Chapter 4 Climate

Overview

This chapter explains the concept of climate and the factors influencing India's climate, particularly the monsoon type. It details the seasonal variations in temperature and precipitation across the country and elaborates on the mechanism of the Indian monsoon. The chapter also discusses the role of monsoons as a unifying force and covers various weather phenomena associated with different seasons.

Climatic Controls

The climate of any place is influenced by several factors:

• Latitude: Affects the amount of solar energy received; temperature generally decreases from the equator towards the poles.
• Altitude: As elevation increases, the atmosphere becomes less dense, and temperature decreases.
• Pressure and Wind System: Determined by latitude and altitude, influencing temperature and rainfall patterns.
• Distance from the Sea (Continentality): Coastal areas experience moderate climates, while interior regions have extreme temperatures (hot summers, cold winters).
• Ocean Currents: Onshore winds interacting with warm or cold currents affect coastal climates.
• Relief Features: Mountains can act as barriers to winds and influence precipitation (e.g., leeward sides are drier).

Factors Affecting India’s Climate

India's climate is a blend of tropical and subtropical characteristics due to the Tropic of Cancer passing through its middle.

• Latitude: The Tropic of Cancer divides India into tropical (south) and subtropical (north) regions.
• Altitude: The Himalayas in the north (average 6,000m) protect from cold Central Asian winds, leading to milder winters compared to Central Asia. Conversely, coastal areas have low elevations (around 30m).
• Pressure and Winds: India experiences unique pressure and wind systems. In winter, high pressure north of the Himalayas causes cold, dry winds blowing south. In summer, a low-pressure area develops over northern India and Asia, attracting moisture-laden southwest monsoon winds from the Indian Ocean. The Coriolis force deflects these winds towards the right in the Northern Hemisphere.

Jet Streams: High-altitude westerly winds influence India's weather. The subtropical westerly jet streams blow south of the Himalayas for most of the year, bringing western cyclonic disturbances in winter. In summer, they shift north, allowing the sub-tropical easterly jet stream to blow over peninsular India.

Western Cyclonic Disturbances: These winter phenomena from the Mediterranean region affect northwest India. Tropical cyclones occur during the monsoon and in October-November, impacting coastal regions.

The Indian Monsoon

The term 'monsoon' originates from the Arabic word 'mausim,' meaning season, referring to the seasonal reversal of wind direction. India's climate is predominantly of the monsoon type. Several factors influence its onset and intensity:

• Differential heating and cooling of land and water create low pressure over India.
• The northward shift of the Inter Tropical Convergence Zone (ITCZ) during summer.
• A high-pressure area east of Madagascar influences the monsoon.
• Intense heating of the Tibetan plateau creates low pressure at high altitudes.
• The movement of the westerly and easterly jet streams.

The Southern Oscillation (SO) and El Niño phenomena also impact monsoon intensity. The monsoon arrives in stages around early June, with the Arabian Sea and Bay of Bengal branches advancing rapidly. It retreats gradually from the northwest starting in early September.

The Seasons

India experiences distinct seasons:

• The Cold Weather Season (Winter): From mid-November to February, characterized by decreasing temperatures from south to north. Northern plains experience warm days and cold nights, with frost common. Northeast trade winds are dry, but Tamil Nadu coast receives some rainfall. Western disturbances bring winter rains to the plains and snowfall in the mountains, crucial for Rabi crops.
• The Hot Weather Season (Summer): March to May, marked by rising temperatures and falling pressure in northern India. The 'Loo' (hot, dry winds) prevails in northwest India, and dust storms are common. Pre-monsoon showers ('mango showers') occur in Kerala and Karnataka.
• Advancing Monsoon (The Rainy Season): From June to September, low-pressure over northern plains intensifies, attracting southwest monsoon winds. These winds bring abundant moisture, resulting in heavy rainfall, particularly on the windward side of the Western Ghats and in northeastern India. However, monsoons have "breaks" – periods of intermittent rain interspersed with dry spells, influenced by the monsoon trough's position.
• Retreating/Post Monsoons (The Transition Season): October-November, marked by weakening monsoon trough, clear skies, rising day temperatures, and oppressive humidity ('October heat'). Cyclonic depressions form over the Bay of Bengal, causing heavy rain on the eastern coast.

Distribution Of Rainfall

Rainfall distribution varies significantly across India. Areas like the western coast and northeastern India receive over 400 cm annually, while western Rajasthan and parts of Gujarat receive less than 60 cm. The variability of annual rainfall is high, especially in low-rainfall regions, making some areas prone to floods and others to droughts.

Monsoon As A Unifying Bond

Despite regional climatic variations, the monsoon acts as a unifying force in India. It influences the landscape, flora, fauna, agricultural calendar, and the lives and festivities of people across the subcontinent, bringing a rhythmic cycle of seasons and driving agricultural activities.

Exercise

The exercises at the end of the chapter test understanding of India's climate. Multiple-choice questions cover topics like highest rainfall locations, summer winds, winter rainfall causes, monsoon arrival, and winter season characteristics. Short answer questions delve into climatic controls, reasons for India's monsoon climate, temperature variations, influencing winds, jet streams, monsoon definitions, and the unifying role of monsoons. Students are asked to explain rainfall distribution, discuss monsoon mechanisms, describe seasonal characteristics, and analyze the effects of monsoon rainfall. Map skills require identifying areas with specific rainfall levels and showing monsoon wind directions. Project activities encourage data analysis and comparison of climatic conditions in different Indian cities, promoting a deeper understanding of regional climate variations and their causes.