: They contribute to localized or widespread rainfall, depending on the scale.
• Differential Heating: Both are influenced by differential heating of land and water surfaces.
• Varying Scale: Land and sea breeze are localized, while monsoons are large-scale wind systems.

Why is Thar A Desert?

• The Aravalli range lies parallel to the flow of the Southwest monsoon winds.
• The Bay of Bengal branch has no or very little moisture left by the time it reaches Rajasthan.
• Winter rain is not experienced here, North-east winter monsoon winds originate in the Northern Plains and low offshore. 
• The Lack of water in that contributes nothing to the humidity and hence, air remains dry throughout the year resulting in no rainfall.